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Grokking The Gimp
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9.1.5 Animation Optimization

Saving an animation to GIF format can create large files, especially if the animation has many frames. For example, the space shuttle animation discussed in the previous section consists of an image whose dimensions are $476\times 182$ pixels and has 9 frames. This is an image consisting of 780 thousand pixels. Due to compression built into the GIF image format, this saves to a 389 kilobyte file. However, the file size can still be significantly reduced by taking advantage of the high degree of redundancy in the image.

The redundancy is due to this animation changing very little from frame to frame. In fact, the background is always the same, and only the position and shape of the shuttle is changing. The function Animation Optimize  in the Image:Filters/Animation menu takes advantage of this and can greatly reduce the size of the resulting file. It does this by differencing frames and only saving the non-zero values. The original animation is reconstructed by adding the differenced frames back into the background.

For the space shuttle animation, the file is reduced from 389 to 163 kilobytes if the Animation Optimize option is applied after conversion to Indexed format. If it is applied to the image in RGB mode, before conversion to Indexed format, an additional savings is realized and the resulting file only occupies 106 kilobytes on the disk. This improvement might not be general, and for projects where file size is important (as it is for low bandwidth network connections), it is advisable to experiment with the two methods.

Grokking The Gimp
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