In December 2008, Jeff Rosen wrote a post on the Wolfire blog titled Why you should support Mac OS X and Linux, in which he discussed reasons beyond philosophical and customer satisfaction concerns for supporting additional desktop platforms, focusing on reasons why it makes business sense to support operating systems which have generally been ignored by the broader gaming industry.

Eighteen months later, Jeff coordinated a pay-what-you-want game promotion in conjunction with four other indie developers, which touted cross-platform (Linux, Mac OS and Windows) support as one of its main selling features. This promotion, known as the Humble Indie Bundle, was a resounding success, and Jeff would eventually move away from Wolfire Games to start a company that would focus on publishing game bundles in the Humble Indie Bundle's style.

Since then, Humble Bundle Inc. has overseen 18 additional game bundles (consisting of a cumulative 103 titles), as well as digital music and an ebook bundle, selling over 4.9 million units and raising over $32,000,000 for game developers and charities (see the Humble Visualisations for further statistics, and my other Humble Bundle analyses for interpretations), whilst promoting independent creators, DRM-free stances, the pay-what-you-want model, and platform agnosticism.

Humble Bundle's success has raised the profile of DRM related issues, raised awareness of indie games and developers, and improved industry visibility to Linux and Mac OS as viable gaming platforms, causing many customers to view the Humble Bundle as reliable, consistent sources of activism in these areas.

This has not been without misstep or hurdle though, and a recent bundle which supported only Windows (consisting of titles which all required the Steam platform's DRM, and which were promoted under the banner of an industry recognised publisher of mainstream games) has been confronting for supporters with whom the philosophical ideals seen within previous bundles had resonated. Additionally, this bundle has caused many Linux and Mac OS gamers to feel alienated and abandoned - a concern multiplied by its overwhelming success, both in terms of revenue and sales, as it becomes evident that from a purely economical standpoint, these less principle-oriented bundles are of greater value than the previous, cross-platform bundles.

In the interests of providing some extra perspective and context to discourse, I am going to attempt to present an unbiased account of cross-platform titles throughout Humble Bundle promotions. For my personal opinions of the Humble THQ Bundle, see here.

Stuff we're going to take a look at:

What Exactly Are We Looking At Here?

This bit is what people would generally refer to as a "methodology" statement talking about how I conducted my research, what assumptions are used, and that sort of thing. You can skip it if you like, but if anything doesn't add up or doesn't make sense, here is the place to double check whether that's intentional or not.

Specifically, this article will focus on the details surrounding Linux, Mac OS and Windows releases which have been included in Humble Bundle promotions. Betas and pre-orders that represent an ongoing commitment by the developer to provide a released cross-platform title (Cortex Command, for example) will be counted as though the final release were included in any relevant bundles, and will appear in any charts or graphs, whilst tech demos and abandoned projects which were released "as is" without intent to be further developed/supported (Introversion's Voxel Tech Demo, for example) will be considered to only support the platforms supported at the time of any relevant bundle. Titles which use platform independent technologies are counted as being cross-platform, though specifics will be discussed in the Technologies section. Titles which provide a different experience to users on different platforms will not be counted as being cross-platform (for example, Wolfire's Mojam game The Broadside Express, for which natively compiled Windows and Mac OS versions exist, but Linux is only supported via a separate Flash version).

I've chosen to avoid looking at Android ports for several reasons, most of them relating to the fact that Android is generally recognised as a mature and viable platform in the mobile space, and cross-platform support not being as big an issue compared to the PC gaming industry, which remains comparatively dominated by Windows-only titles. Additionally, any mobile oriented interpretations we could draw would be inherently limited as Humble Bundle promotions have not been cross-platform so far as mobile devices go, with Android being the only mobile platform supported thus far.

To increase visibility of cross-platform releases motivated by Humble Bundle promotions, only the first appearance of titles which have been in multiple bundles are taken into consideration.

The data we're going to be going over has been collected from sources ranging from Wikipedia and developer news posts through to forum threads and email correspondence with developers. Unfortunately, I have been unable to be provide the level of citation that I would normally prefer for an article like this, but I will endeavour to provide updates with URLs and references after publishing (getting this live before another bundle goes off is fairly high priority right now). Dates, revenue totals and averages, for all bundles except the Double Fine Amnesia Fortnight promotion, have been sourced from the Humble Visualisations. Due to the variety of sources involved, accuracy of other dates may vary depending upon time zone of source (also, month precision the best I could narrow down the release dates of Zen Puzzle Garden)

At the time of writing, the Humble Indie Bundle 7 had received a set of bonus titles and had four and a half days remaining. Figures for relating to this bundle should be interpreted as incomplete.

I would normally reserve the following for a footnote, but I would very much like to highlight that I would not have been able to complete my research for this article without the cooperation of a number of developers, porters and Humble Bundle organisers. In particular, I would like to thank Jeff Rosen of Humble Bundle, Joel Kinnunen of Frozenbyte, Nathan Vella of Capybarra Games, Felix Bohatsch and Andrea Schmoll of Broken Rules, Joseph White of Lexaloffle Games, MangoFusion (James Urquhart) and Jimmeh (James Hannet) of Mode 7 Games, David Kalina of Tiger Style Games, Elke Schipper of Two Tribes, and Kyle Pulver of KPULV, as well as porters icculus (Ryan C. Gordon, icculus.org), urkle (Edward Rudd, outoforder.cc), and flibitijibibo (Ethan Lee, flibitijibibo.com) for their assistance and clarification.

