Interpretation of “background colour index” in GIF

Question

If you look in the GIF specification and search for "Background Color Index", you see the following description:

vii) Background Color Index - Index into the Global Color Table for the Background Color. The Background Color is the color used for those pixels on the screen that are not covered by an image. If the Global Color Table Flag is set to (zero), this field should be zero and should be ignored.

There is an ambiguity here, which is that if the "Global Color Table Flag" is set to zero and this field is ignored, then it is undefined what background colour of a GIF actually should be if the image data itself does not cover the entire image area. This possible because every image data block specifies left/right/width/height independently and there is no requirement that every pixel must be encoded by the image data.

Am I misinterpreting this? If not, then in the presence of this ambiguity, what is the de facto behaviour of GIF implementations today?

Answer 1

For the example, Safari uses black as the background. With Win10 photo viewer it's hard to tell, as it uses a black background anyway. Chrome, IE and Edge use white. The spec doesn't define the behaviour when there's no background colour and you need one, so I guess arbitrary choices is what you'll get, though I vote for transparent being the most sensible.

Answer 2

Yes, the ambiguity is indeed present. Background color is largely a derelict of GIF87a, currently only necessary for 'Restore Background' frame blending mode.

By now, all decoders I have seen treat pixels not belonging to any frame as transparent even if transparency flags in all of the frames are zeroed.

Consider this GIF, for example:

See how your browser interprets it? The whole background is transparent.

P.S.: in case it does not loop, reload the page using Ctrl+F5.