Colin Shaw wrote:
> > > What about YUV420 to RGB565 fast conversion routines? Any hints?
>
> > That depends what you mean by fast. My stuff is all C++ but uses
> > precomputed lookup tables to achieve pretty good speed. I havn't
> > benchmarked it against any of the MMX implementations - if you want one
> > of those then any of the MPEG or DVD players is going to include
> > optimized conversion code.
>
> It is clear that a table lookup from, say YUV420 to any of the RGB formats
> would be pretty small, but what do you do about the size of a table for
> converting from say RGB888 to YUV420 where the space spanned by the result
> is significantly smaller than that of the original. It would appear that
> this lookup table would require about 2^24 entries and would only map to
> 64 possible YUV420 colors. Do you apply granularity to the RGB888
> pointers so that it doesn't hog so much memory? Are there any obvious
> shortcuts that I am overlooking?
:-)
The secret is to break down the conversion such that you can express each of
the r/g/b (or y/u/v) conversions as a sum of lookups into mutliple tables
indexed by 8bit values. Thus the table sizes are very small with no loss of
accuracy, and you've still replaced a bunch of nasty floating point equations
with a few array accesses and integer additions.
Check out rgb2yuv.h/cpp in my tarball at:
http://planeta.terra.com.br/informatica/gleicon/video4linux/
Ben
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