Debuts and Existing Cross-platform Titles

Without a doubt, one of the most celebrated aspects of Humble Bundle promotions has been the number of titles which have been released on new platforms in order to participate. Across all 18 game oriented bundles, 58 titles have made their Linux debut, 21 on Mac OS, and 22 on Windows (with Double Fine's Amnesia Fortnight prototypes providing a large portion of the latter). Whilst many of these titles may have been released on their respective platforms even if they had not been included in a Humble Bundle promotion, it is clear that Humble Bundle has provided a level of support and promotion that would otherwise be unattainable.

Of the titles listed above, 21 had not previously been available for a PC operating system, with one of those (Waking Mars) previously released on a non-PC platform.

Aggregated biannual Humble Bundle debuts per platform. Titles may be included twice if they debuted on multiple platforms.

Note that the first data point is not a full six months as the first Humble Indie Bundle occurred during May 2010.

Looking at debuts in a biannual sense, we can get a feel for the changing rate of debuts over time. In this graph, the combined number of debuts across all platforms may outnumber the total count of titles new to Humble Bundle promotions, as games may be debuting on multiple platforms.

Until mid 2011, the Humble Bundle promotions had not hit their now established frequency of approximately one bundle per month (with the Double Fine Amenesia Fortnight promotion not being presented on the main humblebundle.com page, and the THQ bundle being Windows only, the presence of two bundles each in November and December 2012 can be discounted).

Cumulative Humble Bundle debuts per platform. Titles may be included twice if they debuted on multiple platforms.

It's clear to see that Linux has enjoyed the majority of debuts per platform across all Humble Bundles, and though there is a visible drop in debuts for the first half of 2012 even though there were the same number of promotions as the last half of 2011, this is mostly attributable to the Android themed bundles reusing games which have been in previous bundles (many of these titles had not previously been available for Android) as core titles rather than bringing in new (to Humble Bundles) ones, as has been the norm with other bundles. Examining ratios, we can see growth in Linux debuts from the second half of 2011, increasing from 60% to 74% during the first half of 2012. This then falls off to 49% during the last half of 2012, a factor of the Windows-only THQ and Amnesia Fortnight bundles, without which, 86% of new Humble Bundle titles would have also been Linux debuts.

Mac OS and Windows have generally seen the same number of debuts with exceptions during the last halves of 2011 and 2012 creating a difference of one title in favour of Windows. From the second half of 2011, Mac OS debuts have declined from 23% of new titles, dropping to 15% for the entirety of 2012, whilst Windows has seen growth from 13% through 21% to 26% across the same timeframe. Without the THQ and Amensia Fortnight promotions, Windows would have been more neglected of debuts than ever before, garnering a mere 9% of new titles from other bundles in the second half of 2012, and Mac OS would have seen a jump to 28% of new titles making a debut.

Linux Humble Bundle debuts and new-to-Humble titles vs total revenue per promotion.

A more granular look at debuts shows that Linux has received at least 50% of new titles for all bundles outside of the initial Humble Indie Bundle, the Frozen Synapse Bundle, the Introversion Bundle, the Mojam event and the Botanicula Debut (we'll come back to most of those in a moment) as debuts. Also visible is an interesting (though not one to one) correlation between total revenue and linux debuts. With the exceptions of the THQ Bundle, Amnesia Fortnight event, the Frozenbyte Bundle, and the Voxatron Debut, increases and drops have coincided with increases and decreases in the number of Linux debuts per bundle.

Mac OS Humble Bundle debuts and new-to-Humble titles vs total revenue per promotion.

Looking closer at the Mac OS, we can see that only once (The Humble Indie Bundle 4) has Mac OS had more than 50% of new titles making a debut. Excluding the THQ Bundle and Amnesia Fortnight promotion, 8 promotions have not had any Mac OS debuts at all.

Windows Humble Bundle debuts and new-to-Humble titles vs total revenue per promotion.

Similar to Mac OS, the Windows debut count is limited to 10 of the 19 promotions, receiving no more than 2 debuts at a time except for the Mojam and Amnesia Fortnight promotions, both of which were "game jam" events with a level of Windows exclusivity.

The other side of the debut situation is titles which already had cross-platform support. Whilst these may not represent a direct contribution on Humble Bundle's part to increasing the number of cross-platform games in the market, it does help to highlight those developers who have already made some form of commitment to producing cross-platform PC games.

Interestingly, the total number of titles which were already available for Linux, Mac OS and Windows is 22, very close to the number of titles which weren't previously available for a PC operating system at all (and coincidentally 21% of the 103 titles which have been included across all Humble Bundle promotion).

Does not include non-cross-platform titles, or titles that made a PC platform debut in a Humble Bundle.

* A native Linux version of Bridge Builder exists, but is not advertised on the Chronic Logic or Cryptic Sea websites

** Vectorpark and Amanita Design have other Flash based games that can be played in browsers on all three major PC platforms.

NA indicates that the developer has no other released titles.

