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


<file cpp>
#include <raylib.h>

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

#include "vmath.h"

int const Cw = 600;
int const Ch = 600;
float const Vw = 1.0;
float const Vh = 1.0;
float const ProjectionPlaneZ = 1.0;

using vec2 = vmath::vec2<float>;
using vec3 = vmath::vec3<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);
        }
    }
}

vec2 ViewportToCanvas(float x, float y) {
    return {x * Cw / Vw, y * Ch / Vh};
}

vec2 ProjectVertex(vec3 const &v) {
    return ViewportToCanvas(v.x * ProjectionPlaneZ / v.z,
                            v.y * ProjectionPlaneZ / v.z);
}

int main() {
    InitWindow(Cw, Ch, "Perspective Projection");

    // front
    auto vAf = vec3{-1, 1, 4};
    auto vBf = vec3{1, 1, 4};
    auto vCf = vec3{1, -1, 4};
    auto vDf = vec3{-1, -1, 4};

    // back
    auto vAb = vec3{-1, 1, 6};
    auto vBb = vec3{1, 1, 6};
    auto vCb = vec3{1, -1, 6};
    auto vDb = vec3{-1, -1, 6};

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

        ClearBackground(WHITE);

        DrawLine(ProjectVertex(vAf), ProjectVertex(vBf), BLUE);
        DrawLine(ProjectVertex(vBf), ProjectVertex(vCf), BLUE);
        DrawLine(ProjectVertex(vCf), ProjectVertex(vDf), BLUE);
        DrawLine(ProjectVertex(vDf), ProjectVertex(vAf), BLUE);

        DrawLine(ProjectVertex(vAb), ProjectVertex(vBb), RED);
        DrawLine(ProjectVertex(vBb), ProjectVertex(vCb), RED);
        DrawLine(ProjectVertex(vCb), ProjectVertex(vDb), RED);
        DrawLine(ProjectVertex(vDb), ProjectVertex(vAb), RED);

        DrawLine(ProjectVertex(vAf), ProjectVertex(vAb), GREEN);
        DrawLine(ProjectVertex(vBf), ProjectVertex(vBb), GREEN);
        DrawLine(ProjectVertex(vCf), ProjectVertex(vCb), GREEN);
        DrawLine(ProjectVertex(vDf), ProjectVertex(vDb), GREEN);

        EndDrawing();
    }
}
</file>