RGB Perlin

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <SDL2/SDL.h>
#define WIN_W 800
#define WIN_H 600
#define NOISE_W 800
#define NOISE_H 600
void SDLerr(int, char*);
void updateScreen(double[][NOISE_W], double[][NOISE_W], double[][NOISE_W], SDL_Surface*);
void text(SDL_Renderer*, int, int, char*, SDL_Texture**, SDL_Rect*);
void generateNoise(double[][NOISE_W]);
double interpolate(double[][NOISE_W], double, double);
double mipmap(double[][NOISE_W], double, double, double);

int main(void) {
    if (SDL_Init(SDL_INIT_VIDEO) < 0) SDLerr(1, "Unable to initialize SDL!");
    srand(time(NULL));

    SDL_Window* window = SDL_CreateWindow("RGB Perlin", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, WIN_W, WIN_H, SDL_WINDOW_SHOWN);
    if (window == NULL) SDLerr(2, "Unable to create window!");

    SDL_Surface* screen = SDL_GetWindowSurface(window);
    if (screen == NULL) SDLerr(3, "Unable to get window surface");

    double** redNoise = malloc(NOISE_H * sizeof(double*) + NOISE_H * NOISE_W * sizeof(double));
    double** greenNoise = malloc(NOISE_H * sizeof(double*) + NOISE_H * NOISE_W * sizeof(double));
    double** blueNoise = malloc(NOISE_H * sizeof(double*) + NOISE_H * NOISE_W * sizeof(double));
    generateNoise(redNoise);
    generateNoise(greenNoise);
    generateNoise(blueNoise);
    updateScreen(blueNoise, greenNoise, redNoise, screen);

    SDL_Event event;
    do
    {
        SDL_UpdateWindowSurface(window);
        SDL_WaitEvent(&event);
    }
    while (event.type != SDL_QUIT);

    SDL_FreeSurface(screen);
    SDL_DestroyWindow(window);
    SDL_Quit();
    return 0;
}

void SDLerr(int errnum, char* errmsg) {
    fprintf(stderr, "%s SDL_Error: %s\n", errmsg, SDL_GetError());
    exit(errnum);
}

void updateScreen(double blue[][NOISE_W], double green[][NOISE_W], double red[][NOISE_W], SDL_Surface* screen) {
    unsigned char* pixels = screen -> pixels;
    int p;

    for (int x = 0; x < NOISE_W; x++)
        for (int y = 0; y < NOISE_H; y++) {
            p = (y * NOISE_W + x) * 4;
            pixels[p] = (char) (mipmap(blue, y, x, 64));
            pixels[p + 1] = (char) (mipmap(green, y, x, 64));
            pixels[p + 2] = (char) (mipmap(red, y, x, 64));
        }
}

void generateNoise(double noise[][NOISE_W]) {
    for (int x = 0; x < NOISE_W; x++)
        for (int y = 0; y < NOISE_H; y++)
            noise[y][x] = (rand() % RAND_MAX) / (double) RAND_MAX;
}

double interpolate(double noise[][NOISE_W], double x, double y) {
    int x1 = (int) x, y1 = (int) y;
    int x2 = (x1 + NOISE_W - 1) % NOISE_W;
    int y2 = (y1 + NOISE_H - 1) % NOISE_H;
    double fX = x - x1, fY = y - y1;

    double val = fX * fY * noise[y1][x1];
    val += (1 - fX) * fY * noise[y1][x2];
    val += fX * (1 - fY) * noise[y2][x1];
    return val + (1 - fX) * (1 - fY) * noise[y2][x2];
}

double mipmap(double noise[][NOISE_W], double x, double y, double size) {
    double val = 0.;
    for (double d = size; d >= 1; d /= 2.)
        val += interpolate(noise, x / d, y / d) * d;
    return 128. * val / size;
}