feat: improve distance field generator perfs and quality using 1d squared euclidian distance

src/distance_field_generator.c

@@ -1,5 +1,6 @@
 #include <raylib.h>
 #include <stdlib.h>
+#include <string.h>
 #include <math.h>
 #include <omp.h>
 
@@ -7,104 +8,164 @@ static bool is_inside_shape(Color pixel) {
     return pixel.r == 255 && pixel.g == 255 && pixel.b == 255;
 }
 
-static Image generate_distance_field(Image input_texture, int search_radius, int offset_y) {
+// 1D squared Euclidean distance transform (felzenszwalb & huttenlocher)
+static void edt_1d(float *f, int n, int *v, float *z, float *tmp) {
+    int k = 0;
+    v[0] = 0;
+    z[0] = -1e30f;
+    z[1] =  1e30f;
+
+    for (int q = 1; q < n; q += 1) {
+        float fq = f[q];
+        float s = ((fq + (float)(q * q)) - (f[v[k]] + (float)(v[k] * v[k]))) / (float)(2 * (q - v[k]));
+        while (s <= z[k]) {
+            k -= 1;
+            s = ((fq + (float)(q * q)) - (f[v[k]] + (float)(v[k] * v[k]))) / (float)(2 * (q - v[k]));
+        }
+        k += 1;
+        v[k]     = q;
+        z[k]     = s;
+        z[k + 1] = 1e30f;
+    }
+
+    int j = 0;
+    for (int q = 0; q < n; q += 1) {
+        while (z[j + 1] < (float)q) j += 1;
+        float d = (float)(q - v[j]);
+        tmp[q] = d * d + f[v[j]];
+    }
+    memcpy(f, tmp, n * sizeof(float));
+}
+
+static float *compute_edt(bool *seeds, int width, int height) {
+    float large = (float)((width + height) * (width + height));
+    float *grid = malloc(width * height * sizeof(float));
+
+    for (int i = 0; i < width * height; i += 1) {
+        grid[i] = seeds[i] ? 0.0f : large;
+    }
+
+    int maxdim = width > height ? width : height;
+
+    #pragma omp parallel
+    {
+        int   *v   = malloc(maxdim * sizeof(int));
+        float *z   = malloc((maxdim + 1) * sizeof(float));
+        float *tmp = malloc(maxdim * sizeof(float));
+        float *col = malloc(maxdim * sizeof(float));
+
+        // 1D EDT along each column
+        #pragma omp for nowait
+        for (int x = 0; x < width; x += 1) {
+            for (int y = 0; y < height; y += 1) col[y] = grid[y * width + x];
+            edt_1d(col, height, v, z, tmp);
+            for (int y = 0; y < height; y += 1) grid[y * width + x] = col[y];
+        }
+
+        #pragma omp barrier
+
+        // 1D EDT along each row
+        #pragma omp for nowait
+        for (int y = 0; y < height; y += 1) {
+            memcpy(col, &grid[y * width], width * sizeof(float));
+            edt_1d(col, width, v, z, tmp);
+            memcpy(&grid[y * width], col, width * sizeof(float));
+        }
+
+        free(v);
+        free(z);
+        free(tmp);
+        free(col);
+    }
+
+    return grid;
+}
+
+static Image generate_distance_field(Image input_texture, int offset_y) {
     int width  = input_texture.width;
     int height = input_texture.height;
 
-    Image distance_field = GenImageColor(width, height, (Color){128, 128, 128, 255});
     Color *input_pixels = LoadImageColors(input_texture);
-    Color *output_pixels = LoadImageColors(distance_field);
+    bool  *inside_mask  = calloc(width * height, sizeof(bool));
 
