[theora] YCbCr <-> RGB conversion question

[Conrad Parker]

2009/6/16 Ondrej Certik <ondrej at certik.cz>:
> Hi,
>
> I coded some routines for YCbCr <-> RGB conversion, both using
> floating point numbers and using integer arithmetics, code is
> attached.
>
> Let's say I want to convert from RGB to YCbCr and back. First the
> integer arithmetics:
>
> In [1]: from colors import *
>
> In [2]: a = [255, 0, 255]
>
> In [3]: RGB2YCbCr(a)
> Out[3]: (107, 202, 222)
>
> In [4]: YCbCr2RGB(RGB2YCbCr(a))
> Out[4]: array([255,   1, 255])
>
>
> As you can see, the G component was increased from 0 to 1. Those
> routines are the same as here:
>
> http://msdn.microsoft.com/en-us/library/ms893078.aspx
>
> Now let's try floating point conversions:
>
> In [3]: RGB2YCbCr_precise(a)
> Out[3]: array([106, 202, 222], dtype=uint8)
>
> In [4]: YCbCr2RGB_precise(RGB2YCbCr_precise(a))
> Out[4]: array([255,   0, 254], dtype=uint8)
>
> This time the B component was decreased by 1... Those floating point
> routines were done by me, and I followed the theora spec.
>
> So my questions are:
>
> * does anyone here have a code, that can convert back and forth
> without loosing precision?
> * does it make sense for those routines to work back and forth exactly?
>
> I am new to this, so I am not sure what the common practise is, but it
> seems weird to me, that if I convert the picture from YCbCr to RGB and
> back couple times, the image will slowly deteriorate.

Have you compared to the code in liboggplay?

(speaking of which, python bindings to liboggplay would be elite --
that would add audio, sync and seeking support :-)

Conrad.