container doc: move RIFF description to own section

- remove some little used terms
- condense file header description
- add description of FourCC

Change-Id: Ia873652f41789811f3fb2ec97182a068ac727961

doc/webp-container-spec.txt

@@ -93,11 +93,21 @@ _uint32_
 
 : A 32-bit, little-endian, unsigned integer.
 
+_FourCC_
+
+: A _FourCC_ (four-character code) is a _uint32_ created by concatenating four
+  ASCII characters in little-endian order.
+
 _1-based_
 
 : An unsigned integer field storing values offset by `-1`. e.g., Such a field
 would store value _25_ as _24_.
 
+RIFF file format
+----------------
+The WebP file format is based on the RIFF (resource interchange file format)
+document format.
+
 The basic element of a RIFF file is a _chunk_. It consists of:
 
      0                   1                   2                   3
@@ -112,12 +122,12 @@ The basic element of a RIFF file is a _chunk_. It consists of:
 
 Chunk FourCC: 32 bits
 
-: ASCII four character code or _chunk tag_ used for chunk identification.
+: ASCII four-character code used for chunk identification.
 
 Chunk Size: 32 bits (_uint32_)
 
-: The size of the chunk (_ckSize_) not including this field, the chunk
+: The size of the chunk not including this field, the chunk identifier or
-  identifier and padding.
+  padding.
 
 Chunk Payload: _Chunk Size_ bytes
 
@@ -126,43 +136,13 @@ Chunk Payload: _Chunk Size_ bytes
 
 _ChunkHeader('ABCD')_
 
-: This is used to describe the fourcc and size header of individual
+: This is used to describe the _FourCC_ and _Chunk Size_ header of individual
-  chunks, where 'ABCD' is the fourcc for the chunk. This element's
+  chunks, where 'ABCD' is the FourCC for the chunk. This element's
   size is 8 bytes.
 
-: Note that, in this specification, all chunk tag characters are in
+**Note:** RIFF has a convention that all-uppercase chunk FourCCs are standard
-  file order, not in byte order of a uint32 of any particular
+chunks that apply to any RIFF file format, while FourCCs specific to a file
-  architecture.
+format are all lowercase. WebP does not follow this convention.
-
-_list of chunks_
-
-: A concatenation of multiple chunks.
-
-: We will refer to the first chunk as having _position_ 0, the second
-  as position 1, etc. By _chunk with index 0 among "ABCD"_ we mean
-  the first chunk among the chunks of type "ABCD" in the list, the
-  _chunk with index 1 among "ABCD"_ is the second such chunk, etc.
-
-A WebP file MUST begin with a single chunk with a tag 'RIFF'. All
-other defined chunks are contained within this chunk. The file SHOULD
-NOT contain anything after it.
-
-The maximum size of RIFF's _ckSize_ is 2^32 minus 10 bytes. The size
-of the whole file is at most 4GiB minus 2 bytes.
-
-**Note:** some RIFF libraries are said to have bugs when handling files
-larger than 1GiB or 2GiB. If you are using an existing library, check
-that it handles large files correctly.
-
-The first four bytes of the RIFF chunk contents (i.e., bytes 8-11 of the file)
-MUST be the ASCII string "WEBP". They are followed by a list of chunks. As the
-size of any chunk is even, the size of the RIFF chunk is also even.  The
-contents of the chunks in that list will be described in the following sections.
-
-**Note:** RIFF has a convention that all-uppercase chunks are standard
-chunks that apply to any RIFF file format, while chunks specific to a
-file format are all lowercase. WebP does not follow this convention.
-
 
 WebP file header
 ----------------
@@ -183,12 +163,20 @@ WebP file header
 
 File Size: 32 bits (_uint32_)
 
-: The size of the file in bytes starting at offset 8.
+: The size of the file in bytes starting at offset 8. The maximum value of
+this field is 2^32 minus 10 bytes and thus the size of the whole file is at
+most 4GiB minus 2 bytes.
 
 'WEBP': 32 bits
 
 : The ASCII characters 'W' 'E' 'B' 'P'.
 
+A WebP file MUST begin with a RIFF header with the FourCC 'WEBP'. The file size
+in the header is the total size of the chunks that follow plus `4` bytes for
+the 'WEBP' FourCC. The file SHOULD NOT contain anything after it. As the size
+of any chunk is even, the size given by the RIFF header is also even. The
+contents of individual chunks will be described in the following sections.
+
 Simple file format (lossy)
 --------------------------