bilinear.cpp

#include <stdint.h>
#include <math.h>

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

struct rgb {
	uint8_t r;
	uint8_t g;
	uint8_t b;
};

rgb sample(rgb* data, int w, int h, float u, float v) {
	u *= w;
	v *= h;

	if (u < 0.5f) {
		if (v < 0.5f) {
			return data[0];
		}
		else if (v > h - 0.5f) {
			return data[(h - 1) * w];
		}
		else {
			rgb a = data[(int)(v - 0.5f) * w];
			rgb b = data[(int)(v + 0.5f) * w];
			float r = a.r + (b.r - a.r) * (v - (floorf(v - 0.5f) + 0.5f));
			float g = a.g + (b.g - a.g) * (v - (floorf(v - 0.5f) + 0.5f));
			float B = a.b + (b.b - a.b) * (v - (floorf(v - 0.5f) + 0.5f));
			return { (uint8_t)r, (uint8_t)g, (uint8_t)B };
		}
	}
	else if (u > w - 0.5f) {
		if (v < 0.5f) {
			return data[w - 1];
		}
		else if (v > h - 0.5f) {
			return data[(h - 1) * w + w - 1];
		}
		else {
			rgb a = data[(int)(v - 0.5f) * w + w - 1];
			rgb b = data[(int)(v + 0.5f) * w + w - 1];
			float r = a.r + (b.r - a.r) * (v - (floorf(v - 0.5f) + 0.5f));
			float g = a.g + (b.g - a.g) * (v - (floorf(v - 0.5f) + 0.5f));
			float B = a.b + (b.b - a.b) * (v - (floorf(v - 0.5f) + 0.5f));
			return { (uint8_t)r, (uint8_t)g, (uint8_t)B };
		}
	}
	else {
		if (v < 0.5f) {
			rgb a = data[(int)(u - 0.5f)];
			rgb b = data[(int)(u + 0.5f)];
			float r = a.r + (b.r - a.r) * (u - (floorf(u - 0.5f) + 0.5f));
			float g = a.g + (b.g - a.g) * (u - (floorf(u - 0.5f) + 0.5f));
			float B = a.b + (b.b - a.b) * (u - (floorf(u - 0.5f) + 0.5f));
			return { (uint8_t)r, (uint8_t)g, (uint8_t)B };
		}
		else if (v > h - 0.5f) {
			rgb a = data[(h - 1) * w + (int)(u - 0.5f)];
			rgb b = data[(h - 1) * w + (int)(u + 0.5f)];
			float r = a.r + (b.r - a.r) * (u - (floorf(u - 0.5f) + 0.5f));
			float g = a.g + (b.g - a.g) * (u - (floorf(u - 0.5f) + 0.5f));
			float B = a.b + (b.b - a.b) * (u - (floorf(u - 0.5f) + 0.5f));
			return { (uint8_t)r, (uint8_t)g, (uint8_t)B };
		}
		else {
			rgb a = data[(int)(v - 0.5f) * w + (int)(u - 0.5f)];
			rgb b = data[(int)(v + 0.5f) * w + (int)(u - 0.5f)];
			float r1 = a.r + (b.r - a.r) * (v - (floorf(v - 0.5f) + 0.5f));
			float g1 = a.g + (b.g - a.g) * (v - (floorf(v - 0.5f) + 0.5f));
			float b1 = a.b + (b.b - a.b) * (v - (floorf(v - 0.5f) + 0.5f));

			a = data[(int)(v - 0.5f) * w + (int)(u + 0.5f)];
			b = data[(int)(v + 0.5f) * w + (int)(u + 0.5f)];
			float r2 = a.r + (b.r - a.r) * (v - (floorf(v - 0.5f) + 0.5f));
			float g2 = a.g + (b.g - a.g) * (v - (floorf(v - 0.5f) + 0.5f));
			float b2 = a.b + (b.b - a.b) * (v - (floorf(v - 0.5f) + 0.5f));

			float r = r1 + (r2 - r1) * (u - (floorf(u - 0.5f) + 0.5f));
			float g = g1 + (g2 - g1) * (u - (floorf(u - 0.5f) + 0.5f));
			float B = b1 + (b2 - b1) * (u - (floorf(u - 0.5f) + 0.5f));
			return { (uint8_t)r, (uint8_t)g, (uint8_t)B };
		}
	}
}

float remap(float x, float a1, float b1, float a2, float b2) {
	float t = (x - a1) / (b1 - a1);
	return a2 + t * (b2 - a2);
}

int main() {
	const int w = 3;
	const int h = 3;
	rgb pixels[w * h]{
		{255,0,0},{0,255,0},{0,0,255},
		{0,255,0},{0,0,255},{255,0,0},
		{0,0,255},{255,0,0},{0,255,0},
	};

	int w2 = 512;
	rgb* pixels2 = new rgb[w2 * w2];
	for (int i = 0; i < w2; i++) {
		float v = remap(i, 0, w2 - 1, 0, 1);

		for (int j = 0; j < w2; j++) {
			float u = remap(j, 0, w2 - 1, 0, 1);

			rgb c = sample(pixels, w, h, u, v);
			pixels2[i * w2 + j] = c;
		}
	}

	stbi_write_png("out1.png", w, h, 3, pixels, w * 3);
	stbi_write_png("out2.png", w2, w2, 3, pixels2, w2 * 3);

	delete[] pixels2;

	return 0;
}