This section includes a brief overview of the components of a GIF file. The adventurous reader may wish to refer to the complete GIF89a Specification document for further details on the format.
A GIF file is comprised of several components, including
- Logical Screen Descriptor
- Global Color Table
- Image Descriptor
- Local Color Table
- Graphic Control Extension
- Image Data
- Comment Extension
- Application Extension
Logical Screen Descriptor
The Logical Screen Descriptor describes the area that the animation will occupy, ie the canvas size of the animation. It also defines the pixel aspect ratio, although this is currently unsupported by Gif Animator .
If the animation includes a global color table (see below), the Logical Screen Descriptor also includes the color to use as the background color for frames.
Global Color Table
While each frame in an animation can include its own color palette, for the most optimal use you can have one or more frames using a global color table, which is a shared color palette. Color tables (referred to as color palettes within Gif Animator ) can contain up to 256 colors comprised of red, green and blue channels. The GIF format does not support alpha transparency.
Similar to the Logical Screen Descriptor, the Image Descriptor defines the parameters of an individual image, including the location and size of the image. Images do not need to occupy the entire area of the animation, they can be as large or small as required. Again, the most efficient animations will only have their frames use the absolute smallest image size possible for the data that has changed from one frame to the next.
Local Color Table
This component specifies the color table for a frame, if the frame isn't using the global color table.
This component includes the actual data of the image.
This optionally component allows plain text comments to be embedded in a GIF file. Generally, these aren't displayed to end users. Gif Animator supports the displaying and editing of components and will try to associated comments with either the animation or individual frames.
The GIF format was designed to be extensible and to include other data within the file in special application extensions. Gif Animator only supports the NETSCAPE2.0 extension, which allows a GIF file to actually animate.