Lab Curves not precise RESOLVE 14 [BUG]

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Jean Paul Sneider

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Lab Curves not precise RESOLVE 14 [BUG]

PostWed Jul 05, 2017 5:52 pm

I found out the Lab curves in Resolve are not correct/simmetrical.

Procedure:

Create solid color with generator. Create a perfect neutral.

With the help of color space transform and splitter combiner reveal the the Lab channels.

Change simmetrically the slope, moving the endpoints only, in channels "a" and "b". A perfectly simmetrical action would keep the value of the color generated neutral.

With the help of RGB picker I achieve neutral. (Helped with Histogram Scopes).
When a neutral value is achieved, in all the 255 values of luminosity, the curves are blatantly not simmetrical.
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Tom Early

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostWed Jul 05, 2017 9:16 pm

It's the same for rgb, yuv and hsl. Top anchor point is wrong too. Same for all versions of resolve
Do I love colour grading or I am simply tormented by it?
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waltervolpatto

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostWed Jul 05, 2017 11:53 pm

Why you're expecting to be symmetrical?
Walter Volpatto - C.S.I.
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Jean Paul Sneider

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostThu Jul 06, 2017 12:25 am

Tom Early wrote:It's the same for rgb, yuv and hsl. Top anchor point is wrong too. Same for all versions of resolve

Didn't know, wasn't able to check it. For older version, I just recently moved deep into Lab colorspace use.

waltervolpatto wrote:Why you're expecting to be symmetrical?


Well.. cause they are supposed to be. With neutral point in the middle between green/magenta and blue/yellow.
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Rohit Gupta

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostThu Jul 06, 2017 12:24 pm

Set the Make Lum control is Primary palette to 0. This let’s you grade in 3 channel mode vs 4 channel mode. Can you confirm if this gives you the behaviour you expect?
Rohit Gupta

DaVinci Resolve Software Development
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Jean Paul Sneider

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostThu Jul 06, 2017 6:20 pm

Rohit Gupta wrote:Set the Make Lum control is Primary palette to 0. This let’s you grade in 3 channel mode vs 4 channel mode. Can you confirm if this gives you the behaviour you expect?


Not sure what you mean,
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Dwaine Maggart

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostThu Jul 06, 2017 9:29 pm

On the Color Wheels palette in the lower left corner of the Color page, in either the Primaries Wheels or Primaries Bars mode, in the bottom right corner of the palette is a setting called Lum Mix. The default value is 100. Change that to 0.
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Jean Paul Sneider

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostFri Jul 07, 2017 9:19 am

Dwaine Maggart wrote:On the Color Wheels palette in the lower left corner of the Color page, in either the Primaries Wheels or Primaries Bars mode, in the bottom right corner of the palette is a setting called Lum Mix. The default value is 100. Change that to 0.


No it doesn't solves the problem.
To be more precise if I use the lateral slider that inverts the curve, that should keep neutrals as they are changes them a lot.
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PeterMoretti

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostFri Jul 07, 2017 9:48 am

Tom Early wrote:It's the same for rgb, yuv and hsl. Top anchor point is wrong too. Same for all versions of resolve


Tom, can you be a bit more descriptive? While it sounds important, I really don't understand what you're saying.
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PeterMoretti

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostSat Jul 08, 2017 5:46 am

Jean Paul Sneider wrote:I found out the Lab curves in Resolve are not correct/simmetrical.

Procedure:

Create solid color with generator. Create a perfect neutral.

With the help of color space transform and splitter combiner reveal the the Lab channels.

Change simmetrically the slope, moving the endpoints only, in channels "a" and "b". A perfectly simmetrical action would keep the value of the color generated neutral.

With the help of RGB picker I achieve neutral. (Helped with Histogram Scopes).
When a neutral value is achieved, in all the 255 values of luminosity, the curves are blatantly not simmetrical.


Jean, can you explain more what you are saying? Starting with how you are color correcting a generator, AFAIK Resolve does not allow that? And maybe include some screen captures?

Thanks!
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Jean Paul Sneider

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostMon Jul 10, 2017 3:16 pm

PeterMoretti wrote:
Jean, can you explain more what you are saying? Starting with how you are color correcting a generator, AFAIK Resolve does not allow that? And maybe include some screen captures?

Thanks!


Sure. So:

Neutral and space transform.png
Neutral and space transform.png (182.88 KiB) Viewed 212 times


I choose a random clip.

Node 9 I apply a color generator 120 gray. (as shown by the RGB picker value.

Node 2 Color space transform from 709 to Lab.

Node 8 the opposite action Lab to 709

In between a splitter combine with channels L, a and b.

MOdification  b Channel  keeping neutral.png
MOdification b Channel keeping neutral.png (178.05 KiB) Viewed 212 times


With the help of the histograms and the rgb picker value I modify endpoints of the curve keeping the neutral. As shown it isn't symmetrical.

Non neutral.png
Non neutral.png (177.47 KiB) Viewed 212 times


Trying to keep the symmetry it creates an obvious cast
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PeterMoretti

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostMon Jul 17, 2017 3:33 am

Jean Paul, I've replicated what you're describing. Thank you.

I have to say that I'm not familiar with the Color Space Transform FX. It might be doing something wrong, as it adds a cast as soon as I apply it, but I really don't know. Because when I add it again to reverse the transform, the image again looks normal.

But I've found when just changing a node's color space to LAB and not using a splitter combiner, that no cast issues seem to present themselves.

I am curious as to why you aren't using that approach. I guess I don't understood why people use splitter combiners with LAB. Don't the individual R, G and B curves map to L, a and b anyway? Why do people go through the trouble of splitting out and disabling the channels?

And thanks again for the detailed reply above!
Attachments
LABSymetry.png
LAB a and b changes look symmetrical w/o adding color cast.
LABSymetry.png (100.75 KiB) Viewed 133 times
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Jean Paul Sneider

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostMon Jul 17, 2017 3:04 pm

Like sometimes you want to work on a single channell in RGB, for example for controlled noise reduction, or recover details mixing channels for a bad video, or targeted masks, or use channels as masks.
In general the splitter combiner gives you more flexibility and powerful tools, but they need to act reliably, even more in lab than in RGB.

Too bad Dwaine Maggart seems to have left the conversation.
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PeterMoretti

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostMon Jul 17, 2017 9:55 pm

But what does using a splitter combiner give you that just using unganged R, G and B curves don't give you?

(My hesitancy with using splitter combiner with LAB and disabling channels is that I don't know changes in the in Y curve will map in the a and b channels' nodes. As I'm not sure what that means to increase the luminance of an a or b channel when luminance in LAB is a separate channel and defined differently than it is in RGB or YUV. )
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Jean Paul Sneider

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Re: Lab Curves not precise RESOLVE 14 [BUG]

PostTue Jul 18, 2017 12:03 am

For example selective control on opacity, or channel mixing to restore heavily casted videos. Being able to apply diffent masks to L and ab channels.
You can do that with Some parallel combiner, but you get a messier node tree and still wouldn't be the same then a splitter combiner.

The Y works a a master control since the a and b become black and white.

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