AVSubtitle with indexed color conversion to AVFrame

gistfile1.c

// Colorspace conversion macro definitions from ffplay
#define RGBA_IN(r, g, b, a, s)\
{\
	unsigned int v = ((const uint32_t *)(s))[0];\
	a = (v >> 24) & 0xff;\
	r = (v >> 16) & 0xff;\
	g = (v >> 8) & 0xff;\
	b = v & 0xff;\
}

AVFrame* sub_to_frame(AVSubtitle* sub, int width, int height)
{
	if (sub->num_rects == 0)
		return nullptr;

	AVPicture rgba_picture;
	avpicture_alloc(&rgba_picture, AV_PIX_FMT_RGBA, SUB_WINDOW_WIDTH, SUB_WINDOW_HEIGHT);
	memset(rgba_picture.data[0], 0, SUB_WINDOW_WIDTH * SUB_WINDOW_HEIGHT * 4);

	// Copy subtitle before conversion
	int r, g, b, a;
	for (unsigned int i = 0; i < sub->num_rects; i ++)
	{
		for (int j = 0; j < sub->rects[i]->nb_colors; j++)
		{
			AVSubtitleRect* rect = sub->rects[i];
			RGBA_IN(r, g, b, a, (uint32_t*)rect->pict.data[1] + j);

			// Blend rectangle to frame
			for (int y = rect->y; y < rect->y + rect->h; y++)
			{
				uint8_t* pd = rgba_picture.data[0] + (y * rgba_picture.linesize[0]) + (rect->x * 4);
				uint8_t* rd = rect->pict.data[0] + ((y - rect->y) * rect->pict.linesize[0]);
				for (int k = 0; k < rect->w; k++)
				{
					RGBA_IN(r, g, b, a, (uint32_t*)rect->pict.data[1] + rd[0]);
					pd[0] = r;
					pd[1] = g;
					pd[2] = b;
					pd[3] = a;
					pd += 4;
					rd ++;
				}
			}
		}
	}

	AVFrame* sub_frame = avcodec_alloc_frame();
	avpicture_alloc((AVPicture*)sub_frame, AV_PIX_FMT_YUVA420P, width, height);
	sub_rgba_to_yuva_ctx = sws_getCachedContext(sub_rgba_to_yuva_ctx, SUB_WINDOW_WIDTH, SUB_WINDOW_HEIGHT, AV_PIX_FMT_RGBA,
						                       	   	   	   	   	   	     width, height, AV_PIX_FMT_YUVA420P,
						                       	   	   	   	   	   	     SWS_LANCZOS, NULL, NULL, NULL);
	sws_scale(sub_rgba_to_yuva_ctx, (const uint8_t* const*)rgba_picture.data, rgba_picture.linesize, 0, SUB_WINDOW_HEIGHT, sub_frame->data, sub_frame->linesize);
	sub_frame->width = width;
	sub_frame->height= height;
	sub_frame->format = AV_PIX_FMT_YUVA420P;
	avpicture_free(&rgba_picture);
	return sub_frame;
}