Listing 2. Drawing Individual Pixels on the Screen
#include "SDL.h"
#include <stdio.h>
#include <stdlib.h>
Uint16 CreateHicolorPixel(SDL_PixelFormat *fmt,
Uint8 red, Uint8
green, Uint8 blue)
{
Uint16 value;
/* This series of bit shifts uses the
information from the SDL_Format
structure to correctly compose a 16-bit
pixel value from 8-bit red,
green, and blue data.
*/
value = ((red >> fmt->Rloss) << fmt->Rshift) +
((green >> fmt->Gloss) << fmt->Gshift) +
((blue >> fmt->Bloss) << fmt->Bshift);
return value;
}
int main()
{
SDL_Surface *screen;
Uint16 *raw_pixels;
int x,y;
/* Initialization code goes here. It should
create a 256x256, 16-bit display and save
the surface in the screen pointer. See
the previous example. */
/* "Lock" the screen surface so we can draw to
it directly. */
SDL_LockSurface(screen);
/* Get a pointer to the video surface's i
memory. */
raw_pixels = (Uint16 *)screen->pixels;
/* We can now safely write to the video
surface. We'll draw a nice gradient pattern
by varying our red and blue components along
the X and Y axes. */
for (x = 0; x < 256; x++) {
for (y = 0; y < 256; y++) {
Uint16 pixel_color;
int offset;
pixel_color =
CreateHicolorPixel(screen->format,
x,0,y);
/* Calculate the memory offset of the
pixel we wish to change. */
offset = (screen->pitch/2 * y + x);
raw_pixels[offset] = pixel_color;
};
};
/* We're finished drawing, so unlock the
surface. */
SDL:_UnlockSurface(screen);
/* Inform SDL that the screen has been changed.
This is necessary because SDL's screen
surface is not always the real frame buffer;
it is sometimes emulated behind the scenes.
*/
SDL_UpdateRect(screen,0,0,0,0);
/* Pause for a few seconds as the viewer gasps
in awe. */
SDL_Delay(3000);
return 0;
}