Porters: The Unsung Heroes Of Cross-platform Gaming

Supporting additional platforms outside a developer's core target can be daunting. The amount of work involved can vary wildly depending upon a multitude of aspects ranging from the technical (programming styles, library dependencies, support requirements, etc.), to the more abstract (licencing, conflicting information on best practices, the foreign nature of something new, etc.), to the oft misunderstood (packaging, market demand, etc.). Because of this, it can be difficult to understand and budget for additional platform support, and developers are inevitably faced with three options: to support all platforms from the start of the project, avoiding large amounts of porting work by testing against and keeping all platforms supported during the development process; to develop as normal and port the entire codebase to additional platforms at the end of the development process; to engage a third party to handle the porting work.

Outsourcing always adds some communication hurdles and management overhead, and with a good base code for the porting to begin with, it can often actually be faster to just do the port yourself, rather than write all the specifications, handle source code and data accesses, etc. for outsourcing. - Jukka Kokkonen (Frozenbyte)

Each approach has its merits and downfalls. Some developers may be concerned by a lack of internal skill/knowledge and look to a third party porter to bring experience and take responsibility. Others may dislike the idea of granting access to their codebase to external entities and prefer to grow skills in-house. Others still may prefer to incur an initial productivity hit by supporting additional platforms from the outset and identify good cross-platform coding practices as they go so as to avoid significant porting work at the end of the development process. In an interview following the release of Trine 2 for Linux, Joel Kinnunen and Jukka Kokkonen indicate that reducing communication and management overheads as well as being able to provide effective customer support were key motivators for Frozenbyte to begin porting in-house after outsourcing the Linux ports of three successful titles.

Porter breakdown. Whole chart represents total number of titles. Titles which had one platform ported externally are not counted as internal. Titles which received attention from multiple porters are divided between them.

The above chart shows the ratio of internally developed cross-platform titles compared to those that received attention from a third party. For the purposes of this chart, titles which had internally developed ports for some platforms, but contracted external porters to support additional ones are not counted as internal. Titles which had different porters adding support different platforms are divided between them (eg: if porter A worked on a Mac OS port, and porter B provided a Windows port, then each porter would be given 0.5 so that the chart represents the total number of individual titles.

Titles which were ported and then had those ports updated for inclusion in a promotion listed as being ported by the original porter).

Interestingly, the number of cross-platform titles is very close to evenly divided between internal and external, showing that (intentionally or otherwise) Humble Bundle promotions don't favour or prioritise either approach to supporting multiple platforms.

The two largest providers of external porters, urkle (Edward Rudd) and icculus (Ryan C. Gordon), account for nearly a quarter of all Humble Bundle titles. urkle, a software developer and game porter with a history of contributing to Free Software projects, created the bug tracking system used by Humble Bundle's quality assurance team, and is now a full time employee of Humble Bundle, whilst icculus, a freelance software developer and game porter whose porting roots go back to Loki Software, who also runs icculus.org (a hosting project aimed at incubating worthwhile projects) and continues to provide contracted porting services to Humble Bundle participants. Between them, these two porting veterans have been involved in 31 ports across 11 bundles. Several of icculus ports have also been accompanied by some type of associated Free/Open Source Software releases, for example Linux and Mac OS ports of Haaf's Game Engine - used by Hammerfight, and the TheoraPlay and MojoDDS libraries originally developed for Shank's Linux port. Similarly, urkle has had a hand in several Humble Bundle related source releases, including the Penumbra Overture engine and source, Frictional Games OALWrapper, a file IO tracking tool created to assist porting, and the MonoKickstart used by Bastion.

The next largest external porter is Alternative Games (a Finnish company specialising in porting games to Linux and Mac OS), who are responsible for bridging the cross-platform gap for 5 Humble Bundle titles, and together with icculus and urkle, are the only porters to externally work on multiple Humble Bundle titles.

Also worthy of mention is Tim Ambrogi of Final Form, who worked internally on the Linux port of Jamestown, and externally ported Spirits to Windows and Linux for Spaces Of Play.

Some may note the presence of Codeweavers, a US company who provides a commercially supported derivative of Wine, and is also a contributor and sponsor of the Wine project. Given that they actively work to provide a consumer ready experience, and are engaged to fix bugs as any other external porter might be, they deserve inclusion here. Wine and other technologies will be discussed in the Technologies section.

Porter title count. Titles which received attention from multiple porters are counted multiple times.

Titles which were ported internally with external assistance are counted multiple times.

Typically, porters are brought in to work on a title after its completion, and rarely appear in games' credits. Some porters sign platform specific readme documents that are distributed with a title, but many still go by without acknowledgement that is visible to end users. Whilst this may be the norm for contract type work, a 2012 interview in which flibitijibibo (Ethan Lee, porter of Vessel) talked about an overwhelming and encouraging response to a progress post he had written about Vessel's progress suggests that gamers are keen to learn more about game porters and the work they do.

When I posted oh.html for Vessel, I got an overwhelming amount of demand for more posts like it. Every single site I saw that commented on the article had people in the comments demanding more openness in the port development, including on Humble Bundle's blog. - flibitijibibo (Porter)

The table below lists each porter who has had involvement with a Humble Bundle title, and which titles they worked on. Titles may be listed multiple times if multiple porters have worked on them. Titles which were ported internally with assistance/finishing from an external porter (such as Closure, or Dungeons of Dredmor), and titles which had existing ports updated for inclusion may also be listed multiple times.

