GRFX
From Just Solve the File Format Problem
GRFX is a raster graphics file format used for storing logos and splash screens in some AMI BIOSes.
Structure
All number values are little-endian.
The file begins with this structure:
| Name | Length and type | Description |
|---|---|---|
| magic | 4 bytes | Always 'G' 'R' 'F' 'X'.
|
| ??? | 2 bytes | Unknown; always 01 01 in files known to this author.
|
| hlen | uint16_t | Length of this header; usually 8. |
After this header, another structure follows.
| Name | Length and type | Description |
|---|---|---|
| ??? | 4 bytes | Unknown; usually all zero. |
| hlen | uint16_t | Length of this structure. Always 78 (0x4e) in files known to this author. |
| ??? | 2 bytes | Unknown; often zero, but 14 00 and 2e 00 are also known to this author.
|
| ??? | 2 bytes | Unknown; often zero, but 9f 00 and 8c 00 are also known to this author.
|
| width | uint16_t | Image width in pixels. |
| height | uint16_t | Image width in pixels. |
| ??? | 2 bytes | Unknown; probably a bitfield. Often 00 80, although 01 80 and 03 80 are also known to this author.
|
| ??? | 2 bytes | Unknown; often zero, although 0f 00 is also known to this author.
|
| colours | uint16_t | Number of colours. Always 16 in files known to this author. |
| ??? | 1 byte | Unknown; always zero in files known to this author. Most probably just padding. |
| magic | 3 bytes | Always 'I' 'M' 'A' ("AMI" in reverse)
|
| palette | uint16_t [16 * 3] | EGA palette; a series of triples of colour intensities of red, green and blue (in order), each in the range of 0 to 63. |
| ??? | 2 bytes | Unknown; always zero in files known to this author. |
| ident | 4 bytes | Image identifier; four ASCII capital letters stored in reverse order. Common values are 'S' 'I' 'M' 'A' ("AMIS", small AMI logo) and 'L' 'I' 'M' 'A' ("AMIL", large AMI logo)
|
After this structure, the bitmap data follows. Pixels are stored row by row, each row stores pixels left to right. Each byte contains data for two pixels; the more significant 4 bits store the left pixel of the pair.
It is not known to this author how the file format is supposed to behave for bitmap widths not divisible by two, or for palette sizes other than 16.
