Output: validate image format before rendering

src/main.c

@@ -101,6 +101,21 @@ static int parse_moffat_beta(const char *text, double *value) {
   return errno || *end || *value <= 1.0 ? -1 : 0;
 }
 
+static int validate_tonemapped_output_path(const char *path) {
+  const size_t path_length = strlen(path);
+#ifdef ENABLE_PNG
+  if (path_length >= 4 && strcmp(path + path_length - 4, ".png") == 0)
+    return 0;
+  fputs("PNG output is enabled; use a .png output path.\n", stderr);
+#else
+  if (path_length >= 4 && strcmp(path + path_length - 4, ".ppm") == 0)
+    return 0;
+  fputs("PNG output is unavailable; use a .ppm output path or rebuild with libpng.\n",
+        stderr);
+#endif
+  return -1;
+}
+
 static int parse_args(int argc, char **argv, Settings *s,
                       const char **write_path) {
 #ifdef ENABLE_PNG
@@ -505,6 +520,9 @@ int main(int argc, char **argv) {
     return 2;
   }
 #endif
+  if (settings.frames_dir == NULL &&
+      validate_tonemapped_output_path(settings.output_path))
+    return 2;
   StarCatalog catalog = {0};
   if (settings.all_sky_catalog_path != NULL) {
     if (catalog_load_all_sky(&catalog, settings.all_sky_catalog_path)) {