by Antonio Linares » Fri Dec 08, 2023 8:22 am
Dear Oliveiros,
The kind of GIF file that is needed is a standard GIF file that follows the GIF89a specification. This means that the GIF file should have the following characteristics:
It should have a header that identifies the file as a GIF file and specifies the version of the format. The header should be six bytes long and contain the ASCII characters “GIF89a”
It should have a logical screen descriptor that defines the dimensions and properties of the logical screen. The logical screen descriptor should be seven bytes long and contain the following fields: logical screen width, logical screen height, global color table flag, color resolution, sort flag, size of global color table, background color index, and pixel aspect ratio
It should have a global color table that contains the colors used in the image. The global color table should be present if the global color table flag is set to 1 in the logical screen descriptor. The size of the global color table should be 2^(size of global color table + 1) bytes, where each byte represents one color component (red, green, or blue) of a color.
It should have one or more image descriptors that define the dimensions and properties of each image in the file. The image descriptor should start with a byte containing the ASCII character “,”, followed by the following fields: image left position, image top position, image width, image height, local color table flag, interlace flag, sort flag, size of local color table, and local color table (optional).
It should have one or more image data blocks that contain the compressed pixel data of each image in the file. The image data blocks should start with a byte containing the LZW minimum code size, followed by one or more sub-blocks of data. Each sub-block should start with a byte containing the size of the sub-block, followed by the actual data bytes. The last sub-block should have a size of zero, indicating the end of the image data
It should have one or more graphic control extensions that specify the animation parameters of each image in the file. The graphic control extension should start with a byte containing the ASCII character “!”, followed by a byte containing the value 249, followed by a byte containing the block size (usually 4), followed by the following fields: reserved, disposal method, user input flag, transparent color flag, delay time, transparent color index, and block terminator (a byte containing the value 0
It should have an application extension that specifies the looping behavior of the animation. The application extension should start with a byte containing the ASCII character “!”, followed by a byte containing the value 255, followed by a byte containing the block size (usually 11), followed by the application identifier (“NETSCAPE”), followed by the application authentication code (“2.0”), followed by a sub-block containing the loop count, followed by a block terminator (a byte containing the value 0)
It should have a trailer that marks the end of the file. The trailer should be a single byte containing the ASCII character “;”