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APNG Support

on October 9th 2012


First, thanks for this real very good image editor.

I use it to produce animated PNG and when saving,
it seems that the flag 'dispose_op' from chunck fcTL is alway set to APNG_DISPOSE_OP_NONE (0).
To build my APNG, I have imported png image with transparent background.
On the resulting APNG, the flag 'dispose_op' from chunck fcTL should be
APNG_DISPOSE_OP_BACKGROUND to avoid the images to be draw one over the next.
On your example 'clock-opt' all the frames needs to be drawn 'the next over the previous', so it is ok to use APNG_DISPOSE_OP_NONE
But in my case, when composing from distinct images, the flag must be APNG_DISPOSE_OP_BACKGROUND.

Do you agree with me ?

Can this can be fixed ?

Thanks for all (great job you done)


on October 9th 2012


well, it depends. APNG unlike GIF allows to specify a blending mode for a frame in addition to dispose method. This means that it is possible to restore transparency to a region even if using PNG_DISPOSE_OP_NONE.

The encoder tries to produce smallest possible file sizes and depending on the actual image content, it uses different combinations blending and dispose modes.

I am not saying there cannot be an error in the encoder, but only using APNG_DISPOSE_OP_NONE may still be correct in some cases. An easy way to see whether the saved APNG file is OK or not is to open it in Firefox. If there are glitches, please send me a .rli file that when simply saved as animated png produces these glitches and I'll try to diagnose the problem and fix it.


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