Certain gifs have strange negative flashes?

[KasProduct]

I'm using a piece of forum software (Lithium), and its image encoding is backed by ImageMagick although I cannot directly see or access its configuration and parameters. I'm hoping someone here can help me work out what's causing this strange issue so I can push the devs to fix it.

Here's one of the images used as an avatar. It is a 17-frame 8-colour GIF of a ball rotating, with a little glint/sparkle on the 16th frame.



The software encodes each image uploaded, and this is what comes out.



The issue isn't actually on the glint/sparkle frame. It's on the succeeding frame, the final one.

Does anyone know what would cause an issue like this?

[snibgo]

There are 17 frames. Numbering them from 1, number 16 has the glint.

The first GIF you show is good. It is uncoalesced (ie compressed; each frame stores just differences).

The second GIF you show has a bad final frame. It is the same as the final uncoalesced frame from the first GIF. It seems only the first 16 frames were coalesced. I don't know what might cause that.

It seems the software did something like this:

Code: Select all

convert AzYvT8a.gif -layers coalesce a.gif

... but something went wrong.

Can you ask what version of IM they used? If they used a command-line, what was it?