A few days ago, I released a new version of G’MIC (GREYC’s Magic for Image Computing, numbered 1.6.2.0). This is a free and generic image processing framework I’ve been developing since 2008. For this particular occasion, I thought it would be good (for me) to write a quick summary of what features I’ve been working on these last ten months (since the last release of G’MIC in the 1.5.x.x branch). Hopefully, you’ll be interested too ! This project takes a large amount of my free time (basically every week-end and evenings, except Wednesday, table tennis time 🙂 ), so it’s probably good for me to take a little break and analyze what has been done recently around the G’MIC project.

I will summarize the important features added to G’MIC recently, which are: Various color-related filters, a foreground extraction algorithm, new artistic filters, and other (more technical) cool features. As you will see this quick summary has ended up a not-so-quick summary, so you are warned. Just before I start, I’d like to thank Patrick David again a thousand times for his proof reading. Kudos Pat! And now, let’s start !

1. The G’MIC Project : Context and Presentation

The G’MIC project was born in August 2008, in the IMAGE Team of the GREYC laboratory (a public research unit, located in Caen / France, affiliated to the CNRS, the largest public research institute in France), a team to which I belong. Since then, I’ve developed it apace. G’MIC is an open-source software, distributed under the CeCILL license (a French GPL-compatible license).

What G’MIC proposes is basically several different user interfaces to manipulate generic image data (sequences or still 2D/3D images, with an arbitrary number of channels, float-valued , etc., so definitely not only 2D color images). To date, the most popular G’MIC interface is its plug-in for GIMP.

But G’MIC also provides a command-line interface (similar to what ImageMagick has, but more flexible, perhaps?), and a web interface G’MIC Online. Other ways of using G’MIC exist, such as ZArt, a qt-based image stream processor, or a nice plug-in for Krita (developed by Lukas Tvrdy), or through some features implemented in the recent (and nice!) software Photoflow. These latter interfaces are still a bit more confidential, but not for long I guess. Actually, all of these interfaces are based on libgmic. This is the C++, portable, thread-safe and multi-threaded library (in particular through the use of OpenMP) which implements all of the G’MIC core image processing routines and structures. It also embeds its own script language interpreter, so that advanced users are allowed to write and integrate their customized image processing functions into G’MIC easily. Today, G’MIC already understands more than 900 different commands, all configurable, for a libgmic library file that takes a bit less than 5 MB. The available commands cover a wide range of the image processing field, with algorithms for the manipulation of image geometry and colors, for image filtering (denoising, sharpening with spectral, variational or non-local methods, …), for image registration and motion estimation, for the drawing of graphical primitives, for image segmentation and contour extraction, for 3D object rendering, artistic filters, and so on…

The set of tools G’MIC provides is really useful, for both converting, visualizing and exploring image data, and for creating, and applying complex image processing pipelines. In my opinion, this is one piece of free software that image processing savvy folks should have tried at least once. At least for me, there is an everyday use. 🙂

2. New G’MIC features for color processing

So, here is some of the stuff I’ve added to G’MIC recently, related to the manipulation (creation of modification) of the colors in an image.

2.1. Color curves in an arbitrary color space

The concept of color curves is well known for any artist of photograph who wish to modify the colors of their images. Basically, a color curve tool allows you to define and apply on each R,G,B component of your image, a 1d continuous function f:[0,255] -> [0,255]. Each original red, green or blue value x of a pixel (assumed to be in the range 0..255) is then mapped into a new value f(x) (in 0..255 too). To define these transfer functions (one for each of the R,G,B components), the user classically defines a set of key-points that are interpolated by spline functions.

But what happens if you want to define and apply color curves in a color space different than RGB ? Well… mostly nothing, as most of the image retouching tools (GIMP included) only propose to color curves for RGB, unfortunately.

So, one of the things I’ve done recently in G’MIC has been to implement an interactive color curves filter that works in other color spaces than RGB, that is, CMY, CMYK, HSI, HSL, HSV, Lab, Lch and YCbCr. The new command -x_color_curves is in charge of doing that (using the CLI interface). Plug-in users may use it via the filter Colors / Curves [interactive]. Here, using the plug-in is even nicer because you can easily save your favorite curves, as new Favorite filters, and apply the same color curves again and again on hundred of images afterwards. Here is how it looks:

I’ve also recorded a quick video to show the action of this filter live, using it from the G’MIC plug-in for GIMP:

2.2. Comics colorization

I also had the chance to talk with David Revoy, a talented French illustrator who is the man behind the nice Pepper & Carrot webcomics (among other great things). He told me that the process of comics colorization is a really boring and tedious task, even using a computer, as he explains it on his blog. He also told me that there were some helpers for colorizing comics, but all proprietary software. Nice! There was obviously something worth trying for open-source graphics here.

