LazPaint LZP Format Single Image
This description applies for both the flat image and for the images in the layer structure of LZP file format.
There are two possible compressions. Both are lossless i.e. there is no loss of quality.
If a file is saved in this format without any header, it uses ZStream compression.
ZStream
The image has a defined width and height. There is a rectangular area for its content. Pixels that are outside of this area are transparent (all channels equal to zero).
The content is compressed in any number of chunks. The uncompressed data must then be concatenated regardless of where scanlines start or end.
The header of the ZStream image is as follows:
Offset Size Content ------ ---- ---------------------------- 0 4 Width 4 4 Height 8 4 Length of the caption (N) 12 N Caption (UTF8 encoded) 12+N 4 Left bound of the content 16+N 4 Top bound of the content 20+N 4 Right bound of the content 24+N 4 Bottom bound of the content 28+N 4 Line order (0: top to bottom, 1: bottom to top) 32+N 4 Number of chunks
The content contains the lines in the rectangular area, following the specified line order. Each line contains all pixels in BGRA 32-bit format, i.e. 1 byte for each channel and the order of the channels being blue, green, red, alpha.
The chunks follow directly the header. Here is the structure for one chunk:
Offset Size Content ------ ---- ---------------------------- 0 4 Size of the compressed data in the chunk (M) 4 M Compressed data using ZStream (without ZStream header)
RLE
The header of the RLE image is as follows:
Offset Size Content ------ ---- ---------------------------- 0 4 Width 4 4 Height 8 4 Length of the caption (N) 12 N Caption (UTF8 encoded) 12+N 1 Plane flags
The following plane flags are defined:
- $01: the green channel is a copy of the red channel
- $02: the blue channel is a copy of the red channel
- $04: the blue channel is a copy of the green channel
- $08: there is no alpha channel (the image is opaque)
- $10: there is a palette and colors are expressed as indices in this palette
There is then a variable number of RLE streams. The pixels are scanned from top to bottom and from left to right.
Full RLE structure for empty images
If the image is completely transparent, there are no stream for color channels.
Offset Size Content ------ ---- ------------------------- 0 13+N Header 13+N ? Alpha channel stream
The uncompressed stream for the alpha channel contains one byte for each pixel of the canvas. 0 means the pixel is transparent and 255 means opaque. Because the image is empty, it contains only zeros.
Full RLE structure for non paletted images
Offset Size Content ------ ---- ------------------------- 0 13+N Header 13+N ? Alpha channel stream (optional) ? ? Red channel stream ? ? Green channel stream (optional) ? ? Blue channel stream (optional)
Alpha channel is not present if $08 flag is used. Otherwise the uncompressed stream contains one byte for each pixel of the canvas. 0 means the pixel is transparent and 255 means opaque.
The uncompressed stream for the red channel contains one byte for each non-transparent pixel.
Green channel is not present if $01 flag is used. Otherwise the uncompressed stream contains one byte for each non-transparent pixel.
Blue channel is not present if $02 or $04 flag is used. Otherwise the uncompressed stream contains one byte for each non-transparent pixel.
Full RLE structure for paletted images
Offset Size Content ------ ---- ------------------------- 0 13+N Header 13+N ? Alpha channel stream (optional) ? ? Red channel palette stream ? ? Green channel palette stream (optional) ? ? Blue channel palette stream (optional) ? ? Indexed color stream
Alpha channel is not present if $08 flag is used. Otherwise the uncompressed stream contains one byte for each pixel of the canvas. 0 means the pixel is transparent and 255 means opaque.
Each channel palette stream contains a maximum of 256 uncompressed bytes. The streams may be shorter and in that case, the rest is padded with zeros. The position in the uncompressed stream is the color index.
The uncompressed stream for the red channel contains one byte for each entry of the palette. It is padded with zeros to make 256 bytes.
Green channel palette is not present if $01 flag is used. Otherwise the stream contains the green values for the palette.
Blue channel palette is not present if $02 or $04 flag is used. Otherwise the stream contains the blue values for the palette.
The uncompressed indexed color stream contains one byte for each non-transparent pixel, that is the index into the palette.
RLE stream
The structure of one RLE stream is as follows:
Offset Size Content ------ ---- ---------------------------- 0 4 Size of the compressed data (P) 4 P Compressed data
The data is a series of unpacking instructions. Each element of the instruction is a one byte value.
Special instructions:
Sequence Meaning -------------------------- --------------------------------------------------------------- $C1 Reserved (the reader must stop and say the format is not supported) $C2 Reserved (the reader must stop and say the format is not supported) $E0 Ends of stream $E1 Reserved (the reader must stop and say the format is not supported) $FF The reader must ignore and continue to read
Big repetitions instructions:
Sequence Meaning -------------------------- --------------------------------------------------------------- $00 Hi Lo Val Repeats (Hi*256 + Lo) times the value Val $80 Val Repeats the value Val the same number of times that in the previous $00 instruction $40 Count Val Repeats (Count+64) times the value Val $C0 Val Repeats the value Val the same number of times that in the previous $40 instruction
Small repetitions:
Sequence Meaning -------------------------- --------------------------------------------------------------- $01..$3f Val Repeats Val, the number of times being the value of the instruction ($01..$3f) $40+n (n = 1..31) Uncompress n group of packed repetition (see below) $60+n (n = 1..16) Repeats (n+1) times the value 0 $70+n (n = 1..16) Repeats (n+1) times the value 255
Dump:
Sequence Meaning -------------------------- --------------------------------------------------------------- $81 Count The next (Count+64) bytes are copied as they are $80+n (n = 2..63) The next n bytes are copied as they are $C0+n Val (n = 3..31) The next (n-1)/2 bytes must be unpacked to yield n uncompressed bytes (see below) An initial value Val is provided and is put as it is in the uncompressed stream. $E0+n (n = 2..30) The next n/2 bytes must be unpacked to yield n uncompressed bytes (see below) The last unpacked value with instruction $C0 and $E0 is used as an implicit initial value. This initial value is however not put in the uncompressed stream. Its default value is $80 in the case there is no previous unpacking instruction.