perspective.c

#include <stdio.h>

typedef struct mat4 {
    float m[16];
} mat4;

typedef struct vec3 {
    float x;
    float y;
    float z;
} vec3;

typedef struct vec4 {
    float x;
    float y;
    float z;
    float w;
} vec4;

mat4 frustum(float l, float r, float b, float t, float n, float f) {
    return (mat4){
        2 * n / (r - l), 0, -(r + l) / (r - l), 0,
        0, 2 * n / (t - b), -(t + b) / (t - b), 0,
        0, 0, (f + n) / (f - n), -2 * f * n / (f - n),
        0, 0, 1, 0,
    };
}

vec4 mat4_mul_vec4(mat4 m, vec4 v) {
    float* M = m.m;
    return (vec4){
        v.x * M[0] + v.y * M[1] + v.z * M[2] + v.w * M[3],
        v.x * M[4] + v.y * M[5] + v.z * M[6] + v.w * M[7],
        v.x * M[8] + v.y * M[9] + v.z * M[10] + v.w * M[11],
        v.x * M[12] + v.y * M[13] + v.z * M[14] + v.w * M[15],
    };
}

vec4 transform(mat4 m, vec4 v) {
    v = mat4_mul_vec4(m, v);
    v.x /= v.w;
    v.y /= v.w;
    v.z /= v.w;
    v.w /= v.w;
    return v;
}

float l = -1;
float r = 1;
float b = -1;
float t = 1;
float n = 0.1f;
float f = 10;
mat4 proj;

void func(float x, float y, float z) {
    vec4 v = transform(proj, (vec4){x, y, z, 1});
    printf("%f %f %f %f\n", v.x, v.y, v.z, v.w);
}

int main(int argc, char** argv) {
    proj = frustum(l, r, b, t, n, f);

    func(l, b, n);
    func(r, b, n);
    func(l, t, n);
    func(r, t, n);

    func(l, b, f);
    func(r, b, f);
    func(l, t, f);
    func(r, t, f);
}