Purpose of color resolution bits in a gif

Question

I've dissected a gif image and here is the data in the header: ['G', 'I', 'F', '8', '9', 'a', '^', '\x01', '^', '\x01', '\x87', '\x00', '\x00', '\x00', '\x00', '\x00', '\x01', '\x00', '\x00', '\x01']

The 0x87 represents the flag byte in the gif. Converting this to binary results in 0b10000111.

I'm using http://www.matthewflickinger.com/lab/whatsinagif/bits_and_bytes.asp as a reference.

So, according that to that website, the gif has a (zero + 1) colour resolution (2 possible colours, even though the image has many more). I tried editing this number and it didn't seem to change anything. Can someone explain the purpose of the colour resolution bits?

Answer 1

you can edit it without harm because it's not needed to decode the data - all it does is describe the "source" of the data (the last three bits in that byte are what apply to this particular image).

the standard itself http://www.w3.org/Graphics/GIF/spec-gif89a.txt (page 9) says:

This value represents the size of the entire palette from which the colors in the graphic were selected, not the number of colors actually used in the graphic.

and then goes on to add:

This value should be set to indicate the richness of the original palette, even if not every color from the whole palette is available on the source machine.

so it looks like even the original authors didn't have much idea for how it would be used. and since it's not really useful, i imagine it's not set correctly by some software.

[the only use i can see is that if this value were larger than the actual colour depth used you could add a caption/popup saying "the original image was better than this".]