gistfile1.txt

#define _CRT_SECURE_NO_WARNINGS
#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"
#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

int tp[10000];
int tps = 0;

void fillcell(int* img, int y, int x, int w, int fill)
{
	tp[tps] = y * w + x; tps++;
	img[y * w + x] = fill;
	if (x > 0 && img[y * w + x - 1] == 0xffffffff) fillcell(img, y, x - 1, w, fill);
	if (y > 0 && img[y * w + x - w] == 0xffffffff) fillcell(img, y - 1, x, w, fill);
	if (x < w-1 && img[y * w + x + 1] == 0xffffffff) fillcell(img, y, x + 1, w, fill);
	if (y < w-1 && img[y * w + x + w] == 0xffffffff) fillcell(img, y + 1, x, w, fill);
}

void avgcell(int * img, int * tex)
{
	int r = 0, g = 0, b = 0;
	for (int i = 0; i < tps; i++)
	{
		int c = tex[tp[i]];
		r += (c >> 0) & 0xff;
		g += (c >> 8) & 0xff;
		b += (c >> 16) & 0xff;
	}
	int c =	((r / tps) <<  0) |
			((g / tps) <<  8) |
			((b / tps) << 16) |
			0xff000000;

	for (int i = 0; i < tps; i++)
		img[tp[i]] = c;
}

int main()
{
	int x, y, c;
	int* img = (int*)stbi_load("tilefile.png", &x, &y, &c, 4);
	int* tex = (int*)stbi_load("tex.png", &x, &y, &c, 4);

	for (int i = 0; i < y; i++)
		for (int j = 0; j < x; j++)
			if (img[i * x + j] == 0xffffffff)
			{
				int fill = tex[i * x + j];
				if (fill == 0xffffffff) fill = 0xfffffffe;
				tps = 0;
				fillcell(img, i, j, x, fill);
				avgcell(img, tex);
			}
	stbi_write_png("out.png", x, y, 4, img, x * 4);
}