BOOK=" // https://stackoverflow.com/questions/28330642/3d-perspective-projection-coordinates-in-clip-space  #include <stdio.h> #include <stdlib.h> #include <math.h>  #define DEG2RAD 0.01745329f  void m3d_matrixMul(const float matrix1[], const float matrix2[], float destination[]) {     int i, row, col;      for (i = 15; i >= 0; i--)     {         row = i % 4;         col = i / 4 * 4;         destination[i] = matrix1[row] * matrix2[col] + matrix1[row + 4] * matrix2[col + 1] + matrix1[row + 8] * matrix2[col + 2] + matrix1[row + 12] * matrix2[col + 3];     } }  void m3d_matVecMul(const float matrix[], const float vector[], float destination[]) {     float x = vector[0], y = vector[1], z = vector[2], w = vector[3];      destination[0] = x * matrix[0] + y * matrix[4] + z * matrix[8] + w * matrix[12];     destination[1] = x * matrix[1] + y * matrix[5] + z * matrix[9] + w * matrix[13];     destination[2] = x * matrix[2] + y * matrix[6] + z * matrix[10] + w * matrix[14];     destination[3] = x * matrix[3] + y * matrix[7] + z * matrix[11] + w * matrix[15]; }  void m3d_getFrustum(float left, float right, float bottom, float top, float near, float far, float matrix[]) {     matrix[0] = 2.0f * near / (right - left);     matrix[1] = 0.0f;     matrix[2] = 0.0f;     matrix[3] = 0.0f;     matrix[4] = 0.0f;     matrix[5] = 2.0f * near / (top - bottom);     matrix[6] = 0.0f;     matrix[7] = 0.0f;     matrix[8] = (right + left) / (right - left);     matrix[9] = (top + bottom) / (top - bottom);     matrix[10] = -(far + near) / (far - near);     matrix[11] = -1.0f;     matrix[12] = 0.0f;     matrix[13] = 0.0f;     matrix[14] = -2.0f * far * near / (far - near);     matrix[15] = 0.0f; }  void m3d_getPerspective(float vfov, float aspect, float near, float far, float matrix[]) {     float height = tanf(vfov * 0.5f * DEG2RAD) * near;     float width = aspect * height;     m3d_getFrustum(-width, width, -height, height, near, far, matrix); }  void m3d_getIdentity(float matrix[]) {     matrix[0] = 1.0f;     matrix[1] = 0.0f;     matrix[2] = 0.0f;     matrix[3] = 0.0f;     matrix[4] = 0.0f;     matrix[5] = 1.0f;     matrix[6] = 0.0f;     matrix[7] = 0.0f;     matrix[8] = 0.0f;     matrix[9] = 0.0f;     matrix[10] = 1.0f;     matrix[11] = 0.0f;     matrix[12] = 0.0f;     matrix[13] = 0.0f;     matrix[14] = 0.0f;     matrix[15] = 1.0f; }  void m3d_translate(float x, float y, float z, float matrix[]) {     matrix[12] += x;     matrix[13] += y;     matrix[14] += z; }  int main(void) {     float vertex[4] = {0.5f, 0.5f, 0.5f, 1.0f};     float modelView[16];     float projection[16];     float mvp[16];     char buffer[4];      m3d_getIdentity(modelView);     m3d_translate(0.0f, 0.0f, -2.0f, modelView);     m3d_getPerspective(45.0f, 800.0f / 600.0f, -0.5f, -1000.0f, projection);     m3d_matrixMul(projection, modelView, mvp);     m3d_matVecMul(mvp, vertex, vertex);      printf( Projected vertex: %f, %f, %f, %f\n , vertex[0], vertex[1], vertex[2], vertex[3]);     printf( Press ENTER to quit. );     fgets(buffer, 4, stdin);     return EXIT_SUCCESS; } "