Minimal Triangle Rasterizer

#define WIN32_LEAN_AND_MEAN
#define WIN32_EXTRA_LEAN
#include <windows.h>
#include <stdio.h>
#include "rasterizer.h"

extern HDC hdcMemory;
extern int g_nWindowWidth;
extern int g_nWindowHeight;

void sort(tri* t, int* order) {
  // Find the lowest y
	if (t->a.y < t->b.y) {
		if (t->a.y < t->c.y) {
			order[0] = 0;
		} else {
			order[0] = 2;
		}
	} else {
		if (t->b.y < t->c.y) {
			order[0] = 1;
		} else {
			order[0] = 2;
		}
	}

	// Find the highest y
	if (t->a.y > t->b.y) {
		if (t->a.y > t->c.y) {
			order[2] = 0;
		} else {
			order[2] = 2;
		}
	} else {
		if (t->b.y > t->c.y) {
			order[2] = 1;
		} else {
			order[2] = 2;
		}
	}

	order[1] = 3 - (order[0] + order[2]);
}

void draw_triangle(vert v1, vert v2, vert v3) {
	tri t = {v1, v2, v3};
	draw_triangle(&t);
}

void draw_triangle(float x1, float y1, float x2, float y2, float x3, float y3) {
	tri t = { x1, y1, x2, y2, x3, y3 };
	draw_triangle(&t);
}

void draw_triangle(ptri t) { // The triangle should be ordered by the time it gets here
#define x1 t->vtx[0].x
#define x2 t->vtx[1].x
#define x3 t->vtx[2].x
#define y1 t->vtx[0].y
#define y2 t->vtx[1].y
#define y3 t->vtx[2].y

	float slope_left, slope_right;
	float left, right;
	float height = y3 - y1;

	/*char buffer[512];
	memset(buffer, 0, 512);
	sprintf(buffer, "Second\n{%d, %d}\n{%d, %d}\n%{%d, %d}",
		(int)x1, (int)y1, (int)x2, (int)y2, (int)x3, (int)y3);
	MessageBox(0, buffer, "Debug", MB_OK);*/

	if (height == 0)
		return;

	slope_left = (x2 - x1) / height;
	slope_right = (x3 - x1) / height;

	left = right = x1;

	if (y1 < y3) {
		for (int y = y1; y < y3; ++y) {
			draw_line(left, right, y);
			left += slope_left;
			right += slope_right;
		}
	} else {
		for (int y = y1; y >= y3; --y) {
			draw_line(left, right, y);
			left -= slope_left;
			right -= slope_right;
		}
	}

#undef x1
#undef x2
#undef x3
#undef y1
#undef y2
#undef y3
}

void draw_line(float x1, float x2, float y) {
	if (x1 < 0) x1 = 0;
	if (x1 > g_nWindowWidth) x1 = g_nWindowWidth;

	if (x2 < 0) x1 = 0;
	if (x2 > g_nWindowWidth) x2 = g_nWindowWidth;

	if (y < 0) y = 0;
	if (y > g_nWindowHeight) y = g_nWindowHeight;

	POINT pntArray[2] = { x1, y, x2, y };
	Polyline(hdcMemory, pntArray, 2);
}

void triangle(vert v1, vert v2, vert v3) {
	tri t = {v1, v2, v3};
	triangle(&t);
}

void triangle(float x1, float y1, float x2, float y2, float x3, float y3) {
	tri t = { x1, y1, x2, y2, x3, y3 };
	triangle(&t);
}

void triangle(ptri t) {
	int order[3];
	sort(t, order);

#define x1 t->vtx[order[0]].x
#define x2 t->vtx[order[1]].x
#define x3 t->vtx[order[2]].x
#define y1 t->vtx[order[0]].y
#define y2 t->vtx[order[1]].y
#define y3 t->vtx[order[2]].y

	// If this triangle has no horizontal edges, split it into two triangles
	if (y2 != y1 && y3 != y1 && y3 != y2) {
		float new_x = (x3 - x1) * ((y2 - y1) / (y3 - y1)) + x1;
		if (new_x < x2) {
			draw_triangle(x1, y1, new_x, y2, x2, y2);
			draw_triangle(x3, y3, x2, y2, new_x, y2);
		} else {
			/*char buffer[512];
			memset(buffer, 0, 512);
			sprintf(buffer, "Top\n{%d, %d}\n{%d, %d}\n%{%d, %d}",
				(int)x1, (int)y1, (int)x2, (int)y2, (int)new_x, (int)y2);
			MessageBox(0, buffer, "Triangle debug", MB_OK);*/
			draw_triangle(x1, y1, x2, y2, new_x, y2);

			/*memset(buffer, 0, 512);
			sprintf(buffer, "Bottom\n{%d, %d}\n{%d, %d}\n%{%d, %d}",
				(int)x3, (int)y3, (int)x2, (int)y2, (int)new_x, (int)y2);
			MessageBox(0, buffer, "Triangle debug", MB_OK);*/
			draw_triangle(x3, y3, x2, y2, new_x, y2);
		}
	}
	// If the top is flat
	else if (y1 == y2) {
		if (x1 < x2)
			draw_triangle(x3, y3, x2, y2, x1, y2);
		else
			draw_triangle(x3, y3, x1, y2, x2, y2);
	}
	// If the bottom is flat
	else if (y2 == y3) {
		if (x2 < x3)
			draw_triangle(x1, y1, x2, y2, x3, y2);
		else
			draw_triangle(x1, y1, x3, y2, x2, y2);
	}

#undef x1
#undef x2
#undef x3
#undef y1
#undef y2
#undef y3
}