Canvas file format

Canvas            *.CNV, *.CPT, *.HBL, *.FUL

Canvas is a simple drawing program for the Atari ST. The simplest file
format it uses is .CNV. The format has no info on the resolution of 
the image. The colors are stored in VDI order.

   48 bytes       palette, value 0-7 per byte, RGB in VDI order.
32000 bytes       image data


The CPT format is a compressed format:
   16 words       palette
    1 word        resolution (0=low, 1=med, 2=high resolution)
  ??? words       compressed data, uncompressed 32000 bytes
  
Packing algorithm: first all runlength data is transfered. Then the
gaps that haven't been filled with runlengthdata is filled with the 
remaining data from file.
Runlength data:
    1 word        repeat count
    1 word        offset
    n words       run length data (n=4 low, 2 med, 1 high)
The n words run length data are repeated (repeat count +1) times from 
offset bytes from the start of the file. When repeat count $FFFF signifies 
the end of the run length data (the offset and run length data still folow 
the $FFFF repeat count).
  ??? words       data to fill the gaps that haven't been filled with run 
                  length data.

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*.HBL files:

There's also a special HBL mode with extra colors (ST low and medium only).
Canvas saves the extra colors in a separate file with the same name, but with
an *.HBL file extension. Canvas can change the palette every 4 scan lines, thus
50 palettes can be displayed at once. STe palette is supported.

50 words     palette numbers, 1 per palette [0 to 63 or -1]
             -1 = no change at that scanline, scanline = word index * 4
150 words    unknown [low = all 0's, medium = all -1's]
400 bytes    unknown [all 0's]
48 bytes     default palette when HBLs are off? (RGB in VDI order)
for each flag that is <> -1 {
  48 bytes   palette (RGB in VDI order)
}

Notes: The palettes are stored in reserve. Example: Image uses 4 palettes.
palette ?  (palette when HBLs are off?) <-- offset 800
palette #4 (last palette)
palette #3
palette #2
palette #1 (first palette) <- end of file

The size of the file can be calculated using this method:
count = the number of palette flags that are set to -1
file_size = 800 + ((count + 1) * 48)

Some HBL files contain palette numbers that are clearly out of range. Canvas
manages 64 palettes and seems to dump these values to the HBL file. The palette
number should be ignored and the important value is -1. Canvas dumps all the
palettes in order, regardless of duplicates.

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Canvas v1.17 introduced a new file format referred to as FullPic Picture
Format. These use the file extension FUL. Manual states: This is simply the
Compact Picture Format, HBL file and Animate file all in one file.

? bytes    HBL data, identical to HBL file contents
608 bytes  SEQ data, always 608 bytes (skip)
? bytes    CPT data, identical to CTP file contents
---------
? bytes

Back to ST Picture Formats