The Image Descriptor defines the actual placement and extents of the following image within the space defined in the Screen Descriptor. Also defined are flags to indicate the presence of a local color lookup map, and to define the pixel display sequence. Each Image Descriptor is introduced by an image separator character. The role of the Image Separator is simply to provide a synchronization character to introduce an Image Descriptor. This is desirable if a GIF file happens to contain more than one image. This character is defined as 0x2C hex or “,” (comma). When this character is encountered between images, the Image Descriptor immediately follows.
Below is a table describing the internal contents of the Image Descriptor file component.
|
Bits 7 6 5 4 3 2 1 0 |
Byte # |
Description |
|
0 0 1 0 1 1 0 0 |
1 |
Image separator character “,” (comma) |
|
Image Left |
2 & 3 |
Start of image in pixels from the left side of the screen (LSB first) |
|
Image Top |
4 & 5 |
Start of image in pixels from the top of the screen (LSB first) |
|
Image Width |
6 & 7 |
Width of the image in pixels (LSB first) |
|
Image Height |
8 & 9 |
Height of the image in pixels (LSB first) |
|
M|I|0|0|0| pixel |
10 |
M=0 Use global color map, ignore “pixel” M=1 Local color map follows, use “pixel” I =0 Image formatted in sequential order I =1 Image formatted in interlaced order pixel+1 # of bits per pixel for image |