Precise color correction - using a script to adjust white balance [title edited by moderator]

If the objective is to locate and select those areas in an image that measure highlight values of 5% or less, the first step is to create a 5% value. Since the Lab Lightness scale extremes are 0 to 100, begin by creating the value in Lab on a transparent background, then changing the Mode to RGB. You have to do this only once. It becomes the Reference Patch for all images to be tested.

1. Create 1”x1” patch RGB, Background Transparent
2. Image > Mode > Lab Color
3. In the Info Panel, change one to Lab by clicking on it eye dropper. Lab range is zero percent for black, and 100% as pure white. We want a patch at 95%.
4. In the Color Picker Lab section, set the L to 95, a and b each to 0.
5. Edit > Fill the patch with that Lightness value. It is your 5%.
6. Return to RGB. You now have the Reference Patch   

To use it:

Open an image and add 1 or more inches to one of the sides of the canvas. (Fig 1)

Drag the Reference Patch into that area as an added Layer.  (Fig 1)

Add a Threshold Adjustment Layer, and move its slider all the way to its right (Fig 2)

Drag the slider slowly to the left until the Reference Patch disappears you have now found the areas <= 5% lightness. (Fig 3)

Use the Magic Wand, 3x3 average, 0 Tolerance, Not Contiguous to click on a white area. You have selected the desired areas of lightness. (Fig 4)

Trash the Adjustment Layer. The Selection will appear on the image.

You can Cmd+J to put it on a separate layer, and/or change its value.

Last, return to the original crop.

Hi

I edited your title - there are some script specialists on the forum - changing the title will make it easier for your request to be discovered by those people.

Hope you get an answer soon. 

I wonder if a Photoshop action could do it? Creating an adjustment layer with "apply if" to set the range.

Not my speciality but it may be worth researching.

I hope this helps

thanks
neil barstow, colourmanagement.net :: adobe forum volunteer
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