_adj_x,
                                     int alpha_adj_y)
{
    const sampler_t sampler = (CLK_NORMALIZED_COORDS_FALSE |
                               CLK_FILTER_NEAREST);

    int2 overlay_size = get_image_dim(overlay);
    int2 loc = (int2)(get_global_id(0), get_global_id(1));

    if (loc.x <  x_position ||
        loc.y <  y_position ||
        loc.x >= overlay_size.x + x_position ||
        loc.y >= overlay_size.y + y_position) {
        float4 val = read_imagef(main, sampler, loc);
        write_imagef(dst, loc, val);
    } else {
        int2 loc_overlay  = (int2)(x_position, y_position);
        float4 in_main    = read_imagef(main,    sampler, loc);
        float4 in_overlay = read_imagef(overlay, sampler, loc - loc_overlay);

        int2 loc_alpha    = (int2)(loc.x * alpha_adj_x,
                                   loc.y * alpha_adj_y) - loc_overlay;
        float4 in_alpha   = read_imagef(alpha,   sampler, loc_alpha);

        float4 val = in_overlay * in_alpha.x + in_main * (1.0f - in_alpha.x);
        write_imagef(dst, loc, val);
    }
}