Delay of GIF animation changes

[dloebl]

Sorry for yet another GIF issue 😅

For the following GIF the delay changes when doing a copy via vips. After the first frame, a delay of one second is added.

$ vips copy test.gif[n=-1] out.gif

In the input GIF, the graphic control extension for the first frame is missing.
To me it looks as if the decoder sets some kind of placeholder delay to 100cs when no graphic control extension is present.

$ vips --vips-version
libvips 8.12.1-Wed Nov 24 15:41:04 UTC 2021

[jcupitt]

Hi @dloebl,

You're right, I see:

$ vipsheader -a bugs.gif 
bugs.gif: 500x355 uchar, 4 bands, srgb, gifload
width: 500
height: 355
bands: 4
format: uchar
coding: none
interpretation: srgb
xoffset: 0
yoffset: 0
xres: 1
yres: 1
filename: bugs.gif
vips-loader: gifload
n-pages: 12
loop: 0
delay: 1000 0 0 0 0 0 0 0 0 0 0 0 
background: 20 20 20 

Which is obviously not very useful.

[jcupitt]

Yes, looks like libnsgif defaults to 100cs for the frame delay:

https://github.com/libvips/libvips/blob/master/libvips/foreign/libnsgif/libnsgif.c#L1006

The usual GIF default is 10fps I think, so perhaps this should be 10? I'll patch it.

[jcupitt]

vips copy bugs.gif[n=-1] x.gif

Now makes this: