Module offscreen 

Raster pixels: the Offscreen table and its block data.

An offscreen is a grid of 256×256 blocks. Offscreen.Attribute says how big it is and how a block’s bytes are laid out; Offscreen.BlockData is the id of an external chunk holding the zlib-compressed blocks (spec: docs/spec/04-raster.md).

Attribute (big endian): u32 16, u32 len(Parameter), u32 len(InitColor), u32 len(BlockSize), then three sections, each a UTF-16 name followed by u32 values. The values of Parameter and InitColor are, in order, the columns of the unused VOffscreen table in ParamScheme (BlockDataChannelOrder, BlockDataPixelBytes, … InitColorDataAlphaValue).

A decompressed block is an alpha plane followed by an image plane:

channel_order	kind	block bytes	planes
0x21 (33)	colour	327 680	alpha 256×256, then BGRA 256×256×4 (the 4th byte unused)
0x11 (17), 2 bytes/pixel	gray	131 072	alpha, then gray
0x11 (17), 1-bit	monochrome	16 384	alpha 1 bit/pixel, then 1 bit/pixel
0x01 (1)	mask / alpha only	65 536	one plane

The external payload (big endian): u64 length of the rest, one record per block (u32 record length, name "BlockDataBeginChunk", u32 index, u32 decompressed size, u32 256, u32 256, u32 has data, [u32 4 + n, u32le n, n bytes of zlib], name "BlockDataEndChunk"), then the tail sections BlockStatus and BlockCheckSum (name, u32 12, u32 count, u32 4, count × u32). The checksum is the Adler-32 of u32le n + the zlib bytes, 0 for an empty block. CSP compresses with zlib level 1.

Structs

Attribute

Offscreen.Attribute, kept value for value so it can be written back.

Block

One block record of the payload.

BlockData

The external payload of an offscreen.

InitColor

InitColor: how blocks that are not stored start out.

Offscreen

A decoded offscreen.

Parameter

The fields of the Parameter section, named after VOffscreen.

Section

One named section of the attribute: its u32 values.

Table

A tail table (BlockStatus, BlockCheckSum).

Enums

PixelKind

Pixel layout of an offscreen.

Constants

BLOCK_PIXELS

Pixels per block.

BLOCK_SIZE

Width and height of a block in pixels.

EMPTY_RECORD_SIZE

Size of a block record without data (the BlockSize entry of empty blocks).

RECORD_OVERHEAD

What a stored block adds to its zlib length in its record.

Functions

compress

zlib with CSP’s settings: the bundled C zlib at level 1, window bits 15, memory level 9 (gives CSP’s bytes exactly).

encode

Encode an image into blocks for parameter (colour, gray or alpha). Blocks whose pixels are all transparent are not stored, as CSP does.