[x264-devel] Question for intra prediction

[Loren Merritt]

On Tue, 26 Jun 2007, Taeyoung Na wrote:

> Why the encoder try to decide a best intra mode same as the 
> i_pred_mode? There is no time saving effect, no PSNR preservation as far 
> as I know.

A block is more likely to use the same mode as its neighbors than any one 
other mode (that's why there is a prediction in the standard). Assuming 
CAVLC, the predicted mode takes 1 bit, and the other modes take 4 bits. 
But the absolute number doesn't matter (it can be compensated by adding a 
constant once after all 16 blocks), so it's faster to subtract the 
difference from the predicted mode instead of adding the bit cost to all 
modes.

--Loren Merritt
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