Header |
|
Bitmap
Data |
|
Optional
Palette |
The first 128 bytes of every PCX file is the header, which has the
following format:
typedef struct _PcxHeader
{
BYTE Identifier; /* 0x00: PCX Id Number (Always 0x0A) */
BYTE Version; /* 0x01: Version Number
0 Version 2.5 with fixed EGA palette information
2 Version 2.8 with modifiable EGA palette information
3 Version 2.8 without palette information
4 PC Paintbrush for Windows
5 Version 3.0 etc. and all 24-bit image files
For version = 5, a 768 byte palette is at the end of the file. */
BYTE Encoding; /* 0x02: Encoding Format
Always 1 for RLE */
BYTE BitsPerPixel; /* 0x03: Bits per Pixel
1, 2, 4 or 8 */
WORD XStart; /* 0x04: Left of image; usually 0 */
WORD YStart; /* 0x06: Top of Image; usually 0 */
WORD XEnd; /* 0x08: Right of Image */
WORD YEnd; /* 0x0A: Bottom of image */
WORD HorzRes; /* 0x0C: Horizontal Resolution
Pixels/line or DPI; not used when decoding */
WORD VertRes; /* 0x0E: Vertical Resolution
Pixels/line or DPI; not used when decoding */
BYTE Palette[48]; /* 0x10: 16-Color EGA Palette */
BYTE Reserved1; /* 0x40: Reserved (Always 0) */
BYTE NumBitPlanes; /*0x41: Number of Bit Planes: 1, 3 or 4
|
Color Planes |
Bits per Pixel per Plane |
Max colors |
Video Mode |
Video date |
|
1 |
1 |
2 |
Monochrome |
1981 |
|
1 |
2 |
4 |
CGA |
1981 |
|
3 |
1 |
8 |
EGA |
1984 |
|
4 |
1 |
16 |
EGA and VGA |
|
|
1 |
8 |
256 |
Extended VGA |
1987 |
|
3 |
8 |
16,777,216 |
Extended VGA and XGA |
|
*/
WORD BytesPerLine; /* 0x42: Bytes per Scan-line */
WORD PaletteType; /* 0x44: Palette Type; usually ignored
1: color
2: gray-scale */
WORD HorzScreenSize; /*0x46: Horizontal Screen Size
Only for version 4.0 and greater. */
WORD VertScreenSize; /* 0x48: Vertical Screen Size
Only for version 4.0 and greater. */
BYTE Reserved2[54]; /* 0x4A: Reserved (Always 0) */
} PCXHEAD;
[Look at MS.PCX]
Identifier: 0x0A for PCX
Version: 5 for Version 3.0
Encoding: 1 for RLE
BitsPerPixel: 8
Xstart: 1
Ystart: 1
Xend: 0x20 = 32
Yend: 0x20 = 32
HorzRes: 0x60 = 96
VertRes: 0x61 = 97
EGA Palette is all 0.
NumBitPlanes = 1 ; with 8 bits/pixel gives 256 color Extended VGA
BytesPerLine = 0x20 = 32
Image always starts at 0x80.
PCX RLE format is described on page 671 of Murray & vanRyper.
Note that there is a typo on the lower half of page 664. The identifier is 0Ah, not 10h.
First word is 0xCa 0x0e. The upper two bits (C0) are set to show a run. CA means a run of 10 pixels; value is 0e
The next bytes, 00 and 01 are uncompressed.
Next is D4 0E (run of 20 with value 0e, which takes us to the end of the first line (32 pixels).
The second line is CA 0E C3 00 01 D2 0E. This is 10 white, 3 black, 1 gray, 18 white.
The third line (starting at 0x8D) is C9 0E 01 00 10 D0 0E for 9 white, 1 gray, 1 black, 1 red, 16 white.
Other colors present in the image have index values 0x14, 0x10, 0x11 and 0x04.
The last 0x300 bytes are the palette, in groups of 3 bytes. The last byte in the file is at 0x54e. Thus the palette, if present, starts at 0x24f. If the palette is present, the byte before it (0x24e in this case) should have the value 0x0C.
We can find the palette entries for the colors used:
|
Index |
Address |
Red |
Green |
Blue |
Color |
|
0 |
24f |
00 |
00 |
00 |
Black |
|
1 |
252 |
80 |
80 |
80 |
Gray |
|
4 |
25b |
00 |
80 |
00 |
Dark green |
|
E |
279 |
ff |
ff |
ff |
white |
|
10 |
27f |
ff |
00 |
00 |
Red |
|
11 |
282 |
ff |
ff |
00 |
Yellow |
|
14 |
28b |
00 |
00 |
ff |
Blue |