So I took my keyboard and started coding a color interpolation algorithm that I hoped to be smart enough for this task. The idea is very simple: Instead of forcing the artist to do all the colorization job by himself, we just ask him to put some colored key-points here and here, inside the different image regions to fill-in. Then, the algorithm tries to guess a probable colorization of the drawing, by analyzing the contours in the image and by interpolating the given colored key-points with respect to these contours. Revoy and I also discussed with Timothée Giet (on irc, channel #krita :)), another French artist, about what could be a nice “minimal” interface to make the process of putting these colored key-points comfortable enough for the artist. And finally, the result of these discussion is the new G’MIC command -x_colorize and the corresponding filter Black & White / Colorize [interactive] in the G’MIC plug-in. Apparently, this seemed to be fair enough to do the job!

From what I’ve seen, this has raised quite a bit of interest among the users of Krita (not so surprising as there are many comics creators among them!). Maybe I’m wrong, but I feel this is one of the reasons the Krita team decided to put some additional efforts on their own G’MIC plug-in (which is something to become an awesome piece of code IMHO, probably even better than the current G’MIC plug-in for GIMP I’ve done).

Here is a quick illustration on how this works for real. The images below are borrowed from the David Revoy‘s web site. He has already written very nice articles about this particular feature, so consider reading them for more details about it.

I believe Revoy is using this colorizing algorithm in a regular manner for his creations (in particular for Pepper & Carrot), as illustrated in Fig.2.2-2.3. One can clearly see the different colored key-points put on the image and the outcome of the algorithm, all this being done under the G’MIC plug-in for Krita. After this first colorization step, the artist may add shadows and lights on the flat color regions generated by the algorithm. The nice thing is he can easily work on each color region separately because it is easy to select exactly one single flat color in the output color layer. And for those who prefer to work on more layers, the algorithm is able to output multiple layers too, one for each single reconstructed color region.

So, again, thanks a lot to David Revoy and Timothée Giet for the fruitful discussions and their feedback. We have now a very cool Comics colorization tool in a free software. This is the kind of academic / artistic collaboration I’m fond of.

Below you can see a small video I’ve made that shows how this filter runs inside the G’MIC plug-in for GIMP :

2.3. B&W picture colorization

But could this also work for old B&W photographs ? The answer is Yes! By slightly modifying the colorization algorithm, I’ve been able to propose a way to reconstruct the color chrominance of a B&W image, exactly the same way, from user-defined colored key-points. This is illustrated in the figure below, with the colorization of an old B&W portrait. The pixel-by-pixel content of a photograph being obviously more detailed than a Comics, we often need to put more key-points to get an acceptable result. So, I’m not sure this speeds up the colorization process so much in this case, but still, this is cool stuff. These two colorization techniques are available from the same filter, namely Black & white / Colorize [interactive].

2.4. Color transfer

Here, what I call « color transfer » is the process of modifying the colors of an image A by replacing them by the colors of another image B (reference image) so that the modified image A’ gets the same “color ambiance” than the reference image B, and this is being done in a fully automatic way. This is an ill-posed problem, quite complex to solve, and there is already a lot of scientific papers published on this topic (like this one for instance). The main issue consists in generating a new image A’ that keeps a “natural” aspect, without creating synthetic-looking flat color areas, or on the contrary high color discontinuities that are not initially present in the original image A. In short, this is far from being trivial to solve.

I’ve worked on this topic recently (with one of my colleagues Julien Rabin), and we’ve designed a pretty cool algorithm for transferring colors from an image to another. It’s not perfect of course, but this is really a good start. I’ve implemented the algorithm and put it in G’MIC, available from the plug-in filter Colors / Transfer colors [advanced]. This filter requires several input layers, one being the reference image B containing the colors to transfer to the other layers. Here is how it looks when run from the G’MIC plug-in for GIMP, with the original color image (on the left in the preview window), the reference image (bottom-left) and the filter outcome (on the right).

Sometimes, this filter gives awesome results! Two color transfer examples are illustrated below. Of course, don’t expect it to work nicely with pathological cases (as transferring a colorful image to a monochrome one for instance). But it’s not lying to say it works already quite well. And the best of all, it is quite fast to render.

