Reported Bug in gif files (or maybe a bug)

[stifani]

hello,

i've been using irfanview since 10 years. and i have to say that it is the absolutely the ONLY and BEST viewer for image (with all functions included (resize, bla bla, and so on). So Irfan, YOU ARE THE MAN!

anyway i never had problems or found a single bug, since today, when i was downloading some gif animations from the TV show "Big Brother".

here is a zip with 2 files included.
The problem is that when i open the gif with IV, it happens to show some glitches that the "standard windows viewer" does not show...

here are the screenshots (a little bit hottie)

http://img405.imageshack.us/img405/3411/noglitches.png

http://img690.imageshack.us/img690/5136/glitches.png

by the way i'm using the latest version (4.27)

i'm sorry about some mistakes in the message but it' 4.40 am here and my head is in the other room LOL

[derniwi]

Hi,

have a look at this thread:

http://en.irfanview-forum.de/vb/showthread.php?6744-Artifacts-and-Slow-Speed-with-Animated-GIFs

Best regards, Nils.

[impdf]

The GIF specification allows these kind of GIFs but you should not generated them.

The problem is that each frame in these GIFs uses its own colortable plus transparency.
This means that a transparent pixel in a frame should show the pixel of the previous frame (simplified).
But if the colortable changed between the frames then there may be a problem. If the decoder just uses the new colortable you will see artifacts (wrong colors).
Possible solutions for decoders:
1) Replace the transparent pixel with a similar color in the new color table (this slows down the decoding and may not be perfect)
2) Render always the frames to 24 bits (I expect that browsers are doing this, but this takes memory).