External Porter Humble Bundles Humble Bundle Titles urkle (Edward Rudd) Humble Indie Bundle

Humble Introversion Bundle

Humble Indie Bundle 4

Humble Bundle for Android

Humble Bundle for Android 2

Humble Indie Bundle V

Humble Indie Bundle 6

Humble Bundle for Android 4

Humble Indie Bundle 7 Penumbra: Overture (Linux, Mac OS)

Darwinia (Linux)

Multiwinia (Linux)

Uplink (Linux)

Defcon (Linux)

Gratuitous Space Battles (Linux)

Edge (Linux)

Toki Tori (Linux)

Swords & Soldiers (Linux)

Zen Bound 2 (Linux)

Bastion (Linux)

Amnesia: The Dark Descent (Linux, Mac OS)

Torchlight (Linux)

Shatter (Linux, Mac OS)

Dustforce (Linux)

Eufloria HD (Linux)

Closure (Linux)

Legend of Grimrock (Linux) icculus (Ryan Gordon) Humble Indie Bundle

Humble Indie Bundle 2

Humble Indie Bundle 3

Humble Bundle for Android 2

Humble Indie Bundle 4

Humble Indie Bundle V

Humble Indie Bundle 6

Humble Indie Bundle 7 Aquaria (Linux, Mac OS)

Lugaru HD (Linux, Mac OS)

Braid (Linux, Mac OS)

Cogs (Linux)

Hammerfight (Linux, Mac OS)

Frozen Synapse (Linux)

Dungeons of Dredmor (Linux, Mac OS)

Super Meat Boy (Linux)

Shank (Linux, Mac OS)

Avadon: The Black Fortress (Linux)

Psychonauts (Linux)

S.P.A.Z. (Linux, Mac OS)

Dungeon Defenders (Linux) Alternative Games Humble Frozenbyte Bundle, Humble Indie Bundle 7 Trine (Linux, Mac OS)

Shadowgrounds (Linux, Mac OS)

Shadowgrounds Survivor (Linux, Mac OS)

Jack Claw (Linux, Mac OS)

Shank 2 (Linux, Mac OS) RedMarble Humble Indie Bundle 4 Gratuitous Space Battles (Mac OS) CodeWeavers Humble Indie Bundle V LIMBO (Linux) Jani Hast Humble Indie Bundle 2 Crayon Physics Deluxe (Unknown) * Simon Roth Humble Indie Bundle 3 VVVVVV (Linux) Patrick Weber Humble Indie Bundle 3 And Yet It Moves (Linux) David Gow Humble Introversion Bundle Multiwinia (Linux) Nick Alexander Humble Introversion Bundle Dungeons of Dredmor (Mac OS) Pat ** Humble Bundle for Android 3 Fieldrunners (Linux) Tim Ambrogi Humble Bundle for Android 3 Spirits (Linux, Windows) flibitijibibo (Ethan Lee) Humble Indie Bundle 6 Vessel (Linux, Mac OS) Chris Kruger Humble Indie Bundle 2 Cortex Command (Mac OS) James Urquhart Humble Frozen Synapse Bundle Frozen Synapse (Mac OS) Nathan Swaney Humble Bundle For Android 4 Waking Mars (Linux)

* Jani Hast's role in porting Crayon Physics is unclear, with this blog post being the only available information at present.

** Details of "Pat" are unclear at the time of writing. Emails to Subatomic Studios are yet to yield a response, but the article will be updated to reflect any new information that arises.

Non-cross-platform Titles

As seen in the Internal vs Externally Ported Titles chart, non-cross-platform titles account for 21% of all Humble Bundle titles. The bulk of these appeared towards the end of 2012 when two Windows only promotions ran concurrently.

The non-cross-platform titles seen within Humble Bundle promotions can be broken down into three types:

Cancelled titles

Internal prototypes, tech demos and game jam titles

Completed titles which were not ported

Non-cross-platform Humble Bundle titles and new-to-Humble titles vs total revenue per promotion.

The only title to fit the first category was seen during the Humble Frozenbyte Bundle, within which an internal Windows build of the cancelled game Jack Claw was included along with source code under a non-permissive licence with the intention that Jack Claw could evolve as a community developed project with support and guidance from Frozenbyte. Jack Claw's status as a non-cross-platform title becomes gray as, as a Linux build was added to the bundle ten days later, and source code for the Mac OS and Linux ports were also added in May.

The Humble Introversion Bundle included two Windows only tech demos (Voxel Tech Demo, City Generator Tech Demo), which showed functionality initially developed for Introversion's cancelled title Subversion. The Humble Bundle Mojam event, for which Mojang, Oxeye Game Studio and Wolfire each created a game within 60 hours for charity also yielded two Windows only titles, Fists of Resistance and The Broadside Express (though there are apparently plans to make a Linux build of The Broadside Express available using Unity 4's Linux publishing features available in the future).

Cumulative non-cross-platform Humble Bundle titles per platform.

In November 2012, Double Fine partnered with Humble Bundle to raise money by offering customers a glimpse into their internal prototyping event Amnesia Fortnight, resulting in a further 8 Windows only prototypes being published as part of a Humble Bundle promotion. It is worth noting that the Amnesia Fortnight promotion did not have the word "humble" in its title, and was available from humblebundle.com/double-fine, rather than the main humblebundle.com page, creating some level of distinction between it and other Humble Bundle run promotions.

