VP8Decimate "RDO/only Distortion" branch

[Pablo Enfedaque]

Hi there,

In the function VP8Decimate() there is an if/else that separates two cases:

1. if (rd_opt > RD_OPT_NONE):

For a given macroblock, the rate and distortion of different prediction modes are measured + reconstruction of that MB.

2. else:

Only distortion measurements are performed + reconstruction of that MB.

The comment before the “else” says that:

// At this point we have heuristically decided intra16 / intra4.

// For method >= 2, pick the best intra4/intra16 based on SSE (~tad slower).

// For method <= 1, we don't re-examine the decision but just go ahead with

// quantization/reconstruction.

There is necessarily a previous decision regarding the prediction mode in that MB? Could not be the first time that it is evaluated??

From what I understand, when inside StatLoop() → OneStatPass() → VP8Decimate, if method < 3 then rd_opt=RD_OPT_NONE, so the evaluation in VP8Decimate goes to the “else” branch. However, this will be the first time that that MB is checked, so no previous decisions have been made. Is this right?

Depending on the method (0..6) used, in the main loop of VP8EncLoop(), it could also be the first time that a MB is visited, if inside StatLoop() not all macroblocks have been evaluated (fast mode).

Can someone please clarify those scenarios?

Thanks!

Pablo

[Pascal Massimino]

Hi Pablo,

On Mon, Jun 27, 2016 at 7:44 PM, Pablo Enfedaque <pablo.e...@gmail.com> wrote:

Hi there,

In the function VP8Decimate() there is an if/else that separates two cases:

1. if (rd_opt > RD_OPT_NONE):

For a given macroblock, the rate and distortion of different prediction modes are measured + reconstruction of that MB.

2. else:

Only distortion measurements are performed + reconstruction of that MB.

The comment before the “else” says that:

// At this point we have heuristically decided intra16 / intra4.

// For method >= 2, pick the best intra4/intra16 based on SSE (~tad slower).

// For method <= 1, we don't re-examine the decision but just go ahead with

// quantization/reconstruction.

There is necessarily a previous decision regarding the prediction mode in that MB? Could not be the first time that it is evaluated??

Actually, the mode is 'decided' to be forced to intra16 during the analysis pass: we call DoSegmentJob() in analysis.c, which in turn calls MBAnalyze(), which calls MBAnalyzeBestIntra16Mode() first. Then, for method >= 5, we refine the decision with MBAnalyzeBestIntra4Mode().

hope it helps,

skal/

From what I understand, when inside StatLoop() → OneStatPass() → VP8Decimate, if method < 3 then rd_opt=RD_OPT_NONE, so the evaluation in VP8Decimate goes to the “else” branch. However, this will be the first time that that MB is checked, so no previous decisions have been made. Is this right?

Depending on the method (0..6) used, in the main loop of VP8EncLoop(), it could also be the first time that a MB is visited, if inside StatLoop() not all macroblocks have been evaluated (fast mode).

Can someone please clarify those scenarios?

Thanks!

Pablo

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[Pablo Enfedaque]

Hi skal,

Ok! That makes sense, much clearer now.  

The analysis pass is the one that is actually implemented with multithreading support, right? Do you know if all the macroblocks could be analyzed in parallel in this stage??

Thank you very much!

Pablo

[Pascal Massimino]

Hi Pablo,

On Tue, Jul 5, 2016 at 5:57 PM, Pablo Enfedaque <pablo.e...@gmail.com> wrote:

Hi skal,

Ok! That makes sense, much clearer now.  

The analysis pass is the one that is actually implemented with multithreading support, right? Do you know if all the macroblocks could be analyzed in parallel in this stage??

yes, they can. They're no dependency between macroblocks, each analysis can be done separately. But keep in mind that

this part (analysis) is only a minor fraction of the overall encoding time (the most consuming part is the later RD evaluation).

hope it helps,

skal/