I’ve also recorded a quick video tutorial that shows how this kind of results can be obtained easily from the G’MIC plug-in for GIMP. We can also think of other usages for this interesting new algorithm, like the homogenization of colors between successive frames of a video, or ensuring the color stability of a stereoscopic pair for instance. Here is the video :

2.5. Website for analog film emulation

I have the pleasure of being a friend with Pat David, an american photograph who is very active in the libre graphics community, sharing his experience and tutorials on his must-read blog. I met him at LGM’2014 in Leipzig, and honestly, many useful G’MIC filters have been suggested or improved by him. Particularly, all the filters in the Film emulation category could not have been done without his work on the design of color transfer functions (a.k.a. CLUTs). Below is a figure that shows a small subset of the 300+ color transformations concocted by Pat that has been made available in G’MIC.

Once added (two years ago), these filters raised much interest in the photo retouching community, and have been re-implemented in RawTherapee. To make these filters even more accessible, we have recently set up a web page dedicated to Film Emulation, where you can see/compare all the available presets and download the corresponding CLUT files. Note that you can also find the G’MIC Film Emulation filters in the G’MIC Online web site, and try them live directly in your web browser. Here again, analog film emulation was somehow a missing feature in the libre software world and the G’MIC infrastructure made it easy to fill this gap (who needs DXO FilmPack anymore now ? ;)).

3. An algorithm for foreground/background extraction.

In photo retouching, it is not uncommon to differentiate the processing of foreground and background objects. So one often needs to select the foreground object first (using the “lasso” tool for instance) before doing anything else. For objects with a complex geometry, this is a tedious task. G’MIC now integrates a user-guided segmentation algorithm to speed up this foreground extraction work. The G’MIC command -x_segment (using the CLI interface) and the plug-in filter Contours / Extract foreground [interactive] let the user play with this algorithm.

This works exactly the same way as the Comics colorization filter, except that the colored key-points are replaced by labelled key-points, with labels being either “foreground” or “background”. Then, the extraction algorithm interpolates those labels all inside the image, taking care of the contours, and deduce a foreground/background binary map. Thus, the filter is able to decompose an image into two layers, one with the foreground pixels only, and the other with the remaining pixels (background). The figure below shows a typical use of this filter : Starting from a color image of a flower (top-left), the user put few keypoints on it (top-right) and the extraction algorithm is able to generate the two background / foreground layers on the last row, from this very sparse set of points.

Doing this takes only few seconds, while the same operation made manually would have been much much longer to perform (the flower’s contour being not simple to cut).

After that, it is quite easy to process the background and foreground separately. Here for instance, I’ve just changed the hue and saturation of the foreground layer a little bit, in order to get a more purple flower without changing the colors of the background. Selecting objects in images is a common operation in photo retouching, I’ll let you imagine in how many cases this filter can be useful.

I’ve done another G’MIC video tutorial to illustrate the whole process in real-time, using the G’MIC plug-in for GIMP:



4. Some new artistic filters.

G’MIC has always been a source of dozens of artistic filters. Below is an overview of some of the newest additions.

4.1. Engrave effect.

The filter Black & White / Engrave tries to transform an image into an etching. The high number of parameters lets you precisely tune the rendering done by the algorithm, and many different effects may be obtained with this single filter.

What I find particularly interesting with this filter is the ability it has (with properly chosen parameters) to convert photographs into comics-like renderings. This is illustrated with the two images below.

I’ve also recorded a video tutorial to show the different steps used to generate this kind of results. It takes only few seconds to achieve (maybe minutes for the slowest of you!).

4.2. Delaunay triangulation.

The algorithm of Delaunay triangulation has been added to G’MIC (through the -delaunay3d command). A new filter, namely Artistic / Polygonize [delaunay] inside the plug-in for GIMP uses it to transform color images into nice geometric abstraction. Each generated triangle color can be random, constant or related to the pixels under the triangle. The triangulation tries to stick to the image contours (more or less, depending on the chosen parameters).

4.3. Other artistic filters.

Blender could be of any interest (even better, a full G’MIC plug-in for Blender, maybe ? :p )

As you can see, G’MIC is a very active project, and the number of available artistic filters increases every day. Thus, I can’t describe in details all the new filters added these ten last months, and because a picture is worth a thousand words, you will find below a quick overview of some of them.

