color.h

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#pragma once
#include <stdint.h>

typedef uint32_t pix_color_t;

#define PIX_COLOR_NONE 0x00000000u

#define PIX_COLOR_RED 0xFF0000FFu
#define PIX_COLOR_GREEN 0x00FF00FFu
#define PIX_COLOR_BLUE 0x0000FFFFu
#define PIX_COLOR_WHITE 0xFFFFFFFFu
#define PIX_COLOR_BLACK 0x000000FFu

#define PIX_COLOR_RGBA(r, g, b, a)                                            \
  ((pix_color_t)(((uint32_t)(uint8_t)(r) << 24) |                            \
                ((uint32_t)(uint8_t)(g) << 16) |                            \
                ((uint32_t)(uint8_t)(b) << 8) |                              \
                (uint32_t)(uint8_t)(a)))

#define PIX_COLOR_RGB(r, g, b) PIX_COLOR_RGBA(r, g, b, 0xFFu)

static inline uint8_t pix_color_r(pix_color_t color) {
  return (uint8_t)(color >> 24);
}

static inline uint8_t pix_color_g(pix_color_t color) {
  return (uint8_t)(color >> 16);
}

static inline uint8_t pix_color_b(pix_color_t color) {
  return (uint8_t)(color >> 8);
}

static inline uint8_t pix_color_a(pix_color_t color) {
  return (uint8_t)color;
}

static inline pix_color_t pix_color_blend(pix_color_t src, pix_color_t dst) {
  uint32_t sa = pix_color_a(src);
  uint32_t da = pix_color_a(dst);
  uint32_t ia = 255u - sa;

  // dst's RGB must be weighted by dst's own alpha too - a partially
  // transparent dst shouldn't contribute its raw RGB at full strength, the
  // same way its own draw would only have partially covered whatever was
  // beneath it. The result is then un-premultiplied by the composited
  // alpha, since pix_color_t stores straight (not premultiplied) RGB.
  uint32_t out_a = sa + (da * ia + 127u) / 255u;
  if (out_a == 0) {
    return PIX_COLOR_NONE;
  }

  uint32_t denom = out_a * 255u;
  uint32_t half = denom / 2u;
  uint8_t r = (uint8_t)((pix_color_r(src) * sa * 255u +
                        pix_color_r(dst) * da * ia + half) / denom);
  uint8_t g = (uint8_t)((pix_color_g(src) * sa * 255u +
                        pix_color_g(dst) * da * ia + half) / denom);
  uint8_t b = (uint8_t)((pix_color_b(src) * sa * 255u +
                        pix_color_b(dst) * da * ia + half) / denom);
  return PIX_COLOR_RGBA(r, g, b, (uint8_t)out_a);
}