Take PNG opacity into account when merging tiles

Author: jbo-ads

include/mapcache.h

@@ -1534,6 +1534,11 @@ void _mapcache_imageio_jpeg_decode_to_image(mapcache_context *ctx, mapcache_buff
  */
 mapcache_image_format_type mapcache_imageio_header_sniff(mapcache_context *ctx, mapcache_buffer *buffer);
 
+/**
+ * \brief lookup the first few bytes of a buffer to check for alpha channel
+ */
+mapcache_image_alpha_type mapcache_imageio_alpha_sniff(mapcache_context *ctx, mapcache_buffer *buffer);
+
 /**
  * \brief checks if the given buffer is a recognized image format
  */

lib/imageio.c

@@ -58,6 +58,30 @@ mapcache_image_format_type mapcache_imageio_header_sniff(mapcache_context *ctx,
   }
 }
 
+mapcache_image_alpha_type mapcache_imageio_alpha_sniff(mapcache_context *ctx, mapcache_buffer *buffer)
+{
+  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* mapcache_imageio_decode(mapcache_context *ctx, mapcache_buffer *buffer)
 {
   mapcache_image_format_type type = mapcache_imageio_header_sniff(ctx,buffer);

lib/imageio_jpeg.c

@@ -243,6 +243,7 @@ void _mapcache_imageio_jpeg_decode_to_image(mapcache_context *r, mapcache_buffer
     return;
   }
 
+  img->has_alpha = MC_ALPHA_NO;
   jpeg_read_header(&cinfo, TRUE);
   jpeg_start_decompress(&cinfo);
   img->w = cinfo.output_width;

lib/imageio_png.c

@@ -408,6 +408,7 @@ void _mapcache_imageio_png_decode_to_image(mapcache_context *ctx, mapcache_buffe
     unsigned char pixel[4];
     uint8_t  alpha;
     unsigned char *pixptr = row_pointers[i];
+    img->has_alpha = MC_ALPHA_NO;
     for(j=0; j<img->w; j++) {
 
       memcpy (pixel, pixptr, sizeof (uint32_t));
@@ -417,10 +418,12 @@ void _mapcache_imageio_png_decode_to_image(mapcache_context *ctx, mapcache_buffe
         pixptr[1] = pixel[1];
         pixptr[2] = pixel[0];
       } else if (alpha == 0) {
+        img->has_alpha = MC_ALPHA_YES;
         pixptr[0] = 0;
         pixptr[1] = 0;
         pixptr[2] = 0;
       } else {
+        img->has_alpha = MC_ALPHA_YES;
         PREMULTIPLY(pixptr[0],pixel[2],alpha);
         PREMULTIPLY(pixptr[1],pixel[1],alpha);
         PREMULTIPLY(pixptr[2],pixel[0],alpha);

lib/tileset.c

@@ -903,8 +903,8 @@ void mapcache_tileset_tile_set_get_with_subdimensions(mapcache_context *ctx, map
         if(GC_HAS_ERROR(ctx))
           goto cleanup;
       }
-      if((subtile->encoded_data && mapcache_imageio_header_sniff(ctx,subtile->encoded_data) == GC_JPEG)||
-         (subtile->raw_image && subtile->raw_image->has_alpha == MC_ALPHA_NO)) {
+      if ((mapcache_imageio_alpha_sniff(ctx,subtile->encoded_data) == MC_ALPHA_NO) ||
+          (subtile->raw_image && subtile->raw_image->has_alpha == MC_ALPHA_NO)) {
         /* the returned image is fully opaque, we don't need to get/decode/merge any further subtiles */
         if(assembled_image)
           assembled_image->has_alpha = MC_ALPHA_NO;