color_blender.c

#define MIN(a,b) (((a)<(b))?(a):(b))

typedef unsigned int uint;

typedef struct {
	uint red:5;
	uint green:6;
	uint blue:5;
} f_pixel;

typedef struct {
	uint red:8;
	uint green:8;
	uint blue:8;
	uint alpha:8;
} m_pixel;

static uint alpha_blend(uint fg, uint bg, uint alpha)
{
	return (alpha * fg + (255-alpha) * bg)/255;
}

void overlay_mouse_pointer(f_pixel *frame_buffer, m_pixel *mouse_pointer_buffer, int x_coordinate, int y_coordinate)
{
    int y_max = MIN(y_coordinate+32, 480);
	int x_max = MIN(x_coordinate+32, 640);
	uint fg, bg;
	uint alpha;
	for (int i=y_coordinate; i<y_max; ++i) {
		for (int j=x_coordinate; j<x_max; ++j) {
			// normalize to 255
			bg = frame_buffer[i*640+j].red << (8-5);
			fg = mouse_pointer_buffer[(i-y_coordinate)*32+(j-x_coordinate)].red;
			alpha = mouse_pointer_buffer[(i-y_coordinate)*32+(j-x_coordinate)].alpha;
			
			frame_buffer[i*640+j].red = alpha_blend(fg, bg, alpha) >> (8-5);
			
			bg = frame_buffer[i*640+j].green << (8-6);
			fg = mouse_pointer_buffer[(i-y_coordinate)*32+(j-x_coordinate)].green;
			alpha = mouse_pointer_buffer[(i-y_coordinate)*32+(j-x_coordinate)].alpha;
			
			frame_buffer[i*640+j].green = alpha_blend(fg, bg, alpha) >> (8-6);
			
			bg = frame_buffer[i*640+j].blue << (8-5);
			fg = mouse_pointer_buffer[(i-y_coordinate)*32+(j-x_coordinate)].blue;
			alpha = mouse_pointer_buffer[(i-y_coordinate)*32+(j-x_coordinate)].alpha;
			
			frame_buffer[i*640+j].blue = alpha_blend(fg, bg, alpha) >> (8-5);
							
		}
    }
}