Changing the pallete in allegro?

[Brian]

Well I've made some code for the menu background to my game, which I will double buffer the menu text above, It generates a random set of blue lines (looks better than it sounds). Well, anyway, I want to cycle through the colors of the background [as an animation], instead of it being blue, by changing the pallete.

Can anybody help me?

#include <allegro.h>  // Uses allegro.
#include <time.h>     // Time.h needed for random numbers.
#include <stdlib.h>   // Stdlib needed for rand()
#define SW 640        // Screen width
#define SH 480        // Screen height

int main( void )
{ 
 int y = 0;
 int color = 0;
 int z;
 srand(time(NULL));                          // Initialise randomness.
 allegro_init();                             // Initialise the
 set_color_depth(32);                        // graphics
 set_gfx_mode(GFX_AUTODETECT, SW, SH, 0, 0); // mode.
 install_keyboard();                         // Get keyboard ready.
 BITMAP *bmp = create_bitmap(SW, SH);        // Make the bitmap.
 clear_bitmap(bmp);                          // Clear it.
 while(y < SH)                               // Loop while y is less
 {                                           // Than screen height.
  hline(bmp, 0, y, SW, color);               // Draw line to bmp.
  y++;                                       // Increase y.
  z = rand() % 30;                           // Generate random number.
  if(z <= 15)
  {
   if(color < 255)
    color+= 3;
   else
    color-=3;
  }
  else
  {
   if(color > 0)
    color-= 3;
   else
    color+=3;
  }
 }
 blit(bmp, screen, 0, 0, 0, 0, SW, SH);    // Draw BMP to screen.
 while(!key[KEY_ESC])                      // Wait till esc, then quit.
  poll_keyboard();                         // This is needed for some.
 allegro_exit();                           // Bye!
 return 0;     
}     

END_OF_MAIN();

[Justin W]

Unfortunately, the "pallette" does not exist in any color depth higher than 8 bit (256 color). This is a hardware restraint, not software. A pallette in 8 bit modes resides in the video card, storing information for how each 8 bit value is interpreted as a color. For true color modes, the information on how to represent each pixel is stored within the pixel. There are 8 bits used to represent shades of red, 8 to represent shades of green, and 8 to represent shades of blue, giving us a total of 24 bits.

8 bits is 256 shades of each color, which scientists tell us is the most the human eye can distinguish between. So why 32 bit color, if the human eye can only see 24 bits worth of color? The extra 8 bits are for hardware accelerated "alpha". Think, transparency. If I have a blue pixel that is opaque, and I place a red pixel over it on the display, but it has just half its alpha value set (to 128 in this case), then the video card changes the 32 bit value into a 24 bit value that is a mix of red and blue (purple).

..

But I digress.

In other words, you'll have to make your own palette algorithm and repaint each pixel you want to change for every frame. That can be a bit of a speed killer, especially if you must change every pixel of the screen per frame at a high resolution. (In, say, a fade.) You can either take advantage of that extra 8 bit alpha channel for your 32 bit color depth and/or use this handy little Allegro plugin which takes care of most of that for you and increases the speed dramatically.

Hope that helps.

-Justin