Image Stroke Algorithm

stroke.c

#include <stdint.h>

void stroke(void *srcBmp, void *dstBmp, size_t width, size_t height)
{
	for (size_t x = 0; x < width; x++)
	{
		for (size_t y = 0; y < height; y++)
		{
			uint32_t *src = (uint32_t *)srcBmp + y * width;
			uint32_t currPixel = src[x];

			float a, b;
			if (x < 0 || x >= width || y < 0 || y >= height)
				a = 1.0f;
			else
			{
				uint8_t alpha = ((uint8_t *)&currPixel)[3];
				a = (255.0f - (float)alpha) / 255.0f;
			}

			float c = 1.0f - a;
			for (size_t x1 = x - 1; x1 < x + 2; x1++)
			{
				for (size_t y1 = y - 1; y1 < y + 2; y1++)
				{
					if (x1 != x || y1 != y)
					{
						if (x1 < 0 || x1 >= width || y1 < 0 || y1 >= height)
							b = 1.0f;
						else
						{
							src = (uint32_t *)srcBmp + y1 * width;
							currPixel = src[x1];
							uint8_t alpha = ((uint8_t *)&currPixel)[3];
							b = (255.0f - (float)alpha) / 255.0f;
						}
						a *= b;
					}
				}
			}
			if (a != 1.0f)
				c = c / (1.0f - a);

			uint8_t gray = (uint8_t)floorf((c * 255.0f) + 0.5f);
			uint8_t alpha = (uint8_t)floorf(((1.0f - a) * 255.0f) + 0.5f);
			uint8_t color[4];
			color[0] = color[1] = color[2] = gray;
			color[3] = alpha;
			uint32_t *dst = (uint32_t *)dstBmp + y * width;
			dst[x] = *(uint32_t *)color;
		}
	}
}