Some values unknown but still much more readable than other implementations

The first 12 bytes of 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	Version	HG-3 version number: 0x300
Frame[>]	Frames	A find-by-offset set of frames for this image

The Version field is likely formatted as hex digits, but represented as a decimal version number. Because HG-2 Image's make use of 0x10, 0x20, and 0x25, its possible that 0x300 is used to avoid any sort of conflicts.

Hex	Dec	Includes
0x300	v3.00	Only version, not actually checked by Cs2

StartPosition = stream.Position

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

HG-3 images may have multiple frames of image data, each with their own dimensions and such. Each frame is looked by an ID. These IDs are always printed as a zero-padded 4-digit decimal number (however values larger than 9999 will print with more digits).

Oftentimes, multiple frames are all stored in one image for Menu assets where each frame is a different button or background, etc.

The frame header tells us the ID of the frame, which CatSystem2 looks for, and the offset to the next the next frame in the HG-3 image, if there is a next frame.

Data Type	Value	Description
uint32	OffsetNext	Offset to the next frame
uint32	ID	Identifier for the frame used in game
Tag[>]	Tags	A find-by-offset set of tags for this frame

TagStartPosition = stream.Position

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

Following the Frame Header, is a set of tags specific to the frame. Similar to Frame Header, each tag also has an OffsetNext that points to the next tag until there are no more in the frame. See Tags section for the documented structure of all known tags.

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 in this frame
uint32	Length	Length of this tag's data structure
byte[Tag.Length]	TagData	Tag-specific data

These are all the known tag types that can appear in a frame, some tags have a required order before or after other tags. These tags are only considered part of the current frame. Each tag type has a different way of reading the data. See below for a list of known tags and their structure.

stdinfo contains basic information about the frame, such as dimensions, bit depth, offset, etc. This tag must always be the first tag in a frame (confirmed in assembly).

The tag name (stdinfo) possibly stands for "Standard Info".

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	TotalWidth	Total width of the image with OffsetX applied
uint32	TotalHeight	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

The IsTransparent field is referred to as a Transparency Specifier in 1990's/2000's standard image formats (like TIFF)

Where #### is replaced with a four digit number, padded with zeros. A number other than 0000 has never been observed.
Each numbered tag contains one slice of data for the same image. Each slice defines a range of rows (scan lines) within said image.

The tag name (img####) stands for "Image [IDNo.]".

Specifies a standard image type that both HG-2 and HG-3 images use. The encoding is nearly if not exactly identical to what is described in this patent, filed in 1988.

Data Type	Value	Description
uint32	SliceStart	The starting row of the image slice. (Only observed as zero)
uint32	SliceLength	The number of rows in the image slice. (Only observed as Stdinfo.Height)
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
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.

Specifies JPEG compressed image file data, that can optionally be combined with Tag "img_al".

The tag name (img_jpg) stands for "Image JPEG".

Unlike other tag structures, this structure requires 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.

Specifies image alpha data that is to be added to Tag "img_jpg".
This tag is only looked for after reading an img_jpg tag.

The tag name (img_al) stands for "Image Alpha".

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

Data Type	Value	Description
uint32	CompressedLength	Compressed length of AlphaData bytes
uint32	DecompressedLength	Decompressed length of AlphaData bytes
byte[CompressedLength]	AlphaData	Zlib compressed alpha bytes, one byte per pixel

Undocumented image format. It's possible this is a new form to declare a JPEG image with alpha mask.

Specifies WEBP image file data.

The tag name (img_wbp) stands for "Image WebP".

Unlike other tag structures, this structure requires the Tag.Length field.

Data Type	Value	Description
byte[Tag.Length]	WebPData	Raw WEBP file data

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, Format: #AARRGGBB (TODO: reconfirm)

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

The tag name (cptype) likely stands for "CG Page Type", which would explain its appearance in CG menu thumbnails, only.

Data Type	Value	Description
uint32	Type	Unknown, very rarely non-zero Other known value is 3, found in CG menu images

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.

The tag name (imgmode) likely stands for Image Mode.

Data Type	Value	Description
bool32	Mode	Common in newer games, present on CG. Value is always 0 when present, defaults to 1