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Power windows with concentric banding?

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Michael McCune
Power windows with concentric banding?
on Nov 28, 2016 at 11:59:15 am

Often a shape power window, such as a circular or Bezier shape, shows slight but noticeable concentric banding. Is this a known issue or condition?

For example, a circular or Bezier shape is intended to allow setting the gamma and gain slightly higher or lower. Perhaps we want to lighten a face. The qualifier highlight seems to show the expected smooth gradient change according to softness, inside and outside settings, etc.

Yet the image itself seems to show the lighter central portion, as expected, but surrounding it is a slightly darker concentric band, following the shape of the window, before extending and blending with the outer surrounding image.

Also related, this question: I seem to see distinct concentric line boundaries, as compared to the softer bands of a Bezier shape, when using the circular window ( which had only one softness control). For Bezier shapes it is not a distinct line but softer banding.

In practice this is usually not an major issue. But some images have light surrounding textures and so show the slightly darker banding or the distinct line boundaries clearly.

Any clues? Your thoughts are appreciated.

Thanks, Mike

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Joseph Owens
Re: Power windows with concentric banding?
on Nov 28, 2016 at 4:49:18 pm

[Michael McCune] "In practice this is usually not an major issue."

It would be a major issue, though. Banding, level discontinuity, is normally associated with lower density chroma-sampling and is very common with 8-bit codecs and monitoring. It is not inherent in Resolve process computations which are carried out in 32-bit float.


"I always pass on free advice -- its never of any use to me" Oscar Wilde.

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Michael McCune
Re: Power windows with concentric banding?
on Dec 2, 2016 at 7:28:37 am

Thanks, Joseph. Yes, I understand this issue. Had some of it in the past with hd from a Canon 5D.

But firstly this is uhd recorded prores 422 and also I wasn't clear about the banding. You clarified with the expression, "discontinuity" for that problem.

This it's not a discontinuity characteristic of macro-blocking. Rather it is a band with a continuous shift from light to dark to light. My apologies for the lack of clarity.

Since then I have wondered if this is an artifact arising from interaction with other effects or keys.

But no clear answer yet.

Thanks again for the post.


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Marc Wielage
Re: Power windows with concentric banding?
on Dec 3, 2016 at 3:33:47 am

But is the 422 material 8-bit or 10-bit? If it was 8-bit, you could see some weird effects like this.

My advice would be to take a grayscale ramp, make a compound clip out of it, and apply that same window and see how it looks on the scopes. If there's any "stepping" or any other kind of artifact, you'll see it there. Sometimes, a solid color gray screen can show similar problems if they exist.

Also, try a new version, blow out the correction, and create a new correction and a new window. I have seen rare cases where a node becomes corrupted and starts exhibiting unpredictable behavior. It's very rare and unusual, but it can happen.

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