带alpha通道的图层合并算法，实现多个图层叠加后产生的新图层rgba数据

alpha blend图层合并算法

int clamp(int val) {
  if (val < 0) return 0;
  if (val > 255) return 255;
  return val;
}

unsigned char layerMerge(unsigned char **layers, unsigned int layerCount, unsigned int layerWidth, unsigned int layerHeight) {
	if (1 >= layerCount) return 0;
	unsigned char isNewLayer = 0;
	unsigned int i, j, byteCount;
	byteCount = layerWidth * layerHeight * 4;		/* byte count of each layer */
	unsigned char *result = layers[0];
	for (i = 1; i < layerCount; i++) {
		unsigned char *tile = layers[i];
		for (j = 0; j < byteCount;  j += 4) {
			int r = j; int g = j + 1; int b = j + 2; int a = j + 3;
			if (255 != result[a] && 0 != tile[a]) {
				isNewLayer = 1;
			}
			float alpha1 = result[a] / 255.0f;
			float alpha2 = tile[a] / 255.0f;
			float R = result[r] * alpha1 + tile[r] * alpha2 * (1 - alpha1);
			float G = result[g] * alpha1 + tile[g] * alpha2 * (1 - alpha1);
			float B = result[b] * alpha1 + tile[b] * alpha2 * (1 - alpha1);
			float A = 1 - (1 - alpha1) * (1 - alpha2);
			if (0 == result[a] && 0 == tile[a]) {
				result[r] = result[g] = result[b] = result[a] = 0;
			} else {
				result[r] = clamp((int)(R / A) + 0.5);
				result[g] = clamp((int)(G / A) + 0.5);
				result[b] = clamp((int)(B / A) + 0.5);
				result[a] = clamp((int)(A * 255) + 0.5);
			}
		}
	}
	return isNewLayer;
}

/* out put layers[0] */