Alpha flag fix for lossless.

- Make sure alpha flag is set in case of a lossless file with VP8X chunk.
  The semantic of ALPHA_FLAG changes with this: it means the images
  contain alpha (rather than ALPH chunk in particular).
- Update the mux container spec to add 1-line description of alpha
  flag.
- Rename "HasLosslessImages()" to "MuxHasLosslessImages()", and other
  similar function renames.
- Rename FeatureFlags to WebPFeatureFlags
- Elaborated a comment for a special case.
- A misc comment fix.

Change-Id: If212ccf4338c125b4c71c10bf281a51b3ba7ff45

doc/webp-container-spec.txt

@@ -13,7 +13,7 @@ end of this file.
 WebP Container Specification
 ============================
 
-_Working Draft, v0.2, 20120207_
+_Working Draft, v0.3, 20120523_
 
 
 * TOC placeholder
@@ -271,7 +271,7 @@ Extended WebP file header:
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      ChunkHeader('VP8X')                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
-    |  Rsrv |M|I|A|T|                   Reserved                    |
+    |  Rsrv |AL|M|I|A|T|                   Reserved                 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                          Canvas Width                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
@@ -295,11 +295,15 @@ Metadata (M): 1 bit
 
 : Set if the file contains a 'META' chunk.
 
-Reserved (Rsrv): 4 bits
+Alpha (AL): 1 bit
+
+: Set if the file contains images with transparency information ("alpha").
+
+Reserved (Rsrv): 3 bits
 
 : SHOULD be `0`.
 
-Reserved: 16 bits
+Reserved: 24 bits
 
 : SHOULD be `0`.
 

src/mux/muxedit.c

@@ -218,7 +218,7 @@ static WebPMuxError MuxDeleteAllNamedData(WebPMux* const mux,
   if (mux == NULL || name == NULL) return WEBP_MUX_INVALID_ARGUMENT;
   if (IsWPI(id)) return WEBP_MUX_INVALID_ARGUMENT;
 
-  chunk_list = GetChunkListFromId(mux, id);
+  chunk_list = MuxGetChunkListFromId(mux, id);
   if (chunk_list == NULL) return WEBP_MUX_INVALID_ARGUMENT;
 
   return DeleteChunks(chunk_list, kChunks[idx].tag);
@@ -390,7 +390,7 @@ static WebPMuxError MuxAddFrameTileInternal(
   image_info = NULL;  // Owned by 'chunk' now.
   err = ChunkSetNth(&chunk, &wpi.img_, 1);
   if (err != WEBP_MUX_OK) goto Err;
-  ChunkInit(&chunk);  // chunk owned by wpi.vp8_ now.
+  ChunkInit(&chunk);  // chunk owned by wpi.img_ now.
 
   // Create frame/tile data from image_info.
   err = CreateDataFromImageInfo(wpi.img_->image_info_, is_frame,
@@ -452,7 +452,7 @@ WebPMuxError WebPMuxDeleteImage(WebPMux* const mux) {
 
   if (mux == NULL) return WEBP_MUX_INVALID_ARGUMENT;
 
-  err = ValidateForImage(mux);
+  err = MuxValidateForImage(mux);
   if (err != WEBP_MUX_OK) return err;
 
   // All well, delete image.
@@ -623,6 +623,14 @@ static WebPMuxError CreateVP8XChunk(WebPMux* const mux) {
   err = GetImageCanvasWidthHeight(mux, flags, &width, &height);
   if (err != WEBP_MUX_OK) return err;
 
+  if (MuxHasLosslessImages(images)) {
+    // We have a file with a VP8X chunk having some lossless images.
+    // As lossless images implicitly contain alpha, force ALPHA_FLAG to be true.
+    // Note: This 'flags' update must NOT be done for a lossless image
+    // without a VP8X chunk!
+    flags |= ALPHA_FLAG;
+  }
+
   PutLE32(data + 0, flags);   // VP8X chunk flags.
   PutLE32(data + 4, width);   // canvas width.
   PutLE32(data + 8, height);  // canvas height.
@@ -695,7 +703,7 @@ WebPMuxError WebPMuxAssemble(WebPMux* const mux,
   assert(dst == data + size);
 
