ID Number: Q43330
5.10 6.00 6.00a 6.00ax
MS-DOS
Summary:
In Microsoft C versions 5.0, 5.1, 6.0, 6.0a, and 6.0ax, when
attempting to remap the palette in the EGA modes, follow the required
format to form the argument passed to the remapping functions. This
article uses _remappalette as an example. What is said about lColor
(see function prototype below) also applies to the element of the
array slColors for _remapallpalette.
The function prototypes are as follows:
short far _remappalette (nPixel, lColor) ;
short nPixel ; /* Pixel value to reassign */
long lColor ; /* Color number to assign to nPixel */
short far _remapallpalette (slColors) ;
long far * slColors ; Color array
lColor is a long int formed by the following four bytes:
00, blue byte, green byte, red byte
The four bytes are arranged from the most significant byte to the
least significant byte (left to right). The most significant byte is
always 0 (zero). Each of the remaining three bytes can have the value
of either 00, 2a, 15, or 3f. (In VGA mode, each byte can take any
value between 00 and 3f.) Combinations of different values in the
three bytes form the complete palette of 64 colors for the EGA
graphics modes.
The manifest constants for the EGA modes are _MRES16COLOR,
_HRES16COLOR, and _ERESCOLOR, which are used with the function
_setvideomode.
Please refer to the include file GRAPH.H for the default values of
the 16 colors in an EGA mode.
More Information:
An algorithm is provided in this section to convert a color
represented by an arbitrary number from 0 - 63 to the required format.
A sample program is included that implements the algorithm.
Suppose the color is represented by a byte, as follows:
bit7 , bit6 , bit5 , bit4 , bit3 , bit2 , bit1 , bit0
1. bit7 and bit6 can be ignored, because the byte is always less than
64.
2. The remaining bits are grouped in the following three pairs, where
the order of the two bits in the pair is significant:
bit3 , bit0 ;
bit4 , bit1 ;
bit5 , bit2 ;
3. Each pair is mapped to 00, 2a, 15, or 3f, according to the
following table:
bitX bitY MappedXY
0 0 00
0 1 2a
1 0 15
1 1 3f
4. Mapped30 (from pair bit3, bit0) goes to the blue byte.
Mapped41 (from pair bit4, bit1) goes to the green byte.
Mapped52 (from pair bit5, bit2) goes to the red byte.
The order (from high to low) of the three bytes is as follows:
blue byte , green byte , red byte
5. Example:
Color 43 has bit pattern 0 0 1 0 1 0 1 1. This pattern is
transformed to three pairs: 11, 01, 10. The three pairs are mapped
to 3f, 2a, 15. The mapped color represented in long integer format
is 0x3f2a15.
The following program uses _remappalette() to remap the color "0"
(which defaults to black) to the 64 different colors.
For more information on this subject, refer to the "Programmer's Guide
to PC and PS/2 Video Systems" by Richard Wilton, published by
Microsoft Press.
Sample Code
-----------
/*
* Pallette.c
*/
#include <stdio.h>
#include <graph.h>
#include <conio.h>
#define NUMCOLOR 64
#define MASK 0x0001
unsigned long ConvertColNum (unsigned int) ;
unsigned char ConvertArr [4] = { 0x00, 0x2a, 0x15, 0x3f } ;
void main (void)
{
unsigned short nColor = 0 ;
unsigned int i ;
unsigned long lMappedColor ;
if (!_setvideomode (_ERESCOLOR))
return ;
puts ("Press any key to continue ...") ;
getch () ;
for (i = 0 ; i < NUMCOLOR ; i++)
{
lMappedColor = ConvertColNum (i) ;
if (_remappalette (nColor, lMappedColor) == -1L)
puts ("\x007Mapping color failed : ") ; // beep if fails
printf ("Color number = %u, lMappedColor = %08lx\n", i,
lMappedColor);
if (getch () == 'q')
break ;
}
_setvideomode (_DEFAULTMODE) ;
}
unsigned long ConvertColNum (unsigned int nOrgColor)
{
unsigned long lColor, lTemp ;
int j, temp ;
lColor = 0L ;
for (j = 0 ; j < 3 ; j++)
{
// get the pair
temp = ((nOrgColor >> (j + 3) & MASK) << 1 ) |
(nOrgColor >> j & MASK) ;
lTemp = (unsigned long) ConvertArr [ temp ] ;
lColor |= lTemp << ((2-j) << 3) ; // (2-j) * 8
}
return (lColor) ;
}
Additional reference words: 5.00 5.10 6.00 6.00a 6.00ax