BJ>Hi All , hope you are having a nice day
BJ>Recently someone posted some code for loading 320x300x256 PCX to the
BJ>screen.  It was interesting stuff though I couldn't work out what was 
oing
BJ>on.  Surely the easiest to start with would be the BMP format as I thought
BJ>that PCX's were slightly compressed?
BJ>Have I got this right?  If so how's a BMP loaded?
Some BMP are also compressed but it's very easy to decompress them. I'll
give you the structure of the BMP file then you'll be able to write
something to display it.
Bitmap-File Formats
Windows bitmap files are stored in a device-independent bitmap (DIB)
format that allows Windows to display the bitmap on any type of display
device. The term "device independent" means that the bitmap specifies
pixel color in a form independent of the method used by a display to
represent color. The default filename extension of a Windows DIB file is
.BMP.
Bitmap-File Structures
Each bitmap file contains a bitmap-file header, a bitmap-information
header, a color table, and an array of bytes that defines the bitmap
bits. The file has the following form:
BITMAPFILEHEADER bmfh;
BITMAPINFOHEADER bmih;
RGBQUAD          aColors[];
BYTE             aBitmapBits[];
The bitmap-file header contains information about the type, size, and
layout of a device-independent bitmap file. The header is defined as a
BITMAPFILEHEADER structure. The bitmap-information header, defined as a
BITMAPINFOHEADER structure, specifies the dimensions, compression type,
and color format for the bitmap. The color table, defined as an array of
RGBQUAD structures, contains as many elements as there are colors in the
bitmap. The color table is not present for bitmaps with 24 color bits
because each pixel is represented by 24-bit red-green-blue (RGB) values
in the actual bitmap data area. The colors in the table should appear in
order of importance. This helps a display driver render a bitmap on a
device that cannot display as many colors as there are in the bitmap. If
the DIB is in Windows version 3.0 or later format, the driver can use
the biClrImportant member of the BITMAPINFOHEADER structure to determine
which colors are important.
The BITMAPINFO structure can be used to represent a combined
bitmap-information header and color table. The bitmap bits, immediately
following the color table, consist of an array of BYTE values
representing consecutive rows, or "scan lines," of the bitmap. Each scan
line consists of consecutive bytes representing the pixels in the scan
line, in left-to-right order. The number of bytes representing a scan
line depends on the color format and the width, in pixels, of the
bitmap. If necessary, a scan line must be zero-padded to end on a 32-bit
boundary. However, segment boundaries can appear anywhere in the bitmap.
The scan lines in the bitmap are stored from bottom up. This means that
the first byte in the array represents the pixels in the lower-left
corner of the bitmap and the last byte represents the pixels in the
upper-right corner.
The biBitCount member of the BITMAPINFOHEADER structure determines the
number of bits that define each pixel and the maximum number of colors
in the bitmap. These members can have any of the following values:
Value        Meaning
1        Bitmap is monochrome and the color table contains two entries.
Each bit in the bitmap array
represents a pixel. If the bit is clear, the pixel is displayed with the
color of the first entry in the
color table. If the bit is set, the pixel has the color of the second
entry in the table.
4        Bitmap has a maximum of 16 colors. Each pixel in the bitmap is
represented by a 4-bit index
into the color table. For example, if the first byte in the bitmap is
0x1F, the byte represents two
pixels. The first pixel contains the color in the second table entry,
and the second pixel
contains the color in the sixteenth table entry.
8        Bitmap has a maximum of 256 colors. Each pixel in the bitmap is
represented by a 1-byte
index into the color table. For example, if the first byte in the bitmap
is 0x1F, the first pixel has
the color of the thirty-second table entry.
24        Bitmap has a maximum of 2^24 colors. The bmiColors (or
bmciColors) member is NULL,
and each 3-byte sequence in the bitmap array represents the relative
intensities of red,
green, and blue, respectively, for a pixel.
The biClrUsed member of the BITMAPINFOHEADER structure specifies the
number of color indexes in the color table actually used by the bitmap.
If the biClrUsed member is set to zero, the bitmap uses the maximum
number of colors corresponding to the value of the biBitCount member. An
alternative form of bitmap file uses the BITMAPCOREINFO,
BITMAPCOREHEADER, and RGBTRIPLE structures.
Bitmap Compression
Windows versions 3.0 and later support run-length encoded (RLE) formats
for compressing bitmaps that use 4 bits per pixel and 8 bits per pixel.
Compression reduces the disk and memory storage required for a bitmap.
Compression of 8-Bits-per-Pixel Bitmaps
When the biCompression member of the BITMAPINFOHEADER structure is set
to BI_RLE8, the DIB is compressed using a run-length encoded format for
a 256-color bitmap. This format uses two modes: encoded mode and
absolute mode. Both modes can occur anywhere throughout a single bitmap.
Encoded Mode
A unit of information in encoded mode consists of two bytes. The first
byte specifies the number of consecutive pixels to be drawn using the
color index contained in the second byte. The first byte of the pair can
be set to zero to indicate an escape that denotes the end of a line, the
end of the bitmap, or a delta. The interpretation of the escape depends
on the value of the second byte of the pair, which must be in the range
0x00 through 0x02. Following are the meanings of the escape values that
can be used in the second byte:
Second byte        Meaning
0        End of line.
1        End of bitmap.
2        Delta. The two bytes following the escape contain unsigned
        values indicating the horizontal and vertical offsets of the
        next pixel from the current position.
Absolute Mode
Absolute mode is signaled by the first byte in the pair being set to
zero and the second byte to a value between 0x03 and 0xFF. The second
byte represents the number of bytes that follow, each of which contains
the color index of a single pixel. Each run must be aligned on a word
boundary. Following is an example of an 8-bit RLE bitmap (the two-digit
hexadecimal values in the second column represent a color index for a
single pixel):
>>> Continued to next message
 * SLMR 2.1a * We all live in a yellow subroutine.
--- FMail 0.92
---------------