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iamthird36 · 16th May 2007, 16:43

A few questions how x264 is performing mode selection. My overall problem is that I have different macroblock types of varying importance. For one of the types, I really don't care at all about distortion and I'd like to encode it with as few bits as possible. In this case, it seems like all these blocks should be encoded using skip (if its a P slice) or I 16x16 (if its an I slice).

I've adjusted the coder in two ways for these particular macroblocks. I either significantly increase lambda and lambda2 to force the encoder to operate with a very small rate constraint on those blocks or I adjust the ssd cost such that it is 0 for these blocks. My intuition is that by doing this, the coder should be selecting the modes with the lowest possible bitrate. This doesn't seem to be the case. I get a lot of intra 4x4 (even in p slices) and inter 16x16 blocks with pretty small motion vectors. Any thoughts?

Also, a more general question on how skip modes are determined. With fast-p-skip enabled, how does the encoder determine if a block should be a skip or a predicted block? Without any of my modifications, I seem to get a lot of predicted blocks with relatively small motion vectors where there is no change between frames.

Thanks for any feedback.

16th May 2007, 16:43	#1 \| Link
iamthird36 Registered User Join Date: Apr 2007 Posts: 4	x264 questions about mb type selection A few questions how x264 is performing mode selection. My overall problem is that I have different macroblock types of varying importance. For one of the types, I really don't care at all about distortion and I'd like to encode it with as few bits as possible. In this case, it seems like all these blocks should be encoded using skip (if its a P slice) or I 16x16 (if its an I slice). I've adjusted the coder in two ways for these particular macroblocks. I either significantly increase lambda and lambda2 to force the encoder to operate with a very small rate constraint on those blocks or I adjust the ssd cost such that it is 0 for these blocks. My intuition is that by doing this, the coder should be selecting the modes with the lowest possible bitrate. This doesn't seem to be the case. I get a lot of intra 4x4 (even in p slices) and inter 16x16 blocks with pretty small motion vectors. Any thoughts? Also, a more general question on how skip modes are determined. With fast-p-skip enabled, how does the encoder determine if a block should be a skip or a predicted block? Without any of my modifications, I seem to get a lot of predicted blocks with relatively small motion vectors where there is no change between frames. Thanks for any feedback.