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The following is a draft. It is at best incomplete and at worst completely broken. In any case, it is not an "official" Xiph spec/codec, so use with care.


PNG is an open image compression format that is used as the basis for a "timed image" codec in this specification. Recordings of seminars, lectures and presentations generally consist of slides and a very efficient representation of such a recording is as a stream of images with timing information plus the recorded audio underneath.

This specification defines a format to describe a timed image track, including the presentation parameters, the input images and their timing. It then defines a logical bitstream format for encapsulating the images inside Ogg. When multiplexed together with one of the Xiph audio codecs such as Speex, Vorbis, FLAC, and OggPCM2, e.g. using OggSkeleton, you end up with a video format that consists of timed images and audio.

Timed Image Specification Format

The bitstream format this "codec" should be very simple. It should essentially consist only of a sequence of PNG images preceeded by a header with a simple set o fields to set up the decoding. If at all possible, we're trying to avoid giving parameters to the individual images since this will create an additional intermediate decoding step.

The following fields are under discussion:

  • Image-Format

This format can be generalised to cover all kinds of image formats, e.g. JPEG, GIF. We want to avoid stumbling over image formats that cannot be decoded by a client and therefore all images in such a stream need to be of the same format, which is specified here.

  • Display-Width, Display-Height

While it is expected that most of the images in the data packets are of the same size (dimensions, geometry, resolution), variations may occur. A decoder should be given a resolution at which the images are to be presented. Aspect ratio must be kept when images are re-scaled.

NOTE: It may be interesting to keep smaller images at their original size and just put them in the screen centre?

  • Rescaling

While images that are larger than the display are defined by Display-Width and Display-Height must be scaled down to this size, this may not necessarily be desirable for smaller images. This paramter decides whether smaller images should be scaled up to meet the display size, or just be displayed inside it.

  • Align-Horizontal, Align-Vertical

Smaller images that are not rescaled to display size may be aligned at several different areas inside the larger display image:

    • Align-Horizontal: centre/left/right
    • Align-Vertical: centre/top/bottom
  • Background-Colour

For transparent images and for smaller, non-rescaled images, the background colour of the images has to be defined. This may be black, white, gray or whatever.

A rough format specification could be:

Display-Width: 320
Display-Height: 240
Rescaling: False
Background-Colour: Grey
npt:00:00:00.000 /my_slides/image_01.png
npt:00:02:10.000 /my_slides/image_02.png
npt:00:05:02.000 /my_slides/image_03.png
npt:00:06:50.000 /my_slides/image_04.png

Timed Images Mapping into Ogg

The first step towards encapsulating the data into ogg is the definition of packets:

  • There is a OggPNG ident header, which is encapsulated in the bos page.
  • There is a secondary header packet containing the setup parameters, which is encapsulated in a separate page.
  • Each image is mapped into a data packet, which are each encoded in their own packet and inserted at the accurate time.
  • The eos page is empty.

OggPNG ident header

The timed PNG logical bitstream starts with an ident header which is mapped into the OggPNG bos page. The ident header contains all information required to identify the timed PNG bitstream and to set up a timed PNG decoder. It has the following format:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1| Byte
| Identifier 'PNGT\0\0\0\0'                                     | 0-3
|                                                               | 4-7
| Version major                 | Version minor                 | 8-11
| Granulerate numerator                                         | 12-15
|                                                               | 16-19
| Granulerate denominator                                       | 20-23
|                                                               | 24-27
| Granuleshift  |                                                 28
| ...

The OggPNG version as described here is major=0 minor=1.

The granulerate represents the temporal resolution of the logical bitstream in Hz given as a rational number in the same way as the OggSkeleton fisbone secondary header specifies granulerate. It enables a mapping of granule position of the data pages to time by calculating "granulepos / granulerate".

The default granule rate for OggPNG is: 1/30 (30 frames per second resolution).

The granuleshift is a 1 Byte integer number describing whether to partition the granule_position into two for the OggPNG logical bitstream, and how many of the lower bits to use for the partitioning. The upper bits then still signify a time-continuous granule position for a directly decodable and presentable data granule. The lower bits allow for specification of the granule position of a previous OggPNG data packet (i.e. image), which helps to identify how much backwards seeking is necessary to get to the last and still active image. The granuleshift is therefore the log of the maximum possible image spacing.

The default granule shift used is 32, which halfs the granule position to allow for the backwards pointer.

OggPNG secondary header

This header contains all the setup information for the decoder.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1| Byte
| Display width                 | Display height                | 0-3
| ...                                                           |

OggPNG data

The data packets are simple. Each data packet simply contains a PNG image.

The insertion time (and therefore the granule_pos) is given through the specified time.

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