XiaolinWu.h

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#pragma once
#include <SDL2/SDL.h>
#include <algorithm>
#include <functional>
#include "Color.h"

class XiaolinWu
{
private:
    // integer part of x
    static int ipart(float x)
    {
        return (int)x;
    }
    static int round(float x)
    {
        return ipart(x + 0.5);
    }
    // fractional part of x
    static float fpart(float x)
    {
        return x - (int)(x);
    }
    static int rfpart(int x)
    {
        return 1.f - fpart(x);
    }

public:
    static void drawLine(float x0, float y0, float x1, float y1, std::function<void(int, int, float)> plot)
    {
        bool steep = std::abs(y1 - y0) > std::abs(x1 - x0);

        if (steep)
        {
            std::swap(x0, y0);
            std::swap(x1, y1);
        }
        if (x0 > x1)
        {
            std::swap(x0, x1);
            std::swap(y0, y1);
        }

        float dx = x1 - x0;
        float dy = y1 - y0;
        float gradient = dy / dx;
        if (dx == 0.0)
        {
            gradient = 1.0;
        }

        // handle first endpoint
        float xend = round(x0);
        float yend = y0 + gradient * (xend - x0);
        float xgap = rfpart(x0 + 0.5);
        float xpxl1 = xend; // this will be used in the main loop
        float ypxl1 = ipart(yend);
        if (steep)
        {
            plot(ypxl1, xpxl1, rfpart(yend) * xgap);
            plot(ypxl1 + 1, xpxl1, fpart(yend) * xgap);
        }
        else
        {
            plot(xpxl1, ypxl1, rfpart(yend) * xgap);
            plot(xpxl1, ypxl1 + 1, fpart(yend) * xgap);
        }
        float intery = yend + gradient; // first y-intersection for the main loop

        // handle second endpoint
        xend = round(x1);
        yend = y1 + gradient * (xend - x1);
        xgap = fpart(x1 + 0.5);
        float xpxl2 = xend; //this will be used in the main loop
        float ypxl2 = ipart(yend);
        if (steep)
        {
            plot(ypxl2, xpxl2, rfpart(yend) * xgap);
            plot(ypxl2 + 1, xpxl2, fpart(yend) * xgap);
        }
        else
        {
            plot(xpxl2, ypxl2, rfpart(yend) * xgap);
            plot(xpxl2, ypxl2 + 1, fpart(yend) * xgap);
        }

        // main loop
        if (steep)
        {
            for (int x = xpxl1 + 1; x < xpxl2; x++)
            {
                plot(ipart(intery), x, rfpart(intery));
                plot(ipart(intery) + 1, x, fpart(intery));
                intery = intery + gradient;
            }
        }
        else
        {
            for (int x = xpxl1 + 1; x < xpxl2; x++)
            {
                plot(x, ipart(intery), rfpart(intery));
                plot(x, ipart(intery) + 1, fpart(intery));
                intery = intery + gradient;
            }
        }
    }
};