File: mng-play-proposal-20070904

It is proposed to register an playlist MNG chunk: PLAY.

In this proposal, the new chunk name is shown in its private form (second
letter lowercase, PlAY).  Until the proposal is approved, the private form
must be used in any test implementations.

It is proposed to document the PLAY chunk in the
MNG 1.0 Specification, Version 1.0.1, as follows:

A. Change document version to 1.0.1.

B. Add paragraph 4.3.6

4.3.6. PlAY Playlist Chunk

4.3.6.1 Overview

The PlAY chunk contains a compressed "play list" which provides instructions
to the decoder for constructing animation layers from pieces of previously
defined objects.

4.3.6.2 Terminology

The "frame" is the MNG frame where the layers are are to be written.

The "layer" is a pixel array with dimensions specified by
the "frame_width" and "frame_height" parameters of the MHDR chunk.
Conceptually, each tile is constructed in a layer and passed
back to the MNG viewer.

A "tile" is a pixel array copied from an object and used 
to construct a layer.

4.3.6.3 PlAY Chunk structure

The PlAY chunk contains a 1-byte header followed by one or more 26-byte
tile structures.  The header gives the compression method:

       byte
        0   compression_method (byte)         0: deflate
        1   tile_structure_array[n]           (n > 0; n * 26 bytes): tile
            (compressed)                      structures

Each layer is a tile combined with a transparent background
to form a layer of size (frame_width, frame_height), from the MHDR chunk.
Each layer is initialized to a transparent (RGBA(0,0,0,0)) rectangle of
dimensions frame_width by frame_height.

The tile structure array is always compressed according to the specified
compression method.

A tile is a structure of 26 bytes:

       byte
        0    object    (unsigned int)     Object ID of source image
        2    x         (unsigned int)     Left of tile pixels
        6    y         (unsigned int)     Top of tile pixels
       10    width     (unsigned int)     Width of tile pixels
       14    height    (unsigned int)     Height of tile pixels
       18    x_offset  (signed int)       X offset of tile destination
       22    y_offset  (signed int)       Y offset of tile destination

The tile data identifies a rectangle (x,y,width,height) in the object
containing pixels of the tile.  This rectangle is composited
on the layer at top left position (x_offset, y_offset).  Any part of the
tile outside the object must be treated as transparent.  Any
part of the tile which extends outside the layer must be ignored.
The tile width or height may be zero, in which case the tile contains
all transparent pixels.

4.3.6.4 Delays

The decoder must handle the layers as specified in the MNG FRAM chunk.
This might or might not involve inserting background layers between them.

Even if a layer's tile is entirely outside the frame, the delay for the
resulting empty layer is handled in the same way as if it were inside the
frame.

4.3.6.5 PlAY recommendations for encoders

Tall vs wide layouts

Typically the best compression is obtained when a PNG image is
very wide and not very tall, with similar tiles adjacent.

C. Add to the Appendix: Revision History

       * 30 Sept 2007 (version 1.0.1):

             * Added the PlAY animation chunk.