Accessing HP Color Recovery with X Windows

	NOTE: The algorithm discussed in this section, for the Internal Color Graphics device, is different than the algorithm discussed for the HCRX family of graphics devices in the section "Accessing HP Color Recovery with X Windows" found in Chapter 3 “The HCRX Family of Device Drivers”.

The Starbase and HP-PHIGS graphics libraries provide programmers who use these APIs with transparent access to the HP Color Recovery capability of the Internal Color Graphics device. If you are producing graphics using Xlib calls, then your application must perform some of the necessary processing. At server start-up, there are two properties that are defined and placed on the root window if the HP_DISABLE_COLOR_RECOVERY environment variable has not been exported. These properties are:

The existence of these properties indicates that HP Color Recovery is available on this display. These properties are of type RGB_COLOR_MAP and carry pointers to structures of type XStandardColormap. They may be interrogated with calls to XGetRGBColormaps. The color maps in these structures identify color maps which are created at server start-up and are for use with the TrueColor and PseudoColor visuals, respectively. They are both initialized to contain the 3:3:2 ramp of 8-bit TrueColor. Neither of these color maps can be modified as they are read-only. The use of the TrueColor visual and the color map identified in _HP_RGB_SMOOTH_TRUE_MAP will be assumed in the procedures outlined below.

To use this capability through X windows, create a window with the TrueColor visual and give it the color map attribute identified in the property:

_HP_RGB_SMOOTH_TRUE_MAP

HP Color Recovery will be enabled for all windows with this color map.

HP Color Recovery uses all 256 entries of one of the available color maps. The color visual used by HP Color Recovery emulates the 24-bit TrueColor visual, thus, the colors red, green, and blue are typically declared as integers in the range from 0 to 255. Note that each window that uses HP Color Recovery will use the same color map.

For HP Color Recovery to produce the best results, the emulated 24-bit TrueColor data is dithered as explained below.

A pixel to be dithered is sent to the routine provided in this example. Note that the values of the variables RedValue, GreenValue and BlueValue are generated by an application. In this example, the color values are assumed to be in the range

0 <= color_value <= 255.

The given routine receives the color values and the X and Y window address (Xp and Yp) of the pixel. The X and Y address is used to access the dither tables. The values from the dither tables are added to the color values. After the dither addition, the resultant color values are quantized to 3 bits of red and green and 2 bits of blue. The quantized results are packed into an 8-bit unsigned char and then stored in the frame buffer. As the contents of the frame buffer are scanned to the CRT, a special section in the Internal Color Graphics hardware then converts the 8-bit data stored in the frame buffer into a 24-bit TrueColor image for display.

Here is a routine that can be used to dither the 24-bit TrueColor data.

unsigned char dither_pixel_for_CR(RedValue,GreenValue,BlueValue,Xp,Yp)
int RedValue,GreenValueBlueValue,Xp,Yp;
{
static short dither_red[2][16] = {
  {-16,  4, -1, 11,-14,  6, -3,  9,-15,  5, -2, 10,-13,  7, -4,  8},
  { 15, -5,  0,-12, 13, -7,  2,-10, 14, -6,  1,-11, 12, -8,  3, -9} };
 
static short dither_green[2][16] = {
  { 11,-15,  7, -3,  8,-14,  4, -2, 10,-16,  6, -4,  9,-13,  5, -1},
  {-12, 14, -8,  2, -9, 13, -5,  1,-11, 15, -7,  3,-10, 12, -6,  0} };
 
static short dither_blue[2][16] = {
  { -3,  9,-13,  7, -1, 11,-15,  5, -4,  8,-14,  6, -2, 10,-16,  4},
  {  2,-10, 12, -8,  0,-12, 14, -6,  3, -9, 13, -7,  1,-11, 15, -5} };
 
int red, green, blue;
int x_dither_table, y_dither_table;
unsigned char pixel;
 
x_dither_table = Xp % 16;   /* X Pixel Address MOD 16 */
y_dither_table = Yp % 2;    /* Y Pixel Address MOD 2 */
 
red = RedValue;
green = GreenValue;
blue = BlueValue;
 
red += dither_red[y_dither_table][x_dither_table];
if (red > 0xff) red = 0xff;
if (red < 0x00) red = 0x00;
 
green += dither_green[y_dither_table][x_dither_table];
if (green > 0xff) green = 0xff;
if (green < 0x00) green = 0x00;
 
blue += (dither_blue[y_dither_table][x_dither_table]<<1);
if (blue > 0xff) blue = 0xff;
if (blue < 0x00) blue = 0x00;
 
pixel = ((red &0xE0) | ((green &0xE0) >> 3) | ((blue &0xC0) >> 6));
 
return(pixel);
}