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Spatial filter within temporal filter

 
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VHSobsessive



Joined: 31 Aug 2011
Posts: 16

PostPosted: Fri Aug 16, 2013 3:07 pm    Post subject: Spatial filter within temporal filter Reply with quote

It seems that version 3 introduced a (very aggressive) 'hidden' spatial filter which is tied to the temporal filter and is separate from the switchable spatial filter in NV3.

It can be seen in action by setting the switchable spatial filter to 0% and the temporal filter to it's lowest threshold, so that the temporal filter isn't applied to any pixels - the image is only processed by the hidden spatial filter.

In normal use, it seems the motion estimation used by the temporal filter identifies the moving parts of the image and doesn't attempt temporal filtering on those moving areas, so those pixels are only processed by the hidden spatial filter. This very nicely solves the problem of noise in moving areas, which must improve the compressibility of the output video.

Unfortunately it leads to ugly artifacts where there are scene fades and fades to black etc.

These changes to the temporal filter happened in version three. I worry that the new hidden filter was only implemented because the temporal filter is less effective as a result of being made 'GPU friendly'.

Is it possible to add a switch to deactivate or lessen the hidden filter ? I find the artifacts far more obnoxious than the noise that would otherwise be there. I am tempted to live with the noise in moving areas, as seen in version 2.

Is it also feasible to somehow have NV detect fades and deactivate the spatial filter during those fades ?

Thanks.
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NVTeam



Joined: 01 Sep 2005
Posts: 2237

PostPosted: Fri Aug 16, 2013 9:27 pm    Post subject: Re: Spatial filter within temporal filter Reply with quote

VHSobsessive wrote:
It can be seen in action by setting the switchable spatial filter to 0% and the temporal filter to it's lowest threshold, so that the temporal filter isn't applied to any pixels - the image is only processed by the hidden spatial filter.

That is incorrect. If you want to disable the temporal filter you must set the radius to 0 rather than setting the filter to its lowest threshold. The threshold adjusts the sensitivity of the filter to the changes from one frame to another (like movements of objects). It has no relation to disabling of the filter.

If you disable both spatial filter and temporal filter as described above, the frame remains the same, no filtration is applied.

Vlad
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VHSobsessive



Joined: 31 Aug 2011
Posts: 16

PostPosted: Sat Aug 17, 2013 10:35 pm    Post subject: Reply with quote

Thanks for responding, Vlad.

In NV2, switching off spatial filtering and reducing temporal filter threshold to -100 results in a frame which is identical to the input. This does not happen in NV3, the frame is clearly filtered by an undocumented spatial filter. I would like to be able to switch this undocumented filter off as it makes fades look terrible, in my view far worse than if they were merely grainy.

My original post was not about switching off the temporal filter but to reveal what happens when the threshold is set to it's lowest value. When the temporal filter decides that all parts of the image are moving [threshold set to -100%], it will cease all temporal filtering and all that is left is some heavy spatial filtration.

Perhaps you or other NV users could replicate what I have noticed by using the clip below. The filtration doesn't look too bad in freeze frame, but becomes offensive during playback.

http://s000.tinyupload.com/index.php?file_id=33753574806961022279

Set spatial filter to 0%. Temporal radius 1+, threshold -100%.
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