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.