   // Validate mux.
-  err = WebPMuxValidate(mux);
+  err = MuxValidate(mux);
   if (err != WEBP_MUX_OK) {
     free(data);
     data = NULL;

src/mux/muxi.h

@@ -257,19 +257,22 @@ size_t MuxImageListDiskSize(const WebPMuxImage* wpi_list);
 // Write out the given list of images into 'dst'.
 uint8_t* MuxImageListEmit(const WebPMuxImage* wpi_list, uint8_t* dst);
 
+// Checks if the given image list contains at least one lossless image.
+int MuxHasLosslessImages(const WebPMuxImage* images);
+
 //------------------------------------------------------------------------------
 // Helper methods for mux.
 
 // Returns the list where chunk with given ID is to be inserted in mux.
 // Return value is NULL if this chunk should be inserted in mux->images_ list
 // or if 'id' is not known.
-WebPChunk** GetChunkListFromId(const WebPMux* mux, TAG_ID id);
+WebPChunk** MuxGetChunkListFromId(const WebPMux* mux, TAG_ID id);
 
 // Validates that the given mux has a single image.
-WebPMuxError ValidateForImage(const WebPMux* const mux);
+WebPMuxError MuxValidateForImage(const WebPMux* const mux);
 
 // Validates the given mux object.
-WebPMuxError WebPMuxValidate(const WebPMux* const mux);
+WebPMuxError MuxValidate(const WebPMux* const mux);
 
 //------------------------------------------------------------------------------
 

src/mux/muxinternal.c

@@ -422,10 +422,21 @@ uint8_t* MuxImageListEmit(const WebPMuxImage* wpi_list, uint8_t* dst) {
   return dst;
 }
 
+int MuxHasLosslessImages(const WebPMuxImage* images) {
+  while (images != NULL) {
+    assert(images->img_ != NULL);
+    if (images->img_->tag_ == kChunks[IDX_VP8L].tag) {
+      return 1;
+    }
+    images = images->next_;
+  }
+  return 0;
+}
+
 //------------------------------------------------------------------------------
 // Helper methods for mux.
 
-WebPChunk** GetChunkListFromId(const WebPMux* mux, TAG_ID id) {
+WebPChunk** MuxGetChunkListFromId(const WebPMux* mux, TAG_ID id) {
   assert(mux != NULL);
   switch(id) {
     case VP8X_ID:    return (WebPChunk**)&mux->vp8x_;
@@ -437,7 +448,7 @@ WebPChunk** GetChunkListFromId(const WebPMux* mux, TAG_ID id) {
   }
 }
 
-WebPMuxError ValidateForImage(const WebPMux* const mux) {
+WebPMuxError MuxValidateForImage(const WebPMux* const mux) {
   const int num_images = MuxImageCount(mux->images_, IMAGE_ID);
   const int num_frames = MuxImageCount(mux->images_, FRAME_ID);
   const int num_tiles  = MuxImageCount(mux->images_, TILE_ID);
@@ -465,7 +476,8 @@ static int IsNotCompatible(int feature, int num_items) {
 // and feature incompatibility (use NO_FLAG to skip).
 // On success returns WEBP_MUX_OK and stores the chunk count in *num.
 static WebPMuxError ValidateChunk(const WebPMux* const mux, CHUNK_INDEX idx,
-                                  FeatureFlags feature, FeatureFlags vp8x_flags,
+                                  WebPFeatureFlags feature,
+                                  WebPFeatureFlags vp8x_flags,
                                   int max, int* num) {
   const WebPMuxError err =
       WebPMuxNumNamedElements(mux, kChunks[idx].name, num);
@@ -477,7 +489,7 @@ static WebPMuxError ValidateChunk(const WebPMux* const mux, CHUNK_INDEX idx,
   return WEBP_MUX_OK;
 }
 
