Rewrite image alpha detection for better readability

Author: jbo-ads

lib/imageio.c

@@ -62,24 +62,37 @@ mapcache_image_alpha_type mapcache_imageio_alpha_sniff(mapcache_context *ctx, ma
 {
   const unsigned char * b = buffer->buf;
   mapcache_image_format_type t = mapcache_imageio_header_sniff(ctx,buffer);
-  if (t==GC_JPEG) {
-    // A JPG file is opaque
-    return MC_ALPHA_NO;
-  } else if (t==GC_PNG && buffer->size >= 26 && (b[12]|32) == 'i' && (b[13]|32) == 'h' && (b[14]|32) == 'd' && (b[15]|32) == 'r') {
-    // Check color type of PNG file in IHDR chunk
-    if (b[25] == 0 || b[25] == 2) {
-      // Gray or RGB without alpha
-      return MC_ALPHA_NO;
-    } else if (b[25] == 4 || b[25] == 6) {
-      // Gray or RGB with alpha
-      return MC_ALPHA_YES;
-    } else {
-      // TODO: Should check palette index
-      return MC_ALPHA_UNKNOWN;
-    }
-  } else {
-    return MC_ALPHA_UNKNOWN;
+  mapcache_image_alpha_type alpha_type;
+
+  switch (t) {
+    case GC_JPEG:
+      // JPEG files are opaque
+      alpha_type = MC_ALPHA_NO;
+      break;
+    case GC_PNG:
+      if (buffer->size >= 26) {
+        // Check color type of PNG file in IHDR chunk
+        if ( (b[12]|32)=='i' && (b[13]|32)=='h' && (b[14]|32)=='d' && (b[15]|32)=='r' ) {
+          switch (b[25]) {
+            case 4:
+            case 6:
+              // Gray or RGB with alpha
+              alpha_type = MC_ALPHA_YES;
+              break;
+            default:
+              // Other colortypes have no alpha channel
+              alpha_type = MC_ALPHA_NO;
+          }
+        }
+      } else {
+        alpha_type = MC_ALPHA_UNKNOWN;
+      }
+      break;
+    default:
+      alpha_type = MC_ALPHA_UNKNOWN;
+      break;
   }
+  return alpha_type;
 }
 
 mapcache_image* mapcache_imageio_decode(mapcache_context *ctx, mapcache_buffer *buffer)