问Alpha混合澄清

EN

int pixel,vga_pixel;
int alpha, blue, green, red, pixel;

int height = 1296;
int width = 968;
int x, y;

for (y = 0; y <= height; y++){
    for (x = 0; x <= width; x++){
        pixel = *(img.memloc + x + y);
            //0xff gets the first 8 bits, in this case red  
            red = pixel & 0xff; 

            //shift by 8 to get rid of red then AND  to get first 8, here green
            green = pixel >> 8 & 0xff; 

            blue = pixel >> 16 & 0xff;

            alpha = pixel >> 24 & 0xff;
            int inv_alpha = 0xff - alpha; // 1-alpha

            int vga_red = (red*(int)alpha);
            int vga_green = (green*(int)alpha + inv_alpha/256);
            int vga_blue = (blue*(int)alpha);
            int vga_alpha = 0xff;

            int vga_pixel = vga_alpha << 24 | vga_blue << 16 | vga_green << 8 | vga_red;            
    }
}