The final category of non-cross-platform titles consists of full, completed titles which were not ported to all three major desktop operating systems, was first seen in the Humble THQ Bundle in December 2012, which consisted of 9 Windows only titles for which Steam keys were provided rather than DRM-free downloads. Shortly after the conclusion of the promotion, THQ president Jason Rubin stated on Twitter that feedback from the promotion highlighting demand for Linux support had prompted them to look into additional platforms. Several days after the conclusion of the THQ bundle, shortly prior to the launch of the Humble Indie Bundle 7, THQ announced it had filed for Chapter 11 bankruptcy in an attempt to resolve ongoing financial difficulties.

Got the Linux message load[sic] and clear via #HumbleBundle feedback. Evaluating cost/benefit as we speak. - Jason Rubin (THQ)

It is worth noting that no Humble Bundle promotions with "Indie" in the title contained non-cross-platform games.

Post-bundle Availability

In addition to coordinating promotions, Humble Bundle have made their web based purchasing interface, known as Humble Store, available to developers participating in Humble Bundles, allowing them to sell games via an embedded widget in their own sites, and distribute downloads and other digital items such as Steam keys via the Humble Bundle website.

Post-promotion availability for Humble Bundle titles. Whole chart represents total number of titles. Sources providing all three platforms are counted above combinations. Humble store and own storefronts are counted above other sources. Sources which are not listed on an official web site are only counted if no cross-platform sources or combinations are listed on an official website. Titles which are only available for all three platforms through a combination of sources are divided between them.

Currently, 34 titles from Humble Bundle promotions are making use of this facility to make titles available outside of any promotions they were included in, which, aside from Desura, Indivania and Gameolith, seems to be the only cross-platform (and Linux distribution independent) game purchasing/distribution infrastructure in use by titles which have appeared in Humble Bundle promotions.

Of the titles not making use of the Humble Store web app, 11 titles provide cross-platform purchasing options via custom storefronts on their own websites. 1 title (Blocks That Matter) is available for Linux, Mac OS and Windows both from from Indievania and Desura, and another (Avadon: The Black Fortress) is available from Gameolith.

12 more titles provide a limited degree of cross-platform availability via a combination of the Ubuntu Software Center (which is Linux distribution specific) in combination with Steam or storefronts on their own websites.

Interestingly, 18 titles do not have any references on their official sites to availability on all three platforms (for example, the official site for Gratuitous Space Battles lists Mac OS and Windows availability, but does not mention that it is available for Linux, Mac OS and Windows via Desura). 10 of those titles are ones that are available via the Ubuntu Software Centre. Some of this may be in part attributable to a lack of awareness on the part of developers, as highlighted by recent confrontations between Edmund McMillen and USC organisers regarding reasons for rejections of The Binding of Isaac by both sides, payment, and the legitimacy of Super Meat Boy's availability following the Humble Indie Bundle 4. At the time of writing, the publishing of Vessel for Linux on the Ubuntu Software Center appears to have encountered some problems, with converting an icon between JPG and PNG formats being cited by USC organisers as responsible for a two week delay (responses to requests for additional clarification seemed evasive).

Since the recent launch of the Steam for Linux beta, Humble Bundle titles have slowly trickled in. At the time of writing, 19 games (17 from the official list of supported titles, plus two more that install and run) are playable using the native Steam client. As the client is currently in beta, and officially only targets Ubuntu Linux, it is not counted as truly cross-platform availability for the purposes of this article. It is worth noting that of the titles that are not available for all three platforms, only Aquaria is currently listed as an officially supported Steam for Linux title.

I suspect that with Valve moving Steam onto Linux, the value in continuing to support the platform will stay high. We've spent a lot of our time and energy building a cross-platform engine for Mac/PC/Linux and iOS, I think it'd be a shame not to leverage the cross-compatibility in the future... - David Kalina (Tiger Style Games)

Several Humble Bundle promotions have included pre-orders for unreleased games: Cortex Command (Humble Indie Bundle 2), Splot (Humble Frozenbyte Bundle), and Voxatron (Humble Voxatron Debut). Cortex Command had been available for pre-order throughout most of its 11 year development history, with playable test builds being made available to pre-orderers. Cortex Command eventually received a "complete, but unpolished" 1.0 release in September 2012, 10 months and 2 weeks after its appearance in the Humble Indie Bundle 2. At the time of writing, Splot remains unreleased, with Frozenbyte currently estimating an early 2013 release. Humble Bundle customers who obtained Splot during the Humble Frozenbyte Bundle were given access to a Flash based demo showing the game mechanics intended for the final release. This was updated during the Humble Frozen Synapse Bundle, within which a Splot pre-order was included as a bonus title. At the time of writing, Splot has not been made available to pre-order outside of these promotions. Voxatron became available for pre-order along with a playable alpha with the commencement of the Humble Voxatron Debut. Development of Voxatron seen subsequent alpha builds available to pre-orderers, but at the time of writing the game has not yet reached a full release.

