Difference between revisions of "Format:SGX (SymbOS graphic files)"

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Revision as of 10:13, 24 September 2006

This is a description of the SGX graphic format used in SymbOS and in SymSee.

A SGX graphic consists of any number of chunks, which define different picture parts. While there are limitations for the width and the height of a single part, the complete graphic can have any size, as all parts will be linked together.


Chunk types

Simple 4 colour graphic

byte 0: width in bytes (1-63)
byte 1: width in pixel (1-252)
byte 2: height in pixel (1-255)
byte 3-n: linear graphic data (size = byte0 * byte2)

This chunk type can be recognized, if the first byte has a value between 1 and 63. The 4 colour graphic data is encoded in CPC Mode 1 format. Every byte contains 4 pixels: 

bit7 = bit0 of pixel0 (left pixel)
bit6 = bit0 of pixel1
bit5 = bit0 of pixel2
bit4 = bit0 of pixel3 (right pixel)
bit3 = bit1 of pixel0
bit2 = bit1 of pixel1
bit1 = bit1 of pixel2
bit0 = bit1 of pixel3

Or you can see it like this: 

pixel0 = bit7 + 2 * bit3 (left pixel)
pixel1 = bit6 + 2 * bit2
pixel2 = bit5 + 2 * bit1
pixel3 = bit4 + 2 * bit0 (right pixel)


Extended 4 or 16 colour graphic

byte 0: 64
byte 1: type (0=4 colour graphic, 5=16 colour graphic)
byte 2,3: width in bytes (1-255 [4c] or 2-254 [16c])
byte 4,5: width in pixel (1-1020 [4c] or 1-508 [16c])
byte 6,7: height in pixel (1-65535)
byte 8-n: linear graphic data (size = byte2,3 * byte6,7)

This chunk type can be recognized with 64 in the first byte. 4 colour graphics are encoded in CPC Mode 1 format (see above). 16 colour graphics are encoded in MSX Screen 5/7 format. Every byte contains 2 pixels: 

bit7-4 (high nibble): left pixel
bit3-0 (low nibble): right pixel

Please note, that currently there are some limitations:

  • the maximum width for 4 colour graphics is 1020 pixel
  • the maximum width for 16 colour graphics is 508 pixel
  • 16 colour graphics must always have an even width in bytes (2,4,...,254)


Line feed

byte 0: 255
byte 1,2: undefined

This chunk has a length of 3 bytes and can be recognized with 255 in the first byte. Usually every graphic part will be linked right to the previous part. After a line feed the next graphic part will be linked below the first graphic of the previous "line".


EOF

byte 0: 0
byte 1,2: undefined

An EOF is not explicit required, but should be added, if graphics are stored on an Amsdos or CP/M disc, as these file system are not able to store the exact size of a file.


Palette

SGX files don't contain colour palette definitions, as these graphics have been designed in a way, which makes it possible to display them in any environment and colour-depth (2, 4 or 16 colours) on the SymbOS desktop.

For 4 colour graphics it means, that there is just a grayscale definition:

  • Pen 0: white
  • Pen 1: black
  • Pen 2: light gray
  • Pen 3: dark gray

This can be used for any type of colour, like orange, brown, light red, dark red etc.

16 colour graphics have a fixed palette which has the following definition:

  • Pen 00-03: #ff8,#000,#f80,#800
  • Pen 04-07: #0ff,#008,#88f,#00f
  • Pen 08-11: #fff,#080,#0f0,#f0f
  • Pen 12-15: #ff0,#888,#f88,#f00

The values are in CPC Plus RGB format. #ff8 means: red=15, green=15, blue=8.

On the MSX it would mean: red=7, green=7, blue=4.

On the CPC old generation it means: red=2, green=2, blue=1 (= 1*3^0 + 2*3^1 + 2*3^2 = 25).


Web links

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