Can imread input animated gifs written using imwrite?

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Alec Jacobson
Alec Jacobson on 14 Jun 2017
Commented: DGM on 12 Feb 2022
I've written an animated gif test.gif using imwrite:
frame = getframe(gcf);
[SIf,cm] = rgb2ind(frame.cdata,256);
frame = getframe(gcf);
[SIf,cm] = rgb2ind(frame.cdata,256);
imwrite(SIf,cm, 'test.gif','WriteMode','append','Delay',0.5);
If I try to read this back in using
[X,M] = imread('test.gif');
I see that X has 4 dimensions and the size of the fourth dimension is 2 (good, one for each frame). The colormap M has only 2 dimensions #colors by 3.
Plotting the first frame I see the correct image:
Plotting the second frame I see nonsense:
My guess is that the colormap is wrong/shouldn't be the same for every frame of the animation. Any idea what's going on here? The documentation for imread suggests that it should be able to handle animated gifs.

Answers (1)

Alec Jacobson
Alec Jacobson on 14 Jun 2017
It seems like this is broken ... on purpose. In
There's a commented line:
% This only returns the color table for the first frame in the GIF file.
% There is an enhancement (g1055265) to make this return all color tables.
map = info.ColorTable;
I'm not sure why the map is set only to the first frame's colormap. If you're willing to modify built in matlab files then you can "fix" this so that map is a #colors by 3 by #frames matrix then change this line to:
map = reshape([info(:).ColorTable],[],3,n);
DGM on 12 Feb 2022
There is no enhancement. Depending on your version, imread() can no longer correctly read multiframe GIF files.
The legacy method for reading multiframe GIF files is to get the image data from imread() and get the color tables from imfinfo(). If that's too inconvenient, there are tools on the FEX to streamline the process. I'm not going to link to them, because I doubt anyone is still using R2018a or older. If you are, you can find them.
If you're running R2018b or newer, depending on the specific version and the image at hand, imread() and imfinfo() will either give you subtly corrupted images, grossly corrupted images, or it will not even give you a complete image at all.

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