-WebPMuxError WebPMuxValidate(const WebPMux* const mux) {
+WebPMuxError MuxValidate(const WebPMux* const mux) {
   int num_iccp;
   int num_meta;
   int num_loop_chunks;
@@ -541,15 +553,20 @@ WebPMuxError WebPMuxValidate(const WebPMux* const mux) {
   if (num_vp8x == 0 && num_images != 1) return WEBP_MUX_INVALID_ARGUMENT;
 
   // ALPHA_FLAG & alpha chunk(s) are consistent.
+  if (num_vp8x > 0 && MuxHasLosslessImages(mux->images_)) {
+    // Special case: we have a VP8X chunk as well as some lossless images.
+    if (!(flags & ALPHA_FLAG)) return WEBP_MUX_INVALID_ARGUMENT;
+  } 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 check but "flags && ALPHA_FLAG"
+      // 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.
   if (num_tiles > 0 && num_images != num_tiles) {

src/mux/muxread.c

@@ -166,7 +166,7 @@ WebPMux* WebPMuxCreateInternal(const uint8_t* data, size_t size, int copy_data,
       WebPChunk** chunk_list;
       if (wpi->is_partial_) goto Err;  // Encountered a non-image chunk before
                                        // getting all chunks of an image.
-      chunk_list = GetChunkListFromId(mux, id);  // List for adding this chunk.
+      chunk_list = MuxGetChunkListFromId(mux, id);  // List to add this chunk.
       if (chunk_list == NULL) chunk_list = &mux->unknown_;
       if (ChunkSetNth(&chunk, chunk_list, 0) != WEBP_MUX_OK) goto Err;
     }
@@ -179,7 +179,7 @@ WebPMux* WebPMuxCreateInternal(const uint8_t* data, size_t size, int copy_data,
   }
 
   // Validate mux if complete.
-  if (WebPMuxValidate(mux) != WEBP_MUX_OK) goto Err;
+  if (MuxValidate(mux) != WEBP_MUX_OK) goto Err;
 
  Ok:
   MuxImageDelete(wpi);
@@ -234,7 +234,7 @@ WebPMuxError WebPMuxGetImage(const WebPMux* const mux,
     return WEBP_MUX_INVALID_ARGUMENT;
   }
 
-  err = ValidateForImage(mux);
+  err = MuxValidateForImage(mux);
   if (err != WEBP_MUX_OK) return err;
 
   // All well. Get the image.
@@ -381,7 +381,7 @@ WebPMuxError WebPMuxNumNamedElements(const WebPMux* const mux, const char* name,
   if (IsWPI(id)) {
     *num_elements = MuxImageCount(mux->images_, id);
   } else {
-    WebPChunk* const* chunk_list = GetChunkListFromId(mux, id);
+    WebPChunk* const* chunk_list = MuxGetChunkListFromId(mux, id);
     if (chunk_list == NULL) {
       *num_elements = 0;
     } else {

src/webp/mux.h

@@ -78,7 +78,7 @@ typedef enum {
   ICCP_FLAG       = 0x00000004,
   META_FLAG       = 0x00000008,
   ALPHA_FLAG      = 0x00000010
-} FeatureFlags;
+} WebPFeatureFlags;
 
 typedef struct WebPMux WebPMux;   // main opaque object.
 
@@ -430,7 +430,7 @@ WEBP_EXTERN(WebPMuxError) WebPMuxDeleteTile(WebPMux* const mux, uint32_t nth);
 //   mux - (in) object from which the features are to be fetched
 //   flags - (out) the flags specifying which features are present in the
 //           mux object. This will be an OR of various flag values.
-//           Enum 'FeatureFlags' can be used to test for individual flag values.
+//           Enum 'WebPFeatureFlags' can be used to test individual flag values.
 // Returns:
 //   WEBP_MUX_INVALID_ARGUMENT - if mux is NULL or flags is NULL
 //   WEBP_MUX_NOT_FOUND - if VP8X chunk is not present in mux object.