FLIF and Responsive Web Design

Responsive web design is a hot topic, and that is not surprising: websites are viewed on a much larger variety of devices than ever before. Smartphones, tablets, laptops, desktop computers, smart TVs, smart watches, and so on. HTML5 and CSS3 help a lot to make responsive web design possible. There are some remaining issues though. One of the biggest problems is how to deal with images in a responsive design.

State of the art

Typical approaches are to create different variations of the same image (i.e. downscaled to different resolutions and/or lossy encoded at different quality settings) and provide them as separate files. Then the goal is to let the software client load the most fitting file for a situation, while avoiding problems like downloading multiple variations of the same image, content delivery networks caching issues, race conditions, browser preloading issues, and so on. The net result can be that the website actually loads slower instead of faster.

Even if all goes well, there is a significant overhead at the server side since multiple variations of the same image have to be created and cached. Also it may be necessary for the client to download multiple variations of the same image, e.g. when the user wants to zoom in on a page to see more detail.

FLIF: one file per image

With FLIF, this is not necessary, and for every image, only one file is required, ever. The optimization can happen entirely on the client side.

A FLIF image can be loaded in different ‘variations’ from the same source file, by loading the file only partially. This makes it a very appropriate file format for responsive web design. Since there is only one file, the browser can start downloading the beginning of that file immediately, even before it knows exactly how much detail will be needed. The download or file read operations can be stopped as soon as sufficient detail is available, and if needed, it can be resumed when for whatever reason more detail is needed — e.g. the user zooms in or decides to print the page.

Client side knows best

Using FLIF, software clients (web browsers, image viewers, ...) can more effectively use resources such as bandwidth or memory, by limiting the loading of files in consideration of the circumstances of the situation. After all, it is the client side that knows best what those circumstances are. For example:

the proportion between the display resolution (and pixel density) and the image resolution;

transfer speed that is currently available;

available bandwidth (e.g. if the user pays per downloaded kilobyte)

diskspace and memory constraints

is the image currently on-screen or off-screen?

zooming, screen rotation, window resizing;

user preference in the speed vs quality trade-off

No pixel needs to be encoded twice

Only one file is ever required per image, and that can make everything much easier. For example, a typical image gallery could use the same file everywhere: thumbnails, previews, multiple scaled-down versions, and the original files, it could all be the same file. No pixel has to be encoded more than once.

There are also many possibilities in terms of smarter caching. Client-side caches, proxy servers or CDNs could decide to store only the beginning of FLIF files, trimming away bytes at the end as the cached files age. A partially cached file still contains the most useful and most frequently requested image information.

Finally, even when network bandwidth is not an issue, FLIF's “no pixel needs to be encoded twice” philosophy can be useful. Large image files often contain a small preview image in their header. Sometimes more than one, and sometimes (e.g. in many camera RAW formats), there's even a full-resolution ‘preview’ image included (e.g. encoded in a lossy format like JPEG and with less color depth). Also, many operating systems or image viewers produce cached thumbnail files. All these scaled-down versions of the images are redundant, they are only there to make it easier to quickly show icon listings. With FLIF, these tricks are not needed: the beginning of the file is the preview, and previews of any desirable size can be produced this way.