-    TraceLog(LOG_INFO, "Using %d threads for parallel processing", omp_get_max_threads());
-
-#pragma omp parallel for schedule(dynamic, 64)
     for (int y = 0; y < height; y += 1) {
-        if (omp_get_thread_num() == 0 && y % (height / 10) == 0) {
-            TraceLog(LOG_INFO, "Progress: %d%%", (y * 100) / height);
-        }
-
-        for (int x = 0; x < width; x += 1) {
-            int index = y * width + x;
         int src_y = y + offset_y;
-            bool is_inside = (src_y >= 0 && src_y < height)
-                ? is_inside_shape(input_pixels[src_y * width + x])
-                : false;
-
-            float min_distance_sq = search_radius * search_radius;
-            bool found_boundary = false;
-
-            for (int dy = -search_radius; dy <= search_radius; dy += 1) {
-                for (int dx = -search_radius; dx <= search_radius; dx += 1) {
-                    int sample_x = x + dx;
-                    int sample_y = y + dy + offset_y;
-
-                    if (sample_x < 0 || sample_x >= width || sample_y < 0 || sample_y >= height) continue;
-
-                    int sample_index = sample_y * width + sample_x;
-                    bool sample_is_inside = is_inside_shape(input_pixels[sample_index]);
-
-                    if (sample_is_inside != is_inside) {
-                        float distance_sq = dx * dx + dy * dy;
-                        if (distance_sq < min_distance_sq) {
-                            min_distance_sq = distance_sq;
-                            found_boundary = true;
+        if (src_y < 0 || src_y >= height) continue;
+        for (int x = 0; x < width; x += 1) {
+            inside_mask[y * width + x] = is_inside_shape(input_pixels[src_y * width + x]);
         }
     }
-                }
-            }
-
-            float distance = found_boundary ? sqrtf(min_distance_sq) : (float)search_radius;
-            float normalized_distance = distance / (float)search_radius;
-
-            unsigned char distance_value = is_inside ?
-                (unsigned char)(127.0f - 127.0f * normalized_distance) :
-                (unsigned char)(128.0f + 127.0f * normalized_distance);
-
-            output_pixels[index] = (Color){distance_value, distance_value, distance_value, 255};
-        }
-    }
-
-    for (int i = 0; i < width * height; i += 1) {
-        ImageDrawPixel(&distance_field, i % width, i / width, output_pixels[i]);
-    }
-
     UnloadImageColors(input_pixels);
-    UnloadImageColors(output_pixels);
 
-    return distance_field;
+    bool *outside_mask = malloc(width * height * sizeof(bool));
+    for (int i = 0; i < width * height; i += 1) outside_mask[i] = !inside_mask[i];
+
+    TraceLog(LOG_INFO, "Computing inside EDT...");
+    float *dist_to_outside = compute_edt(outside_mask, width, height);
+
+    TraceLog(LOG_INFO, "Computing outside EDT...");
+    float *dist_to_inside = compute_edt(inside_mask, width, height);
+
+    free(outside_mask);
+
+    // pixel_value = clamp(sdf, -128, 127) + 128
+    Color *out_pixels = malloc(width * height * sizeof(Color));
+    for (int i = 0; i < width * height; i += 1) {
+        float sdf = inside_mask[i]
+            ? -sqrtf(dist_to_outside[i])  // negative = inside
+            : +sqrtf(dist_to_inside[i]);  // positive = outside
+
+        float clamped = fmaxf(-128.0f, fminf(127.0f, sdf));
+        unsigned char v = (unsigned char)(clamped + 128.0f);
+        out_pixels[i] = (Color){v, v, v, 255};
     }
 
-int main(int argc, char *argv[])
-{
+    free(inside_mask);
+    free(dist_to_inside);
+    free(dist_to_outside);
+
+    Image out = {
+        .data    = out_pixels,
+        .width   = width,
+        .height  = height,
+        .format  = PIXELFORMAT_UNCOMPRESSED_R8G8B8A8,
+        .mipmaps = 1,
+    };
+    return out;
+}
+
+int main(int argc, char *argv[]) {
     if (argc < 3) {
-        TraceLog(LOG_ERROR, "Usage: %s <input_image> <output_image> [search_radius] [ball_radius]", argv[0]);
+        TraceLog(LOG_ERROR, "Usage: %s <input_image> <output_image> [ball_radius]", argv[0]);
         return 1;
     }
 
     const char *input_path  = argv[1];
     const char *output_path = argv[2];
-    int search_radius = (argc >= 4) ? atoi(argv[3]) : 100;
-    int ball_radius   = (argc >= 5) ? atoi(argv[4]) : 0;
-    int offset_y      = ball_radius / 2;
+    int ball_radius = (argc >= 4) ? atoi(argv[3]) : 0;
+    int offset_y    = (ball_radius + 10) / 2;
 
     Image input_image = LoadImage(input_path);
     if (input_image.data == NULL) {
-        TraceLog(LOG_ERROR, "Error: Failed to load input image '%s'", input_path);
+        TraceLog(LOG_ERROR, "Failed to load input image '%s'", input_path);
         return 1;
     }
 
-    Image distance_field_image = generate_distance_field(input_image, search_radius, offset_y);
-    if (!ExportImage(distance_field_image, output_path)) {
+    TraceLog(LOG_INFO, "Generating distance field (%dx%d, offset_y=%d)...", input_image.width, input_image.height, offset_y);
+
+    Image distance_field = generate_distance_field(input_image, offset_y);
+
+    if (!ExportImage(distance_field, output_path)) {
         TraceLog(LOG_ERROR, "Failed to save output image '%s'", output_path);
         UnloadImage(input_image);
-        UnloadImage(distance_field_image);
+        UnloadImage(distance_field);
         return 1;
     }
 
     TraceLog(LOG_INFO, "Distance field saved to '%s'", output_path);
 
     UnloadImage(input_image);
-    UnloadImage(distance_field_image);
+    UnloadImage(distance_field);
 
     return 0;
 }