[FFmpeg-devel] [RFC] AVComponentDescriptor and bit-streamed formats

[Stefano Sabatini]

Hi all,

I'm trying to figure out how to manage the bit-streamed formats using
pixdesc.

I'll use the term "bit-streamed format" and "byte-streamead format" to
distinguish between the two types of formats.

These are the concepts used for describing a component:

* plane: this applies perfectly well to both bit-streamed and
  byte-streamed formats

* step (range: 1 - 8): it's the number of bytes between 2 horizontally
  consecutive pixels (byte-streamed formats).

  For bit-streamed formats it may be both the number of *bits* between
  two consecutive pixels (only problem is that the range may not
  suffice)
  *or*
  the number of bytes between the pixels, if the number of bytes
  between two pixels is not an integer then we need to add to this
  value another value.

* offset (range: -1 - 6): number of bytes before the component of the
  first pixel, same consideration as for the step field applies.

* shift (range: 0 - 7): it's the number of least significant bits that
  must be shifted away to get the value.

  This doesn't make sense for bit-streamed filters, but may be used in
  combination with step or offset.

* depth: number of bits in the component, works fine with both bit and
  byte-streamed formats.

So the problem I'm having is with step+offset+shift.

The variables I need for describing a bit-streamed format are:
* plane
* step (bits or bytes+remaining bits)
* offset (bits or bytes+remaining bits)
* depth

Currently the only bit-streamed formats supported in FFmpeg are:
BGR4, RGB4, MONOWHITE, MONOBLACK, and for these the ranges used for
step and offset may suffice for interpreting those values as a number
of *bits*, but for extensibility sake I think we should:

* extend the range of the step and offset fields
or
* introduce new fields to express the step/offset remaining bits
  (maybe reusing shift)

Once this is clear it should be fairly easy to complete the
read_line() function in pixdesc.h and write a corresponding
write_line() functions (which I almost completely did in my working
copy, but the missing bits for dealing with bit-streamed formats).

Regards.
-- 
FFmpeg = Funny and Fostering Mythic Picky Extravagant Game