CODEWITHSUNDEEP
gifdecoding
Martin Konecny
Martin Konecny

Reputation: 59601

Decoding a GIF question

For a personal project I'm creating a static gif decoder. Everything works well so far, except I'm having trouble understanding one concept from the specification:

This block is REQUIRED for an image. Exactly one Image Descriptor must be present per image in the Data Stream. An unlimited number of images may be present per Data Stream. (Spec)

Unlimited images per data stream? There can be more than 1? I'm assuming they don't mean an animated GIF because in the same specification they state, that they don't recommend gif's be used for animation.

I've read a lot of other sites documenting the gif format and they all state something similar. Heres another link:

There might be an unlimited number of images present in a GIF.

Any ideas?

Upvotes: 1

Views: 779

Answers (2)

Egor Skriptunoff
Egor Skriptunoff

Reputation: 23737

GIF87a does not allow animation, but enables multiple images in GIF.
The idea was to decompose (for better total compression ratio) large picture into several rectangular subpictures drawn on a common canvas initially filled with background color. Every subpicture has its own x, y, width, height and palette. For example, if you are drawing astronomic picture: Earth and Moon, these two objects may be represented as two small subpictures on common large black canvas. It is not an animation: both subpictures are visible simultaneously.
But this beautiful idea was not supported by software (browsers, picture viewers, etc.), probably due to such non-standard approach.

Since GIF89a these subpictures were transformed into animation frames.

Upvotes: 1

Therealstubot
Therealstubot

Reputation: 757

There can be more than one image present in a GIF file. Yes, GIF files can perform crude animation, but it's usually not pretty.

Upvotes: 0

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