5. A quick view of the other improvements

Of course, the few new filters I’ve presented above are only a small part of the overall work that has been done on G’MIC (the most visible part). Here is a list of other notable (mostly technical) improvements:

5.1. Global improvements and the libgmic library.

A lot of work has been done to clean and optimize the entire source code. The size of the libgmic library has been drastically reduced (currently less than 5 MB), with an improved C++ API. Using some of the C++11 features ( rvalue-references ) allows me to avoid temporary buffer copies which is really nice in the context of image processing as the allocated memory may become huge quickly. With the help of Lukas Tvrdy, one of the Krita developers, we have been able to improve the compilation of the project a little bit on Windows (with Visual Studio), and run various code analysis tools to detect and fix memory leaks and strange behavior. Many sessions of valgrind, gprof, g++ with the -fsanitize=address option, PVS Studio has successfully led to effective code improvements. Not very visible or exciting for me, but probably very satisfying for the final user 🙂 As a consequence, I’ve decided the code was clean enough to regularly propose G’MIC pre-releases that are considered as stable enough while getting the latest features developed. The method of releasing G’MIC is now closer to a Rolling Release scheme.

) allows me to avoid temporary buffer copies which is really nice in the context of image processing as the allocated memory may become huge quickly. With the help of Lukas Tvrdy, one of the Krita developers, we have been able to improve the compilation of the project a little bit on Windows (with Visual Studio), and run various code analysis tools to detect and fix memory leaks and strange behavior. Many sessions of valgrind, gprof, g++ with the -fsanitize=address option, PVS Studio has successfully led to effective code improvements. Not very visible or exciting for me, but probably very satisfying for the final user 🙂 As a consequence, I’ve decided the code was clean enough to regularly propose G’MIC pre-releases that are considered as stable enough while getting the latest features developed. The method of releasing G’MIC is now closer to a Rolling Release scheme. The compilation of the default G’MIC binaries on Windows has been improved. It now uses the latest g++-4.9.2 / MinGW as the default compiler, and new installers for Windows 32 bits / 64 bits are now available.

G’MIC has new commands to compress/uncompress arbitrary data on fly (through the use of the zlib). So now, the embedded G’MIC commands are stored in a compressed form and the Internet updates are now faster.

The G’MIC project got its own domain name http://gmic.eu, independent from Sourceforge. We have improved the web pages a lot, particularly thanks to the wonderful tutorials on the use of the G’MIC command-line tool, written by Garry Osgood (again, thanks Garry!). This is clearly a page to bookmark for people who slowly want to learn the basics of G’MIC.

5.2. New possibilities for image filtering.

A bunch of new commands dedicated to image filtering, and their associated filters in the G’MIC plug-in for GIMP have been added, for image sharpening (Mighty Details), deconvolution by an arbitrary kernel (Richardson-Lucy algorithm), guided filtering, Fast Non-Local Means, Perona-Malik anisotropic diffusion, box filtering, DCT transforms, and so on. Honestly, G’MIC now implements so many different image filtering techniques that it is a must-have for any image processing person (even only for the denoising techniques it provides). For the users of the plug-in, this means potentially more diverse filters in the future. The figure below shows an application of the Mighty Details filter on a portrait, to simulate a kind of Dragan effect.

5.3. Improvements of the G’MIC plug-in for GIMP.

The plug-in for GIMP being the most used interface of the G’MIC project, it was necessary to make a lot of improvements:

The plug-in now has an automatic update system which ensures you always get the latest new filters and bug fixes. At the date of the 1.6.2.0 release, the plug-in counts 430 different filters, plus 209 filters still considered in development (in the Testing/ category). For a plug-in taking 5.5 Mb on the disk, let us say this makes an awesome ratio of number of filters / disk space. A complete list of the filters available in the plug-in for GIMP can be seen here.

release, the plug-in counts 430 different filters, plus 209 filters still considered in development (in the Testing/ category). For a plug-in taking 5.5 Mb on the disk, let us say this makes an awesome ratio of A complete list of the filters available in the plug-in for GIMP can be seen here. During the execution of a filter that takes a bit of time, the plug-in now displays information about the elapsed time and the used memory in the progress bar.

The plug-in GUI has been slightly improved: the preview window is more accurate when dealing with multi-layer filters. The filters of the plug-in can be interactive now: they can open their own display window, and handle user events. A filter can also decide to modify the value of its parameters. All of this allowed me to code the new interactive filters presented below (as the one for the Comics colorization, for the color curves in an arbitrary color space, or for the interactive foreground extraction). This is something that was not possible before, except when using the command-line interface of G’MIC.

