Improve the API for the public-facing noise function.

include/heman.h

@@ -59,8 +59,12 @@ heman_image* heman_color_apply_gradient(heman_image* heightmap, float minheight,
 // distance field to generate an interesting height map.
 heman_image* heman_generate_island_heightmap(int width, int height, int seed);
 
-// High-level function that computes several octaves of noise for demo purposes.
-heman_image* heman_generate_island_noise(int width, int height, int seed);
+// High-level function that sums up a number of noise octaves, also known as
+// Fractional Brownian Motion.  Taken alone, Perlin / Simplex noise are not
+// fractals; this makes them more fractal-like. A good starting point is to use
+// a lacunarity of 2.0 and a gain of 0.5, with only 2 or 3 octaves.
+heman_image* heman_generate_simplex_fbm(int width, int height, float frequency,
+    float amplitude, int octaves, float lacunarity, float gain, int seed);
 
 // Apply ambient occlusion and diffuse lighting to the given heightmap.
 heman_image* heman_lighting_apply(heman_image* heightmap,

src/generate.c

@@ -1,14 +1,15 @@
 #include "image.h"
 #include "noise.h"
 #include <math.h>
+#include <memory.h>
 
 static const float SEALEVEL = 0.5f;
 
 #define NOISE(U, V) open_simplex_noise2(ctx, U, V)
 #define MAX(a, b) (a > b ? a : b)
 #define MIN(a, b) (a > b ? b : a)
 
-heman_image* heman_generate_island_noise(int width, int height, int seed)
+static heman_image* generate_island_noise(int width, int height, int seed)
 {
     struct osn_context* ctx;
     open_simplex_noise(seed, &ctx);
@@ -42,7 +43,7 @@ heman_image* heman_generate_island_noise(int width, int height, int seed)
 
 heman_image* heman_generate_island_heightmap(int width, int height, int seed)
 {
-    heman_image* noisetex = heman_generate_island_noise(width, height, seed);
+    heman_image* noisetex = generate_island_noise(width, height, seed);
     heman_image* coastmask = heman_image_create(width, height, 1);
     float* data = coastmask->data;
     float invh = 1.0f / height;
@@ -100,3 +101,34 @@ heman_image* heman_generate_island_heightmap(int width, int height, int seed)
     heman_image_destroy(heightmap);
     return result;
 }
+
+heman_image* heman_generate_simplex_fbm(int width, int height, float frequency,
+    float amplitude, int octaves, float lacunarity, float gain, int seed)
+{
+    struct osn_context* ctx;
+    open_simplex_noise(seed, &ctx);
+    heman_image* img = heman_image_create(width, height, 1);
+    float* data = img->data;
+    float invh = 1.0f / height;
+    float invw = 1.0f / width;
+    float ampl = amplitude;
+    float freq = frequency;
+    memset(data, 0, sizeof(float) * width * height);
+
+    while (octaves--) {
+#pragma omp parallel for
+        for (int y = 0; y < height; ++y) {
+            float v = y * invh;
+            float* dst = data + y * width;
+            for (int x = 0; x < width; ++x) {
+                float u = x * invw;
+                *dst++ += ampl* NOISE(u * freq, v * freq);
+            }
+        }
+        ampl *= gain;
+        freq *= lacunarity;
+    }
+
+    open_simplex_noise_free(ctx);
+    return img;
+}

test/main.c

@@ -37,9 +37,17 @@ static void test_noise()
 {
     printf("Generating noise.\n");
     double begin = omp_get_wtime();
-    heman_image* img = heman_generate_island_noise(SIZE, SIZE, 7000);
+    float frequency = 4;
+    float amplitude = 1;
+    int octaves = 10;
+    float lacunarity = 2;
+    float gain = 0.65f;
+    int seed = 5000;
+    heman_image* img = heman_generate_simplex_fbm(
+        SIZE, SIZE, frequency, amplitude, octaves, lacunarity, gain, seed);
     double duration = omp_get_wtime() - begin;
     printf("Noise generated in %.3f seconds.\n", duration);
+    write_image(OUTFOLDER "noise.png", img);
     heman_image_destroy(img);
 }