Finally, there are 13 titles which have not been made available for all three platforms outside the Humble Bundle promotions they participated in. Included in that 13 is Catacomb Snatch, for which the game source (but not the cross-platform builds) is publicly available, and Aquaria, (for which Linux users must purchase a Windows version and for use with the freely downloadable Linux binaries). It appears that there are also developers who have been actively disinterested in making titles available for all platforms outside of Humble Bundle promotions, with Super Meat Boy developers Team Meat being particularly vocal (note that the below quote was made at a time when Team Meat was apparenlty unaware that Super Meat Boy was available via the Ubuntu Software Center).

the thought of maintaining and updating another version of the game makes me want to kill myself. - @SuperMeatBoy (Team Meat)

Technologies

There are a great many tools, libraries, frameworks and middlewares available to game developers, some of which are more cross-platform friendly than others. As icculus' Linux port of Dungeon Defenders (a title using the Unreal Engine 3, which has long a history relating to Linux and Mac OS clients for its flagship title, UT3, which were promised prior to release but never appeared. The closest it's had to official cancellation has been a "Sorry but I don't think this is going to happen anymore," comment from Vice President Mark Rein almost three years after release - a lingering reminder of a once steadfast stalwart's exit from cross-platform PC gaming) for the Humble Indie Bundle 7 proves, no hurdles are uncrossable, but with the rate at which Humble Bundle promotions come and go, time and budget constraints can become very real issues (as exemplified by problems encountered with Wwise when LIMBO was being prepared for inclusion in the Humble Indie Bundle V preventing it from being ported natively).

Thanks to @humble & @icculus Dungeon Defenders is also the first native UE3 game on Linux @unrealengine @MarkRein http://bit.ly/hvcsBL - Trendy Entertainment

Some of the technologies used in Humble Bundle titles have also become points of contention for purchasers on some platforms. Most visibly, LIMBO was poorly received when its Linux version was found to be the Windows version making use of a technology called Wine to allow it to run under Linux and Mac OS. For many Linux users who credit the Humble Bundle promotions with helping shift the balance of Linux gaming away from running Windows games via Wine by being a reliable source of native titles, this came as a shock. The Wine wrapper, which had been customised and tailored to suit LIMBO, was provided by CodeWeavers, a supporter/contributor to the Wine project who also offers a commercial supported Wine derivative called CrossOver. Unlike the community maintained Wine project, CodeWeavers have the same kinds of obligations and requirements for robustness and reliability that a porter might have when providing a native version of a title, potentially demonstrating some committment on the part of those engaging CodeWeavers' services to the platforms being targeted (arguably moreso than a developer who fends off requests for native support with a, "I hear it runs with Wine," response). That said, a number of Linux users had reported better performance using the community maintained version of Wine than with the Humble Bundle LIMBO package. During a IAmA event (a question and answer session) on Reddit, Jeff Rosen stated that Humble Bundle were watching carefully and taking feedback on LIMBO, and hoped that Linux users would not be dismissive of the title just because of the involvement of Wine.

The LIMBO Linux build was created by CodeWeavers who basically take a custom version of Wine and tune the game to make sure it runs flawlessly. This is our first experiment with CodeWeavers and we are watching carefully. - Jeff Rosen (Humble Bundle)

Adobe's Flash and AIR platforms have also caused problems. In June 2011, Adobe announced that they would no longer be releasing AIR for Linux, instead making porting kits with source available to Open Screen Project partners, who would be able to make Linux versions available. At the time of writing, no subsequent Linux builds of AIR have been made available. In February 2012, Adobe announced that they would no longer be releasing Flash builds for Linux, instead offering Linux support as a part of Google's Chrome browser, making current versions of Flash inaccessible to Linux users not using Chrome. Across all Humble Bundle promotions, there have been 10 titles making use of Flash, and 3 using AIR. When the Humble Botanicula Debut launched, many Linux users experienced problems with the AIR runtime, which could be resolved by installing the AIR SDK.

A number of Humble Bundle titles making use of Microsoft's .NET Framework have been made available for Linux using a cross-platform open source implementation of the framework known as Mono. When Atom Zombie Smasher was added to the Humble Indie Bundle 3, some customers were disappointed at what they perceived to be a lack of native support, but this concern does not seem to have been raised for subsequent titles making use of Mono or MonoGame (an open source implementation of Microsoft's XNA Framework based on Mono).

Several Humble Bundle titles have made use of Java, a cross-platform runtime which champions a "write once, run anywhere" philosophy aiming to make portability as easy as possible for developers. At least two titles make use of Lightweight Java Game Library (LWJGL), a performance oriented Java framework initially created to handle animation for television by Caspian Rychlik-Prince, lead developer of Humble Indie Bundle 2 title Revenge of the Titans.

In June 2012, Unity Technologies announced that they would be adding a preview of a Linux Publishing feature to their game engine and development environment. At the time of writing, 2 titles (Rochard and Splice) using the Unity engine have been included in Humble Bundle promotions.

Beyond frameworks and platforms, underlying cross-platform libraries such as OpenGL, OpenAL, SDL, ODE, and Freetype can assist developers with ensuring the portability of titles. Forward thinking developers with cross-platform support in mind gravitate towards them to aid in-development or post-development porting efforts. The impact of Humble Bundle promotions in encouraging prospective developers to target additional desktop platforms and choose cross-platform libraries is evident, as highlighted by Waking Mars developer David Kalina mentioned via email, when he stated that their choice of exclusively cross-platform libraries was driven by a desire to participate in a Humble Bundle promotion.

As a side note, I'll mention that there is probably zero chance there would be a Linux version of the game if not for Humble. My entire approach to building our engine was based on the goal of getting into a Bundle someday, which encouraged full cross-platform Win/Mac/Linux support.- David Kalina (Tiger Style Games)

Closing Thoughts

So, what does all this mean? What answers can we derive from the assembled information we've just walked through?