A G’MIC filter now has knowledge about all the input layer properties, such as their position, their opacities and their blending mode. A filter can modify these properties for the output layers too.

A new filter About / User satisfaction survey has been added. You can give some anonymous information about your use of G’MIC. The result of this survey is visible here (and updated in almost real-time). Of course, this result image is generated by a G’MIC command itself :).

5.4. G’MIC without any limits : Using the command-line interface.

gmic is the command-line interface of G’MIC, usable from a shell. That is undoubtly the most powerful G’MIC interface, without any limitations inherent to the input/output constraints of the other interfaces (like being limited to 2D images, 8bits / channels, 4 channels max on GIMP, or a single input image in the G’MIC Online site, etc.). With the CLI interface, you can load/save/manipulate sequences of 3D volumetric images, with an arbitrary number of channels, float-valued, without limits other than the available memory. The CLI interface of G’MIC has also gotten a lot of improvements:

It now uses by default OpenCV to load/save image sequences. We can then apply image processing algorithms frame by frame on video files (command -apply_video), on webcam streams (command -apply_camera), or generate video files from still images for instance. New commands -video2files and -files2video have been added to easily decompose/recompose video files into/from several frames. Processing video files is almost childs play with G’MIC now. The G’MIC manual (command -help) has been also improved, with a colored output, proposed corrections in case of typos, links to tutorials when available, etc..

The invokation of gmic on the command line without any arguments makes it enter a demo mode, where one can select between a lot of different small interactive animations from a menu, to get an idea of what G’MIC is capable of. A good occasion to play Pacman, Tetris or the minesweeper on the pretext to test a new serious image processing software!

5.5. Other G’MIC interfaces being developed.

ZArt is another available G’MIC interface initially developed as a demonstration platform, for processing images from the webcam. We use it classically for public exhibitions like the “Science Festival” we have once a year in France. It is very convenient to illustrate what image processing are, and what they can do, for the general public. ZArt has been significantly improved. First, it is now able to import any kind of video files (instead of supporting only webcam streams). Second, most of the filters from the GIMP plug-in has been imported to ZArt. This video shows these new possibilities, for instance with real-time emulation of analog films of a video stream. This could be really interesting to get a G’MIC plug-in working for video editing software, as the brand new Natron for instance. I’m already in touch with some of the Natron developers, so maybe this is something doable in the future. Note also that the code of ZArt have been upgraded to be compatible with the newest Qt 5.

It has been a long time now since Lukas Tvrdy, member of the Krita devs, has started to develop a G’MIC plug-in for Krita. This plug-in is definitely promising. It contains all of the elements of the current G’MIC plug-in for GIMP, and it already works quite well on GNU/Linux. We are discussing to try solving some problems remaining for the compilation of the plug-in on Windows. Using the Krita plug-in is really nice because it is able to generate 16bits/channel images, or even more (32bits float-valued), whereas the plug-in for GIMP is currently limited to 8bits/channel for the plug-in API. This may change in the future with GEGL, but right now, there is not enough documentation available to ease the support of GEGL for the G’MIC plug-in. So now, if you want to process 16bits/channel images, you can do this only through the command-line tool gmic, or the G’MIC plug-in for Krita.

The integration of some of the G’MIC image processing algorithms has also begun in PhotoFlow. This is a quite recent and promising software which focuses on the development of RAW images into JPEG, as well as on photo retouching. This is a project done by Carmelo DrRaw, check out this project, it is really nice.

6. Perspectives and Conclusions.

Here we are! That was a quite complete summary of what happened the ten last months around the G’MIC project. All this has been made possible also thanks to many different contributors (coders, artists, and users in general), whose number is increasing everyday. Thanks again to them!

In the future, I still dream of more G’MIC integration in other open-source software. A plug-in for Natron could be really great to explore the possibilities of video processing with G’MIC. A plug-in for Blender would be really great too! Having a G’MIC-based GEGL node is again a great idea, for the upcoming GIMP 3.0, but we have probably a few decades to decide 🙂 If you feel you can help in these topics, feel free to contact me. One thing I’m sure: I won’t be bored by G’MIC in the next few years!

Having said that, I think it’s time to go back working on G’MIC again. See you next time, for other exciting news about the G’MIC project! (and if you appreciate what we are doing or just the time spent, you are free to send us a nice postcard from your place, or give a few bucks for the hot chocolates I need to keep my mind clear working on G’MIC, through the donation page). Thank you!