Mux: Allow only some frames/tiles to have alpha.

And related const fixes.

Change-Id: I79f6f1b9f8c6faac8cc4ef24b58edff6d90de045

doc/webp-container-spec.txt

@@ -300,9 +300,7 @@ multiple frames. Data for each frame consists of:
     present at the beginning of each tile for a frame that is represented as
     multiple tile images.
 
-  * An optional 'ALPH' chunk with alpha bitstream of the frame/tile. If the
+  * An optional 'ALPH' chunk with alpha bitstream of the frame/tile.
-    file contains transparency, this chunk MUST be present for each frame/tile
-    containing a 'VP8 ' chunk. It MUST NOT be present otherwise.
 
   * A 'VP8 ' or a 'VP8L' chunk containing compressed image data of the
     frame/tile.
@@ -355,7 +353,8 @@ Metadata (M): 1 bit
 
 Alpha (L): 1 bit
 
-: Set if the file contains images with transparency information ("alpha").
+: Set if the file contains some (or all) images with transparency information
+("alpha").
 
 Rotation and Symmetry (R): 3 bits
 
@@ -540,10 +539,10 @@ Alpha bitstream: _Chunk Size_ - `1` bytes
 : Encoded alpha bitstream.
 
 This optional chunk contains encoded alpha data for a single frame/tile.
-Either **ALL or NONE** of the frame/tiles must contain this chunk. However,
+For images with transparency, some of the frames/tiles may contain this chunk.
-there is one exception to this rule: a frame/tile containing a 'VP8L' chunk
+However, a frame/tile containing a 'VP8L' chunk SHOULD NOT contain this chunk.
-SHOULD NOT contain this chunk. **Rationale**: the transparency information of
+**Rationale**: the transparency information of a frame/tile is already part of
-a frame/tile is already part of the 'VP8L' chunk.
+the 'VP8L' chunk.
 
 The alpha channel data is losslessly stored as raw data (when
 compression method is '0') or compressed using the lossless format

src/mux/muxedit.c

@@ -613,9 +613,8 @@ static WebPMuxError CreateVP8XChunk(WebPMux* const mux) {
     }
   }
 
-  if (images->alpha_ != NULL && images->alpha_->data_.bytes_ != NULL) {
+  if (MuxImageCount(images, WEBP_CHUNK_ALPHA) > 0) {
-    // This is an image with alpha channel.
+    flags |= ALPHA_FLAG;  // Some images have an alpha channel.
-    flags |= ALPHA_FLAG;
   }
 
   if (flags == 0) {

src/mux/muxi.h

@@ -194,7 +194,7 @@ WebPMuxImage* MuxImageDelete(WebPMuxImage* const wpi);
 void MuxImageDeleteAll(WebPMuxImage** const wpi_list);
 
 // Count number of images matching the given tag id in the 'wpi_list'.
-int MuxImageCount(WebPMuxImage* const wpi_list, WebPChunkId id);
+int MuxImageCount(const WebPMuxImage* wpi_list, WebPChunkId id);
 
 // Check if given ID corresponds to an image related chunk.
 static WEBP_INLINE int IsWPI(WebPChunkId id) {
@@ -208,14 +208,14 @@ static WEBP_INLINE int IsWPI(WebPChunkId id) {
 }
 
 // Get a reference to appropriate chunk list within an image given chunk tag.
-static WEBP_INLINE WebPChunk** MuxImageGetListFromId(WebPMuxImage* wpi,
+static WEBP_INLINE WebPChunk** MuxImageGetListFromId(
-                                                     WebPChunkId id) {
+    const WebPMuxImage* const wpi, WebPChunkId id) {
   assert(wpi != NULL);
   switch (id) {
     case WEBP_CHUNK_FRAME:
-    case WEBP_CHUNK_TILE:  return &wpi->header_;
+    case WEBP_CHUNK_TILE:  return (WebPChunk**)&wpi->header_;
-    case WEBP_CHUNK_ALPHA: return &wpi->alpha_;
+    case WEBP_CHUNK_ALPHA: return (WebPChunk**)&wpi->alpha_;
-    case WEBP_CHUNK_IMAGE: return &wpi->img_;
+    case WEBP_CHUNK_IMAGE: return (WebPChunk**)&wpi->img_;
     default: return NULL;
   }
 }

src/mux/muxinternal.c

@@ -262,9 +262,9 @@ WebPMuxImage* MuxImageRelease(WebPMuxImage* const wpi) {
 //------------------------------------------------------------------------------
 // MuxImage search methods.
 
-int MuxImageCount(WebPMuxImage* const wpi_list, WebPChunkId id) {
+int MuxImageCount(const WebPMuxImage* wpi_list, WebPChunkId id) {
   int count = 0;
-  WebPMuxImage* current;
+  const WebPMuxImage* current;
   for (current = wpi_list; current != NULL; current = current->next_) {
     const WebPChunk* const wpi_chunk = *MuxImageGetListFromId(current, id);
     if (wpi_chunk != NULL) {
@@ -558,13 +558,6 @@ WebPMuxError MuxValidate(const WebPMux* const mux) {
   } else {
     err = ValidateChunk(mux, IDX_ALPHA, ALPHA_FLAG, flags, -1, &num_alpha);
     if (err != WEBP_MUX_OK) return err;
-
-    // num_images & num_alpha_chunks are consistent.
-    if (num_alpha > 0 && num_alpha != num_images) {
-      // Note that "num_alpha > 0" is the correct test but "flags && ALPHA_FLAG"
-      // is NOT, because ALPHA_FLAG is based on first image only.
-      return WEBP_MUX_INVALID_ARGUMENT;
-    }
   }
 
   // num_tiles & num_images are consistent.