[theora] No quality advantage with two-pass?

Torrance torrance123 at gmail.com
Sat Aug 29 16:24:22 PDT 2009

On 30/08/2009, at 9:39 AM, Remco wrote:

> And just out of curiosity, how are the quality levels translated into
> an encoding strategy? Are these quality levels mapped to advanced
> internal encoder options, or is the encoder basically one big
> mathematical equation that takes one parameter? (besides the video
> input, obviously)

IANAE, but this is from http://web.mit.edu/xiphmont/Public/theora/demo6.html 
  and should be reliable.
> lambda/qi mapping and rate/distortion modeling
> In order for a video encoder to be easy to use, it needs simple  
> tuning knobs to effect 'simple' behavioral changes, such as a single  
> master adjustment for output bitrate or constant output quality,  
> although this might internally translate into hundreds of parameter  
> changes.
> With the unification of all the rate-distortion lambdas into a  
> single unified lambda value, output quality is governed by two  
> master values, the quantizer index (in Theora, an integer between 0  
> and 63) and the rate-distortion lambda. Although the two values are  
> not directly fundamentally related, an efficient encoder chooses  
> lambda and qi such that the output distortion is minimized for a  
> given bitrate much the same way token or block coding decisions are  
> made by trading the bitrate and distortion costs.
> Once it was possible to choose an appropriate lambda for a given qi  
> and vice versa, Thusnelda finally had a single master knob to allow  
> a user to set a constant quality mode. Constant and average bitrate  
> modes (rate control) is implemented in terms of varying qi according  
> to predicted bit usage and distortion over a set of frames.  
> Thusnelda, as it turns out, follows classical predictions for bit  
> usage according to 'rho-domain analysis', which predicts that bit  
> usage in any given frame is roughly linearly related to the number  
> of nonzero DCT coefficients in that frame.
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