With this method, transparent pixels serve the purpose of preserving the color of the pixel from the previous frame. The second disposal method is called “Do not dispose,” and it turns out that using the “Do not dispose” (or DND) method, GIF viewers can, instead, build a new frame using the last frame as the starting point. It does what you’d expect: when a new frame is drawn, the area of the image is set to be completely transparent and only non-transparent pixels get drawn on top of that.
The first disposal method is called “Restore to background color” in the GIF specification. However, it turns out that if there are multiple frames, (i.e., an animated GIF), there are several possible ways to handle transparencies. If the GIF is a single-frame (i.e., a still image), how to handle the “transparent” color is pretty obvious: the tool rendering the GIF just needs to know to not draw that pixel. Non-transparencies are 100% opaque, meaning the GIF format does not support an “alpha” in the way that some formats, like PNG, do. Optionally, one of those colors can be transparent. In a GIF, any pixel can take on any one of 256 colors defined in a palette. To understand the issue that FFMPEG had writing transparent GIFs, you need to understand exactly how transparencies work in the GIF format, and how FFMPEG was handling it. Why FFMPEG Doesn’t Support Transparencies For this reason, we decided to tackle the issue ourselves. Although we have other tools at our disposal, such as Gifsicle, we want to use as many of the same tools as possible, especially ones that are powerful, popular and fast like FFMPEG. Until recently, however, FFMPEG did not properly support writing GIFs that are animated and have transparent pixels. At GIPHY, we use FFMPEG to process all uploaded content – including GIFs and Stickers.įFMPEG is fast, high quality, and open source. One of those tools is FFMPEG, an extremely popular package for working with video in a wide variety of formats. The distinction exists in our engineering toolchain as well – some tools struggle to correctly support Stickers (usually due to transparency). However, we still use the GIF format to store stickers – all they really are is GIFs with transparent pixels. Here at GIPHY, we differentiate between GIFs and Stickers in our business language, as the two products are served to different searches and customers. A sticker (left) is just a GIF (right) with transparent pixels.
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