The obvious takeaway is that non-cross-platform issues have been present in Humble Bundle's offerings for much longer than the recent single platform promotions (even beyond the scope of what we've been looking at, almost half of the Humble Store widgets are for non-cross-platform titles), but never to this level. Whilst this doesn't necessarily detract from the range of cross-platform products Humble Bundle offer, precedent can be a powerful thing, and it would be a loss to the gaming industry as a whole should Humble Bundle's cross-platform ideals fall by the wayside after being championed for so long.

That said, things rarely seem as bad in hindsight. SpaceChem's use of Mono, LIMBO's use of Wine, the announcement of venture capital funding appearing around the same time as a "Humble tip" contribution allocation option, the decline of Desura keys, the decline of source releases, outstanding issues with Torchlight and Psychonauts, and even the confusion surrounding the differentiation between "Indie" and "Non-indie" bundles have all faded into the past and are rarely brought up as important issues. There is danger in this though, as situations which are potentially harmful for cross-platform gaming pass by without significant question - the poor maintenance of ports (that it took Super Meat Boy nearly six months to have significant issues in its Linux port resolved, and perhaps only then because it was featured in a subsequent Humble Bundle promotion is disgraceful, and perhaps an echo of disdain for and misconception of some platforms on the part of Team Meat), and the titles which made cross-platform debuts in a Humble Bundle promotion but are no longer available, for example.

At the time of writing, emotions have begun to cool, and the currently running Humble Indie Bundle 7 has brought 8 Linux and 2 Mac OS debuts to the table. Whilst it remains to be seen whether the recent non-cross-platform bundles will have any lasting impact, in-progress numbers for this promotion suggest that Mac OS purchases, and both Linux and Mac OS revenue are down compared to previous "Indie" bundles. The Windows purchases ratio seems much higher than ever seen before in a Humble Bundle promotion (those for which per platform stats have been made available for, at least), potentially indicating that the Humble Bundle audience has been expanded as a result of these recent promotions.

To date, Humble Bundle has been responsible for bringing a huge number of titles to additional desktop platforms and highlighted some great developers with firm committments to Linux, Mac OS and Windows (as my research came together, I was surprised to discover that nearly half of all Humble Bundle titles had been internally ported - a fantastic situation that combined with the number of debuts highlights how many may developers have grown these skills internally as a result of Humble Bundle's efforts). As the visualisations we've just gone through show, this is now happening faster than ever. Though many chose to be dismissive of the figures to come out of Humble Bundle promotions, saying that certain aspects prevent those types of purchasing behaviours from being applicable to the broader industry, the impact of Humble Bundle on the perception of cross-platform gaming is undeniable.

Only the passage of time will reveal whether Humble Bundle's commitment to cross-platform gaming is diminished, but the presentation of deviations from the established "Indie", "Android" and (for want of a better term) cross-platform "Non-indie" promotions has been and will remain a clear issue regardless of any principles if better distinction is not maintained. If something is an experiment, tell people beforehand - feedback from the THQ bundle and LIMBO's Wine wrapper should be enough to highlight how much transparency is valued when things become uncertain. If something is different, it needs to be highlighted - we know there have been many prospective Humble Bundle purchasers who misinterpreted the "Android" bundles' title, assuming that the offerings were exclusively for Android devices (a "they'll pick up on it eventually" approach only tells your customers that they're not important). If elements of your community have concerns, respond to them respectfully, openly and honestly. As Jeff once said, it's good to be a big fish in a small pond, more platforms means more opportunities, vocal minorities [can be a blessing], you can't choose your evangelists, and you can't choose your power users.

To conclude, if you're not supporting Linux and Mac OS X from a philosophical standpoint or for the fans, at least do it for the money. If you don't support non-Windows platforms, you're leaving a lot of cash on the table. I don't know about you, but I'm not in a position to just say f--- it to a large community of people who want to support us. - Jeff Rosen (Wolfire)

I, for one, look forward to future offerings from Humble Bundle, and hope to see more transparency, continued commitment to cross-platform gaming, DRM-free software, worthwhile charities, great developers and fantastic games.

In the month after this article first went live, I continued to follow up on outstanding leads, work on an additional visualisation, monitor the performance of the Humble Indie Bundle 7, and even work towards building a community project to expand the usefulness of the research I've done. You can read more on those in the sections below.

Whilst following up on outstanding leads, I discovered that I had recorded an incorrect date for the Mac OS release of Fieldrunners, citing it as a debut in the Humble Bundle for Android 3 when it had in fact been released several months earlier alongside the Windows version. The charts and values in this article have been updated to reflect the corrected date.

We were using Unity 4 beta release and our job was to port Rochard to Unity 4 in the first place, as we had used some 3.x features in a bit unorthodox way. Then, we reported The Unity3D Linux team every problem we encountered in the Linux build. They fixed the beta release and eventually we were able to ship the Linux build. - Jan Achrenius (Recoil Games)

I also received an email from Jan Achrenius of Recoil Games, who relayed some of their experiences with getting Rochard ready to become the first commercial Unity 4 Linux title (a second first for Rochard, which also happens to have been the first Unity title released on the Playstation 3).

