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Discovery of technique for setting shadow and highlight points needs verification

New Here ,
Sep 19, 2020

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I scan and edit artwork (among other things) and made this discovery by accident. I was trying to find a method of setting an exact shadow and highlight point for artwork that had no black or white in it. I have since found that using auto levels will set a rough estimate of a shadow/highlight point in these cases but my technique, in general seems to set them better. However I need verification (preferably mathematical) to prove this is a legitimate technique.

First, I create a curves layer and use auto curves to estimate overall brightness and contrast. Then I create a second curves layer using monochromatic contrast. I have found that the technique sometimes works best using the default clipping and sometimes best with the clipping set to zero. At this point the image does not look identical to the original at all.

I then delete the original brightness and contrast auto curves layer and boom! the image looks identical to the original for shadow/highlight. I have noticed on the histogram that the auto levels setting for shadow in these cases (no black in the image) always sets itself on 25 however, with my technique, the shadow point varies depending on the artwork.

Please can someone tell me if this technique has any legitimacy or have I been very lucky although I find it hard to believe. The auto levels will not always completely remove a white cast from the image whereas with my technique, the cast always seems to be removed. There remains the issue of whether or not to include clipping on the monochromatic contrast layer which at present I am unable to resolve except by trial and error. Can any Photoshop experts help me with this please?

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Discovery of technique for setting shadow and highlight points needs verification

New Here ,
Sep 19, 2020

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I scan and edit artwork (among other things) and made this discovery by accident. I was trying to find a method of setting an exact shadow and highlight point for artwork that had no black or white in it. I have since found that using auto levels will set a rough estimate of a shadow/highlight point in these cases but my technique, in general seems to set them better. However I need verification (preferably mathematical) to prove this is a legitimate technique.

First, I create a curves layer and use auto curves to estimate overall brightness and contrast. Then I create a second curves layer using monochromatic contrast. I have found that the technique sometimes works best using the default clipping and sometimes best with the clipping set to zero. At this point the image does not look identical to the original at all.

I then delete the original brightness and contrast auto curves layer and boom! the image looks identical to the original for shadow/highlight. I have noticed on the histogram that the auto levels setting for shadow in these cases (no black in the image) always sets itself on 25 however, with my technique, the shadow point varies depending on the artwork.

Please can someone tell me if this technique has any legitimacy or have I been very lucky although I find it hard to believe. The auto levels will not always completely remove a white cast from the image whereas with my technique, the cast always seems to be removed. There remains the issue of whether or not to include clipping on the monochromatic contrast layer which at present I am unable to resolve except by trial and error. Can any Photoshop experts help me with this please?

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How to, Performance

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Sep 19, 2020 0
Most Valuable Participant ,
Sep 19, 2020

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If you are interested in locating and marking the tonal range extreme points in artwork so that you may place those two points on a Curve for adjustment consider the following technique:

 

From the symbols at the bottom of the Layers panel, select Adjustment Layer (half-black-half-white circle) and from its drop-down menu choose Threshold. You will be presented with a histogram, and below it, a slider. Move the slider all the way to the right; the image will go completely black. Inch the slider to the left until the first white non-spectral patch appears. It is the location of the highlight extreme. Choose the Color Sampler tool. Click in that highlight location. The Color Sampler will mark the site.


Next, move the slider all the way to the left; the image will go completely white. Inch the slider to the right until the first meaningful black patch appears. Choose the Color Sampler tool. Click in the shadow location. The Color Sampler will mark the site.

 

You no longer need the Threshold layer, so drag it to the Trash. The image with the points you marked will reappear. You have now successfully located and marked the art end points.  To place those points on a curve, choose the Image > Adjustments > Curves. Use the Eyedropper tool to Cmd-click on each of the points shown on the image.

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Sep 19, 2020 1
Adobe Community Professional ,
Sep 19, 2020

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Don't know if this will help now, but for art reproduction there's no way around a colorchecker. Each patch has standardized Lab values (convertible to any RGB color space), and you use the six neutral patches to set endpoints and basic contrast curve:

colorchecker_1.jpg

colorchecker_2.jpg

 

There's one little gotcha, if the end result is a printed paper copy: you need to compensate for paper color and maximum ink in your print process. They set their own endpoints and compress available contrast range a bit. In practice this means you'll need to push the endpoints a little bit, increasing contrast. If you don't do that the result will look a little flat. But exactly how much depends and you'll have to work it out case by case.

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Sep 19, 2020 0
New Here ,
Sep 21, 2020

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Thank you to both contributors. Much appreciated, however, I am still keen to know if my discovery is legitimate or just a theory. Will keep looking.

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Sep 21, 2020 0