{{:cgfs:filled_triangles-000.png?400|}}

<file cpp>
#include <raylib.h>

#include <cassert>
#include <cmath>
#include <iterator>
#include <vector>

#include "vmath.h"

int const Cw = 600;
int const Ch = 600;

using vec2 = vmath::vec2<float>;

void PutPixel(int x, int y, Color const &color) {
    DrawPixel(x + Cw / 2, -y + Ch / 2, color);
}

template <typename Iter>
    requires std::output_iterator<Iter, float>
void Interpolate(int i0, float d0, int i1, float d1, Iter out) {
    if (i0 == i1) {
        *out = d0;
        return;
    }

    auto a = (d1 - d0) / (i1 - i0);
    auto d = d0;

    for (int i = i0; i <= i1; ++i) {
        *out++ = d;
        d += a;
    }
}

void DrawLine(vec2 const &p0, vec2 const &p1, Color const &color) {
    auto x0 = p0.x;
    auto y0 = p0.y;
    auto x1 = p1.x;
    auto y1 = p1.y;

    if (std::abs(x1 - x0) > std::abs(y1 - y0)) {
        if (x0 > x1) {
            std::swap(x0, x1);
            std::swap(y0, y1);
        }
        auto i0 = static_cast<int>(std::floor(x0));
        auto i1 = static_cast<int>(std::floor(x1));
        auto ys = std::vector<float>(i1 - i0 + 1);

        Interpolate(i0, y0, i1, y1, ys.begin());

        for (int i = i0; i <= i1; ++i) {
            PutPixel(i, ys[i - i0], color);
        }
    } else {
        if (y0 > y1) {
            std::swap(y0, y1);
            std::swap(x0, x1);
        }
        auto i0 = static_cast<int>(std::floor(y0));
        auto i1 = static_cast<int>(std::floor(y1));
        auto xs = std::vector<float>(i1 - i0 + 1);

        Interpolate(i0, x0, i1, x1, xs.begin());

        for (int i = i0; i <= i1; ++i) {
            PutPixel(xs[i - i0], i, color);
        }
    }
}

void DrawWireframeTriangle(vec2 const &p0,
                           vec2 const &p1,
                           vec2 const &p2,
                           Color const &color) {
    DrawLine(p0, p1, color);
    DrawLine(p1, p2, color);
    DrawLine(p2, p0, color);
}

void DrawFilledTriangle(vec2 const &p0,
                        vec2 const &p1,
                        vec2 const &p2,
                        Color const &color) {
    vec2 P0{p0}, P1{p1}, P2{p2};
    if (P1.y < P0.y) {
        std::swap(P1, P0);
    }
    if (P2.y < P0.y) {
        std::swap(P2, P0);
    }
    if (P2.y < P1.y) {
        std::swap(P2, P1);
    }

    auto i0 = static_cast<int>(std::floor(P0.y));
    auto i1 = static_cast<int>(std::floor(P1.y));
    auto i2 = static_cast<int>(std::floor(P2.y));
    auto x01 = std::vector<float>(i1 - i0 + 1);
    auto x12 = std::vector<float>(i2 - i1 + 1);
    auto x02 = std::vector<float>(i2 - i0 + 1);

    Interpolate(i0, P0.x, i1, P1.x, x01.begin());
    Interpolate(i1, P1.x, i2, P2.x, x12.begin());
    Interpolate(i0, P0.x, i2, P2.x, x02.begin());

    x01.pop_back();

    auto x012 = std::vector<float>(x01.size() + x12.size());
    assert(x012.size() == x02.size());

    auto corner = std::copy(x01.begin(), x01.end(), x012.begin());
    std::copy(x12.begin(), x12.end(), corner);

    auto &x_left = x012;
    auto &x_right = x02;
    auto m = x012.size() / 2;
    if (x02[m] < x012[m]) {
        std::swap(x_left, x_right);
    }

    for (int y = i0; y < i2; ++y) {
        auto x_l = static_cast<int>(std::floor(x_left[y - i0]));
        auto x_r = static_cast<int>(std::floor(x_right[y - i0]));
        for (int x = x_l; x <= x_r; ++x) {
            PutPixel(x, y, color);
        }
    }
}

int main() {
    InitWindow(Cw, Ch, "Filled Triangles");

    vec2 p0{-100, 200};
    vec2 p1{100, 100};
    vec2 p2{-200, -200};

    while (!WindowShouldClose()) {
        BeginDrawing();

        ClearBackground(RAYWHITE);

        PutPixel(p0.x, p0.y, RED);
        PutPixel(p1.x, p1.y, RED);
        PutPixel(p2.x, p2.y, RED);

        DrawFilledTriangle(p0, p1, p2, BLUE);
        DrawWireframeTriangle(p0, p1, p2, PURPLE);

        EndDrawing();
    }
}
</file>