Image Formats

Weissblatt uses custom image formats for it's sprites, graphics and textures, which it inherited and extended from Doom (1993). All of these formats are paletted, meaning they require a Color Palette to be rendered properly.

Flat

Flats are textures used for floors and ceilings. The file format is a simple dump of palette indices. Doom originally required these to be 64x64 pixels in size, but Weissblatt supports any square size between 1x1 and 2048x2048.

Fade

The screen fade format is identical to flats; a simple dump of palette indices. Sizes are required to be one of the following:

Image Size
80x50
160x100
320x200
640x400

When rendered in-game, only the red channel is considered when dereferencing colors, with the value of the red channel serving as transparency or alpha channel for rendering the screen transition. For convenience, most artists prefer to draw screen fades as paletted grayscale images.

When importing screen fades into your PK3, it must be rendered into individual frames and in-order under Fades/. The file name must match FADExxyy, with xx standing for the screen fade number and yy standing for the frame.

Picture Format

Reference: Picture format on Doom Wiki

Doom pictures are used for UI graphics, patches, sprites and wall textures. The file format is column-oriented, with each column being composed of zero or more posts, which are basically arrays of pixels. These posts are assembled into a column based on their lenghts and offsets. Each column starts at an offset defined in the header and is terminated by a byte containing 0xFF.

Headers

Each Doom picture starts with a header, indexing the offsets of each column. When a picture is rendered, the engine starts drawing each column at the offset given in columnofs and draws all posts it finds until the next column starting with 0xFF (column terminator byte).

Patch header data structure

Field	Type	Byte size	Byte offset	Description
width	uint16_t	2	0	Graphic width (Little Endian)
height	uint16_t	2	2	Graphic height (Little Endian)
leftoffset	int16_t	2	4	Pixel offset from the left side (Little Endian)
topoffset	int16_t	2	6	Pixel offset from the top (Little Endian)
columnofs	uint32_t[]	4 * width	8	Array of column offsets relative to the beginning of this header (Little Endian)

Posts

Posts are one-dimensional strips of palette indices with a length and a top offset or top delta attached:

Post data structure

Field	Type	Byte size	Byte offset	Description
topdelta	uint8_t	1	0	Post offset from top
length	uint8_t	1	1	Post length
padding	uint8_t	1	2	Unused padding byte.
data	uint8_t[]	length	3	Array of palette indices
padding	uint8_t	1	3 + length	Unused padding byte

Tall patches

Tall patches are a backwards-compatible hack to allow doom pictures to be larger than 256 pixels in height. Since doom pictures can theoretically be up to 65536x65536 pixels large, yet posts can only be up to 254 pixels long (with the byte value 255 serving as a, hack to allow for Doom Pictures larger than 256 pixels in height.

Whenever a post's top delta is less than it's predecessor's top delta, an overflow is detected and both values are added together for the new top delta to allow for the tall patch. This makes top offsets beyond 254 effectively relative to one another, rather than absolute as before.

When a Doom engine does not support tall patches, the posts in question will simply wrap and draw over the existing sprite space.

Many tools like SLADE and PK3Make will also automatically insert zero-length pseudo patches at top delta 254 during image conversion to guarantee a reliable detection of tall patches.