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BGST

The background format for Kirby's Epic Yarn.

General Information

Byte order: big-endian
Sections: 3
- header
- grid entries
- image data

Format documentation

Size = 0x40

field	offset	size	data type	description
magic	`0x0`	`0x4`	`char[0x4]`	"BGST"
flags	`0x4`	`0x4`	`u32`?	Often seen to be `0x10` or `0x11`.
image width	`0x8`	`0x4`	`u32`	The width of each image, in pixels.
image height	`0xC`	`0x4`	`u32`	The height of each image, in pixels.
grid width	`0x10`	`0x4`	`u32`	The number of rows in the grid.
grid height	`0x14`	`0x4`	`u32`	The number of columns in the grid.
image count	`0x18`	`0x4`	`u32`	The number of total images in the file.
layer enabled	`0x1C`	`0xC`	`bool[0xC]`	Indicates which rendering layers are available for the file to use.¹
grid entry offset	`0x28`	`0x4`	`u32`	Offset to grid entries.
image data offset	`0x2C`	`0x4`	`u32`	Offset to image data array.
scale modifier	`0x30`	`0x4`	`float`	Used in-game when rendering the cells.² This value is set to `1.0f` if the flags are `0x10`.

1 - Below is an enum of rendering layers as they are called by the game. Relative to the viewer, they're sorted from furthest to nearest:

enum Layer {
	Far05  = 0,
	Far04  = 1,
	Far03  = 2,
	Far02  = 3,
	Far01  = 4,
	Map    = 5,
	Game   = 6,
	Near01 = 7,
	Near02 = 8,
	Near03 = 9,
	Near04 = 10,
	Near05 = 11
};

2 - When the game renders images, the output X position is 46.0f / scale modifier and the output Y position is 1 / (46.0f / scale modifier). 46.0f seems to be some scale factor.

Grid Entries

Grid entries can be thought of as information for rendering a grid cell. Every field here is treated by the game as a signed 16-bit integer.

field	offset	description
entry enabled	`0x0`	Indicates whether or not this entry should be rendered.
layer	`0x2`	The layer this entry will be rendered on.
row	`0x4`	The row this entry will be rendered on.
column	`0x6`	The column this entry will be rendered on.
main image index	`0x8`	The image this entry will render.³
mask image index	`0xA`	The mask this entry will apply to this image, if any.³
unknown	`0xC`	This field is almost always `0xFFFF`.
unknown	`0xE`	This field is almost always `0xFFFF`.

3 - Image indices can be assigned a certain value.

A value greater than or equal to 0 is an image index.
A value equal to -1 indicates that no image is to be rendered.
The meaning of a value equal to -2 is currently unknown. Additionally, fields that appear to be grid entries do not follow the regular grid entry pattern -- an example can be found in stage/stage000/section001.bgst3. It's unknown if this is an entry, some other data, or garbage.

Image data

A "main image" is a CMPR-compressed image. Because transparency is not supported in that format, grids that want to render a specific part of the image must use an additional I4-encoded alpha mask.

These compressed data blocks are 0x20000 bytes in size.