Alpha blending (over) with pre-multiplied ARGB pixels

alpha-blend.c

/*
 * Copyright (c) 2023 Jani Juhani Sinervo
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include <stdint.h>

void
premul_argb_blend(uint8_t out[static restrict 4],
                  const uint8_t a[const restrict 4],
                  const uint8_t b[const restrict 4])
{
    uint8_t blue_a = a[0];
    uint8_t green_a = a[1];
    uint8_t red_a = a[2];
    uint8_t alpha_a = a[3];

    uint8_t blue_b = b[0];
    uint8_t green_b = b[1];
    uint8_t red_b = b[2];
    uint8_t alpha_b = b[3];

    uint8_t blue_out = (255 * blue_a + 255 * blue_b - (blue_b * alpha_a))/255;
    uint8_t green_out = (255 * green_a + 255 * green_b - (green_b * alpha_a))/255;
    uint8_t red_out = (255 * red_a + 255 * red_b - (red_b * alpha_a))/255;
    uint8_t alpha_out = (255 * alpha_a + 255 * alpha_b - (alpha_b * alpha_a))/255;

    out[0] = blue_out;
    out[1] = green_out;
    out[2] = red_out;
    out[3] = alpha_out;
}