The biggest obstacle was the way how the deferred shading was implemented in Unity. The performance was inferior with AMD's OpenGL driver and we asked AMD to fix it. Eventually AMD recommended some other order of operations and Unity Linux team fixed the problem themselves just two days before HiB6 release. - Jan Achrenius (Recoil Games)

Update: Non-cross-platform Impact

When I initially published this article, the Humble Indie Bundle 7 was still live, and per-platform statistics suggested that the preceding non-cross-platform promotions hay have increased the number of Windows purchasers and impacted on Linux and Mac OS purchaser behaviour.

At the conclusion of the Humble Indie Bundle 7, total revenue and total purchases were on par with previous "Indie" bundles (excluding the Humble Indie Bundle V), coming in at $2.65m and 396,953 respectively. The per-platform purchase ratio also seems in line with previous "Indie" bundle trends, though Linux purchasers are slightly down (though still above the preceding Humble Bundle For Android 4), and Windows purchasers dramatically up since the Humble Indie Bundle 6.

Most interestingly, is the revenue ratios for each platform, which are more in line with what was seen during the Humble Indie Bundle V, which had an outstanding number of Windows purchasers and a higher than usual average purchase price across all platforms. Unexpectedly, whilst Mac OS and Linux averages have remained consistent, the Windows average purchase price has pushed higher, coming to within $0.47 of the cross platform, which is the closest to platform average parity a Humble Bundle promotion has ever come.

After publishing my little commentary on the behaviours surrounding the Humble THQ Bundle, I received some feedback from a number of people who said they were paying higher than usual for the Humble Indie Bundle 7 to reinforce that cross-platform bundles are more appreciated, though without a dramatic shift in Linux averages, it's difficult to say whether those sorts of purchases had an impact on the final figures.

Does this mean that the non-cross-platform promotions have had a positive or negative impact? It's difficult to say. It seems that new Windows purchasers may have been brought in, and it's possible that some Linux purchasers may have been alienated, but the net change seems to be negligible at this point.

A Very Big Chart

When I first conceptualised this article, I wanted to do a large scale visualisation which highlighted how and where Humble Bundle promotions intersected with the lifespans of included titles. In the interests of getting the article published before the Humble Indie Bundle 7 ended, I put it aside.

Over the past several weeks, I've had the opportunity to revisit this idea. It's been an interesting challenge to wrestle with, with finding a balance between readability and accuracy proving to be quite a hurdle. After lots of back and forthing on aspect ratios, spacing and scale, I've wound up with something that I think makes the best compromise.

Before delving deeper, I need to warn you all that it's big. Big enough that when I exported it at the resolution I was hoping to publish it at, I couldn't find anybody who could open it in a browser. The chart is best viewable at 1:1, but still vaguely interpretable when zoomed out for an overview.

I've provided a PNG and an SVG (best viewed with Inkscape) here, and whilst both are most likely viewable in a web browser, I'd recommend downloading one and panning around in an appropriate image viewer/editor (on a side note, I'm told it looks fantastic on one of Apple's super high resolution displays).

The first thing that is immediately apparent when looking at the chart in full, is just how recently the first Humble Indie Bundle launched. Within 32 months, Humble Bundle have published/distributed over 100 titles.

Excluding titles from the THQ bundle and Psychonauts, no Humble Bundle promotions since the Introversion Bundle had titles released on PC prior to December 2008.

A dramatic decline in inclusion of titles with existing Linux builds seems visible, but is exaggerated by the recent non-cross-platform promotions.

Over half of Humble Bundle titles appeared in a bundle within a year of their first PC release.

Looking at individual titles, Zen Puzzle Garden and Psychonauts stand out as the two titles with the longest delay between their initial PC releases, and supporting all three major desktop platforms.

The oldest title (Uplink) appeared in a bundle over 10 years after its first PC release, was originally released for Windows and Linux in October 2001, with Mac OS appearing in May 2003.

Cortex command, Splot and Vessel stand out as titles which had post-bundle releases, with Cortex Command receiving its 1.0 release nearly two years after its appearance in the Humble Indie Bundle 2. Splot remains unreleased, with Frozenbyte anticipating an early 2013 launch. Vessel's Linux build made its first appearance nearly three months after the Humble Indie Bundle 6, and its Mac OS version is nearing release.

Sharing Knowledge (Wiki Enhancement Project)

One of the most rewarding aspects of the writing I do is the contact and conversations I get to have as my diverse readers share their thoughts and feedback both with me and the broader community.

When this article was originally published, I received a lot of requests for further detail on the quality of ports and the status of outstanding bugs. This was something I had hoped to touch on, but it proved to be a topic that warranted more time and research to be adequately explored than I was in a position to give.

Following discussion in assorted forums and IRC channels, PCGamingWiki.com contributor soeb and myself have started a wiki enhancement project which aims to provide additional details on Humble Bundle titles on Windows, Mac and Linux.

Much of the initial data that we're looking at contributing to PCGamingWiki overlaps with information found in my articles (in particular, the source spreadsheet for this one). It's great to know that some of my work will be contributing to a resource that's far beyond anything I could have done on my own.

This is a fantastic project that not only has the potential to help out gamers who are looking for workarounds, but also to highlight that cross-platform consistency is valuable to gamers, and in some small way highlight games with "better" cross-platform support.

If you're interested to find out more or get involved, you can read my overview, or jump across to soeb's progress page.