3D Engine book excerpt

// Building a 3d game engine in c++ by brian hook


// Constructor
void Triangle(vertex a, vertex b, vertex c, color) {
    // Local cache variables
    int x1 = a.x;
    int x2 = b.x;
    int x3 = c.x;
    int y1 = a.y;
    int y2 = b.y;
    int y3 = c.y;
    int tmp;
    
    // Sort from top to bottom
    if (y3 < y1) {
        tmp = y3;
        y3 = y1;
        y1 = tmp;
        tmp = x3;
        x3 =  x1;
        x1 = tmp;
    }
    
    if (y2 < y1) {
        tmp = y2;
        y2 = y1;
        y1 = tmp;
        tmp = x2;
        x2 = x1;
        x1 = tmp;
    }
    
    if (y3 < y2) {
        tmp = y3;
        y3 = y2;
        y2 = tmp;
        tmp = x3;
        x3 = x2;
        x2 = tmp;
    }
    
    // If the triangle has no horizontal edges split it into two triangles
    if (y2 != y1 && y3 != y1 && y3 != y2) {
        int xn;
        // find new x value
        xn = (x3 - x1) * ((y2 - y1) / (y3 - y1)) + x1;
        
        // Draw sub triangles
        if (xn < x2) {
            DrawTriangle(x1, y1, xn, y2, x2, y2, color);
            DrawTriangle(x3, y3, x2, y2, xn, y2, color);
        } else {
            DrawTriangle(x1, y1, x2, y2, xn, y2, color);
            DrawTriangle(x3, y3, y2, y2, xn, y2, color); 
        }
    }
    // Top is flat
    else if (y1 == y2) {
        if (x1 < x2)
            DrawTriangle(x3, y3, x2, y2, x1, y2, color);
        else
            DrawTriangle(x3, y3, x1, y2, x2, y2, color);
    }
    // Bottom is flat
    else if (y2 == y3)  {
        if (x2 < x3)
            DrawTriangle(x1, y1, x2, y2, x3, y2, color);
        else
            DrawTriangle(x1, y1, x3, y2, x2, y2, color);
    }
}

// Actually rasterise the triangle
void DrawTriangle(int x1, y1, x2, y2, x3, y3, color) {
    float dx_left, dx_right;
    float curx_left, curx_right;
    float height = y(2 or 3) - y1;
    
    // make sure height is not 0
    if (height == 0)
        return;
    
    // determine the slope for the left and right sides
    dx_left = (x2 - x1) / height;
    dx_right = (x3 - x1) / height;
    
    curx_left = curx_right = x1;
    
    if (y1 < y(2 or 3)) {
        for (int y = y1; y < y(2 or 3); ++y) {
            DrawLine(curx_left, curx_right, y, color);
            // adjust x coords
            curx_left += dx_left;
            curx_right += dx_right;
        }
    } else {
        for (int y = y1; y >= y(2 r 3); --y) {
            DrawLine(curx_left, curx_right, y, color);
            curx_left -= dx_left;
            curx_right -= dx_right;
        }
    }
}