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HG‐2 Image
All values and versions are known, with exception to the 8 bytes before compressed length. The same as with HG-3 Image's img#### tag.
The file structure and image encoding for HG-2 Images is over 20 years old now, as they existed in the original CatSystem (1) engine in the late 1990's. And the image encoding is identified as the same encoding from a patent filed in 1988.
NOTE: Although HG-2 shares the same set of 6 fields with HG-3 for its native image encoding, these fields are ignored by CatSystem 2 (and presumably 1) in assembly. Thus HG-2 images do not support multiple slices for a single image.
This header uses the same structure as HG-3 Image's header.
| Data Type | Value | Description |
|---|---|---|
char[4] |
"HG-2" | File Signature |
uint32 |
HeaderSize | Size of this header, always 12 |
uint32 |
Version | HG-2 version number: 0x10, 0x20, or 0x25
|
| Frame[>] | Frames | A find-by-offset set of frames for this image |
Note: Version is read as a uint16
The Version field is likely formatted as hex digits, but represented as a decimal version number.
| Hex | Dec | Includes |
|---|---|---|
0x10 |
v1.0 | Single frame, no ID |
0x20 |
v2.0 | Multiple frames, ID, Total Size, Offset XY, Transparent fields |
0x25 |
v2.5 | Base XY fields |
StartPosition = stream.Position
Version 0x10 HG-2 images have only one frame, and no ID. These have never been seen in the wild, but discovered in the assembly instructions for reading HG-2 images.
Position = StartPosition + Frame.OffsetNext
StartPosition = stream.Position
do while Frame.OffsetNext != 0
Note: BitDepth is read as a uint16 - Looking at GARbo's implementation, they list the fiend as u16 BPP followed by u16 Depth. But use of this has not yet been observed in assembly. If it does have significance, then it may be similar to the 'Planes' field used by bitmaps, or unlikely but may be used for actual slice implementation.
| Data Type | Value | Description |
|---|---|---|
uint32 |
Width | Condensed width of the image (without transparency) |
uint32 |
Height | Condensed height of the image (without transparency) |
uint32 |
BitDepth | Number of bits per pixel, 24 or 32 |
uint32 |
Reversed1 | Unused, not supported (would be the slice index) |
uint32 |
Reserved2 | Unused, not supported (would be the slice length) |
uint32 |
CompressedDataLength | Compressed length of Zero Run-length copy data |
uint32 |
DecompressedDataLength | Decompressed length of Zero Run-length copy data |
uint32 |
CompressedCmdLength | Compressed length of Zero Run-length copy command bits |
uint32 |
DecompressedCmdLength | Decompressed length of Zero Run-length copy command bits |
Frame versions 0x20 and up include Version 0x10 data for every frame in the HG-2 image.
| Data Type | Value | Description |
|---|---|---|
| Info 0x10^ | FrameInfo 1.0 | HG-2 version 0x10 frame info |
uint32 |
OffsetData | Offset from start of Version 0x20 to Image Data Can also be additional length of Frame structure Always >= 0x20
|
uint32 |
ID | Identifier for the frame used in game |
uint32 |
TotalWidth | Total width of the image with OffsetX applied |
uint32 |
TotalHeight | Total height of the image with OffsetY applied |
int32 |
OffsetX | Horizontal offset of the image from the left |
int32 |
OffsetY | Vertical offset of the image from the top |
bool32 |
IsTransparent | True if transparency is used in the image |
uint32 |
OffsetNext | Offset from start of Version 0x10 to next frame Value is 0 if no more frames |
The IsTransparent field is referred to as a Transparency Specifier in 1990's/2000's standard image formats (like TIFF)
| Data Type | Value | Description |
|---|---|---|
| Info 0x20^ | FrameInfo 2.0 | HG-2 version 0x10+0x20 frame info |
int32 |
BaseX | Horizontal center of the image, used for drawing in-game |
int32 |
BaseY | Vertical center of the image, used for drawing in-game |
This always immediately follows the Frame Info for Version == 0x10, otherwise it has an additional offset of Frame.OffsetData.
Position = StartPosition + 0x24 + Frame.OffsetData
| Data Type | Value | Description |
|---|---|---|
| byte[CompressedDataLength] | RleData | Compressed Zero Run-length copy data |
| byte[CompressedCmdLength] | RleCmd | Compressed Zero Run-length copy command bits |
See HG‐X ProcessImage for how standard HG-2 and HG-3 image data is made readable.