Triangle fill and line draw algorithms with SDL2 using SDL_Surface

trianglefill_and_stuff.cpp

// Untested code, but should work with some tweaks (more or less)

struct Point{
    double x, y;
};

struct Line{
    Point p1, p2;
};

void DrawLine(Line l, SDL_Surface* dst, Uint32 color){
    Uint32* pixels = (Uint32*)dst->pixels;
    int width = dst->w;
    int height = dst->h;
    // differences
    double dx = l.p2.x - l.p1.x;
    double dy = l.p2.y - l.p1.y;
    // slope
    double k = dy / dx;
    // based on slope value, fill pixel-by-pixel either horisontally or vertically
    if(abs(k) <= 1.0){
        if(l.p1.x > l.p2.x){
            Point temp = l.p1;
            l.p1 = l.p2;
            l.p2 = temp;
        }
        // fill horisontally
        for(int x=l.p1.x; x<l.p2.x; x++){
            int y = l.p1.y + (x - l.p1.x) * k;
            if(x >= 0 && x < width && y >= 0 && y < height)
                pixels[y*width + x] = color;
        }
    }
    else{
        if(l.p1.y > l.p2.y){
            Point temp = l.p1;
            l.p1 = l.p2;
            l.p2 = temp;
        }
        // fill vertically
        for(int y=l.p1.y; y<l.p2.y; y++){
            int x = l.p1.x + (y - l.p1.y) / k;
            if(x >= 0 && x < width && y >= 0 && y < height)
                pixels[y*width + x] = color;
        }
    }
}

void DrawHorisontalLine(int y, int x1, int x2, SDL_Surface* dst, Uint32 color){
    Uint32* pixels = (Uint32*)dst->pixels;
    if(x2 < x1)
        swap(x2, x1);
    // clamp co-ordinates to screen size
    if(x1 < 0) x1 = 0;
    if(x2 >= dst->w) x2 = dst->w - 1;
    for(int x=x1; x<x2; x++){
        pixels[y*dst->w+x] = color;
    }
}

void FillTriangle(Point p1, Point p2, Point p3, SDL_Surface* dst, Uint32 color){
    // sort points by height
    if(p3.y < p2.y) swap(p3, p2);
    if(p2.y < p1.y) swap(p2, p1);
    if(p3.y < p2.y) swap(p3, p2);
    // calculate differences
    double dx1 = p3.x - p1.x;
    double dx2 = p2.x - p1.x;
    double dx3 = p3.x - p2.x;
    double dy1 = p3.y - p1.y;
    double dy2 = p2.y - p1.y;
    double dy3 = p3.y - p2.y;
    // line slopes
    double k1 = dy1 / dx1;
    double k2 = dy2 / dx2;
    double k3 = dy3 / dx3;
    // draw line by line from top to bottom
    for(int y=p1.y; y<p3.y; y++){
        int x1 = (y - p1.y) / k1 + p1.x;
        int x2;
        // determine if lower or upper half
        if(y < p2.y){
            x2 = (y - p1.y) / k2 + p1.x;
        }else{
            x2 = (y - p2.y) / k3 + p2.x;
        }
        DrawHorisontalLine(y, x1, x2, dst, color);
    }
}