Output: default to PNG with PPM fallback

src/main.c

@@ -96,6 +96,11 @@ static int parse_moffat_beta(const char *text, double *value) {
 
 static int parse_args(int argc, char **argv, Settings *s,
                       const char **write_path) {
+#ifdef ENABLE_PNG
+  const char *default_output_path = "output/imgs/minkowski_sky.png";
+#else
+  const char *default_output_path = "output/imgs/minkowski_sky.ppm";
+#endif
   *s = (Settings){.width = 1280,
                   .height = 720,
                   .coarse_cell_pixels = 16,
@@ -106,7 +111,7 @@ static int parse_args(int argc, char **argv, Settings *s,
                   .observer_radius = 30.0,
                   .psf = {2.7, 4.5},
                   .catalog_path = "assets/sky_grid_5deg.csv",
-                  .output_path = "output/imgs/minkowski_sky.ppm",
+                  .output_path = default_output_path,
                   .frames_prefix = "frame",
                   .movie_duration = 2.0,
                   .movie_fps = 30.0,
@@ -246,13 +251,18 @@ static int render_frame(const Settings *s, StarCatalog *catalog,
 
 static int frame_output_path(char path[PATH_MAX], const Settings *s,
                              size_t frame_id) {
+#ifdef ENABLE_PNG
+  const char *extension = "png";
+#else
+  const char *extension = "ppm";
+#endif
   struct stat st;
   if (s->frames_dir == NULL || s->frames_prefix == NULL ||
       strchr(s->frames_prefix, '/') != NULL || stat(s->frames_dir, &st) ||
       !S_ISDIR(st.st_mode))
     return -1;
-  const int written = snprintf(path, PATH_MAX, "%s/%s_%06zu.png", s->frames_dir,
-                               s->frames_prefix, frame_id);
+  const int written = snprintf(path, PATH_MAX, "%s/%s_%06zu.%s", s->frames_dir,
+                               s->frames_prefix, frame_id, extension);
   return written < 0 || written >= PATH_MAX ? -1 : 0;
 }
 

src/optics.c

@@ -120,6 +120,7 @@ static unsigned char tonemap_channel(double hdr_value)
     return (unsigned char)lround(255.0 * clamp(display, 0.0, 1.0));
 }
 
+#ifndef ENABLE_PNG
 static int write_tonemapped_ppm(const char *path, const double *hdr, int width,
                                 int height)
 {
@@ -132,6 +133,7 @@ static int write_tonemapped_ppm(const char *path, const double *hdr, int width,
     }
     return fclose(file) == 0 ? 0 : -1;
 }
+#endif
 
 #ifdef ENABLE_PNG
 static int write_tonemapped_png(const char *path, const double *hdr, int width,
@@ -171,13 +173,16 @@ done:
 int write_tonemapped_image(const char *path, const double *hdr, int width, int height)
 {
     const size_t path_length = strlen(path);
-    if (path_length >= 4 && strcmp(path + path_length - 4, ".png") == 0) {
 #ifdef ENABLE_PNG
+    if (path_length >= 4 && strcmp(path + path_length - 4, ".png") == 0)
         return write_tonemapped_png(path, hdr, width, height);
+    fputs("PNG output is enabled; use a .png output path.\n", stderr);
+    return -1;
 #else
-        fputs("PNG output is disabled; rebuild with make ENABLE_PNG=1.\n", stderr);
-        return -1;
+    if (path_length >= 4 && strcmp(path + path_length - 4, ".ppm") == 0)
+        return write_tonemapped_ppm(path, hdr, width, height);
+    fputs("PNG output is unavailable; use a .ppm output path or rebuild with libpng.\n",
+          stderr);
+    return -1;
 #endif
-    }
-    return write_tonemapped_ppm(path, hdr, width, height);
 }

src/optics.h

@@ -13,7 +13,7 @@ LinearRgb blackbody_to_linear_rgb(double temperature_K);
 void splat_moffat(double *hdr, int width, int height, double x, double y,
                   LinearRgb color, double flux,
                   const PointSpreadFunction *psf);
-/* Writes PPM by default; a .png path requires a build with ENABLE_PNG=1. */
+/* Writes PNG when built with libpng; non-libpng builds use PPM fallback. */
 int write_tonemapped_image(const char *path, const double *hdr, int width,
                            int height);