Code fragment blending ASS_Images onto an RGB surface

render_subtitles_rgb24.c

#include <stddef.h>
#include <stdint.h>
#include <stdlib.h>

#define dither() (rand() / (RAND_MAX + 1.0L))

typedef struct
{
	size_t w, h;
	struct rgb24_pixel { uint8_t r, g, b; } samples[];
} rgb24;

void render_subtitles_rgb24(rgb24 *restrict buffer, ASS_Image *restrict img)
{
	for (; img; img = img->next)
	{
		if (!img->w || !img->h) continue;
		
		uint_fast8_t r = img->color >> 24,
		             g = img->color >> 16 & 255,
		             b = img->color >> 8 & 255,
		             a = 255 - (img->color & 255);
		
		for (int y = img->dst_y; y < img->dst_y + img->h; ++y)
		{
			const unsigned char *restrict map =
				img->bitmap + (y - img->dst_y) * img->stride;
			for (int x = img->dst_x; x < img->dst_x + img->w; ++x)
			{
				uint_fast32_t alpha = (unsigned) a * *map++;
				struct rgb24_pixel *restrict p =
					&buffer->samples[y * buffer->w + x];
				p->r = (p->r * (65025 - alpha) + r * alpha) / 65025.0L + dither();
				p->g = (p->g * (65025 - alpha) + g * alpha) / 65025.0L + dither();
				p->b = (p->b * (65025 - alpha) + b * alpha) / 65025.0L + dither();
			}
		}
	}
}

render_subtitles_rgbld.c

#include <stddef.h>
#include <stdint.h>

typedef struct
{
	size_t w, h;
	struct rgbld_pixel { long double r, g, b; } samples[];
} rgbld;

void render_subtitles_rgbld(rgbld *restrict buffer, ASS_Image *restrict img)
{
	for (; img; img = img->next)
	{
		if (!img->w || !img->h) continue;
		
		long double r = (img->color >> 24) / 255.0L,
		            g = (img->color >> 16 & 255) / 255.0L,
		            b = (img->color >> 8 & 255) / 255.0L;
		uint_fast8_t a = 255 - (img->color & 255);
		
		for (int y = img->dst_y; y < img->dst_y + img->h; ++y)
		{
			const unsigned char *restrict map =
				img->bitmap + (y - img->dst_y) * img->stride;
			for (int x = img->dst_x; x < img->dst_x + img->w; ++x)
			{
				unsigned alpha = (unsigned) a * *map++;
				struct rgbld_pixel *restrict p =
					&buffer->samples[y * buffer->w + x];
				p->r = (p->r * (65025 - alpha) + r * alpha) / 65025;
				p->g = (p->g * (65025 - alpha) + g * alpha) / 65025;
				p->b = (p->b * (65025 - alpha) + b * alpha) / 65025;
			}
		}
	}
}