On Wed, 28 Mar 2001, Colin Shaw wrote:
> 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 standard way it to use 9 tables of 256 entries, one for each combination
of (R, G, B) with (Y, U, V). Then you do something like
Y = R_to_Y[R] + G_to_Y[G] + B_to_Y[B];
U = R_to_U[R] + G_to_U[G] + B_to_U[B];
V = R_to_V[R] + G_to_V[G] + B_to_V[B];
So instead of one table lookup, in a huge table, you do 9 lookups plus a few
additions. For more accuracy you would probably want to use fixed point
arithmetic. You don't need all 9 tables either, as some of them will end up
being the same.
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