surfarce

// Initialization
Width = 640; Height = 480;
Texture = surface_create(Width, Height);

// Alpha blending
surface_set_target(Texture);
    // Prepare the surface for drawing
    draw_clear_alpha(0, 0);
    // Texture to be alpha blended
    draw_sprite(rockBackground, 0, 0, 0);
    // Prepare the surface for alpha blending
    draw_set_blend_mode(bm_subtract);
    // Alpha blending!
    draw_surface(mySurface[0], 0, 0);
    // Done with blending
    draw_set_blend_mode(bm_normal);
// Done with the surface
surface_reset_target();