HG‐3 Image

Some values unknown but still much more readable than other implementations

File Structure

This header uses the same structure as HG‐2 Image's header.

Data Type	Value	Description
char[4]	"HG-3"	File Signature
uint32	HeaderSize	Size of this header, always 12
uint32	Format(?)	Some sort of format applied to the file structure, always 0x300

StartPosition = stream.Position

Frame

Position = StartPosition + FrameHeader.OffsetNext
StartPosition = stream.Position do while FrameHeader.OffsetNext != 0

FrameHeader

This always immediately precedes the start of the Frame.

Data Type	Value	Description
uint32	OffsetNext	Offset to the next frame
uint32	ID	Identifier for the frame used in game

Tag "stdinfo"

This always immediately precedes the FrameHeader.

TagStartPosition = stream.Position

Data Type	Value	Description
char[8]	"stdinfo"	Name of the data structure following this tag
uint32	OffsetNext	Offset to the next tag
uint32	Length	Length of this tag's data structure

If the char[8] field is not "stdinfo", then this file is invalid or corrupted.

Stdinfo

This always immediately precedes Tag "stdinfo".

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
int32	OffsetX	Horizontal offset of the image from the left
int32	OffsetY	Vertical offset of the image from the top
uint32	Width	Total width of the image with OffsetX applied
uint32	Height	Total height of the image with OffsetY applied
bool32	IsTransparent	True if transparency is used in the image
int32	BaseX	Horizontal center of the image, used for drawing in-game
int32	BaseY	Vertical center of the image, used for drawing in-game

Note: The IsTransparent field does not mean the BitDepth will be 24 bits. Sometimes IsTransparent is true because the pixels around the border of a solid image are slightly transparent (Observed with Grisaia Title background).

Frame Tags

Both Tag "stdinfo" and Frame Tags are of the same structure, and are both assigned to Tag.

Position = TagStartPosition + Tag.OffsetNext
TagStartPosition = stream.Position
do while Tag.OffsetNext != 0

Data Type	Value	Description
char[8]	TagName	Name of the data structure following this tag
uint32	OffsetNext	Offset to the next tag, 0 if no more tags until next frame
uint32	Length	Length of this tag's data structure

Frame Tag Structure

This always immediately precedes Frame Tags.

Each tag type has a different way of reading the data. See the Tags section for a list of known tags and their structure.

Data Type	Value	Description
byte[Tag.Length]	TagName	Data for this tag

Tags

These are all the known tag types that can appear after the Stdinfo structure of a frame. These tags are only considered part of the current frame.

Tag "img####"

Where "####" is replaced with a four digit number, padded with zeros.
A number other than "0000" has yet to be encountered, and it is unknown what this number is used for.
The binaries for reading HG-3 images don't hardcode the tag as "img0000" so it must have some sort of use.

Specifies a standard image type that both HG-2 and HG-3 images use.

Data Type	Value	Description
uint32	Reserved	Seems to always be zero
uint32	Height	Height of the image, same as Stdinfo.Height
uint32	CompressedDataLength	Compressed length of unrle copy data
uint32	DecompressedDataLength	Decompressed length of unrle copy data
uint32	CompressedCmdLength	Compressed length of unrle copy cmd bits
uint32	DecompressedCmdLength	Decompressed length of unrle copy cmd bits
byte[CompressedDataLength]	Compressed unrle copy data
byte[CompressedCmdLength]	Compressed unrle copy cmd bits

See ProcessImage for how standard HG-2 and HG-3 images are processed.

Tag "img_al"

Specifies image alpha data that is to be added to Tag "img_jpg".
This tag has yet to be encountered without a Tag "img_jpg".

Unlike the output of Tag "img####" + ProcessImage, there is no padding for the stride of the alpha bytes.

Data Type	Value	Description
uint32	CompressedLength	Compressed length of alpha bytes
uint32	DecompressedLength	Decompressed length of alpha bytes
byte[CompressedLength]	Compressed alpha bytes

Tag "img_jpg"

Specifies JPEG compressed image data, that can optionally be added to Tag "img_al".

Unlike other tag structures, this structure require's the Tag.Length field.

Data Type	Value	Description
byte[Tag.Length]	JpegData	Raw JPEG file data

When combined with Tag "img_al", all alpha bytes for the JPEG image overwritten by Tag "img_al"'s alpha bytes.

Tag "ats####"

Where "####" is replaced with a four digit number, padded with zeros.
The "####" represents the ID of the attribute which is used to reference it in-game.

Specifies an Attribute, which is a set of coordinates, size, and color for drawing sprites on top of other sprites, such as the 💢 (anger) manpu.

Data Type	Value	Description
int32	X	X coordinate of the attribute relative to the left of the current frame
int32	Y	Y coordinate of the attribute relative to the top of the current frame
uint32	Width	Width to draw the image with
uint32	Height	Height to draw the image with
uint32	Color	Color to draw the image with, #AARRGGBB

Tag "cptype"

It is unknown what this tag specifies, but it is always present at the end of every frame's list of tags.

Data Type	Value	Description
uint32	CpType	Unknown, very rarely non-zero

Tag "imgmode"

It is unknown what this tag specifies. This tag is very uncommon, and may only be present in newer commercial versions of the CatSystem2 engine.

Data Type	Value	Description
uint32	ImageMode	Unknown