Wondering how to blend splodgy colors into uniform color photoshop

Question

Hi,I am setting up a business, aiming to do scanning of artwork, photos etc at clients homes. One of my scanners is a Fujitsu SV600 overhead scanner.  I can get good results from the scanner using an advanced ICC profile, with Levels. The trouble is that the scanner picks up every last crease and imperfection in the paper on artwork and on some non-white paper scans the colours come out blotchy. I have a Spectro 1 Pro spectrophotometer to get exact results on the background colour (in conjunction with a Hue/Saturation layer or Replace Color command) but in using these techniques, the colours remain blotchy. What i want to do is unify the blotchy colours with a single new colour (as proposed by the spectrophotometer on the original artwork) without upsetting any of the detail in the artwork (drawing, painted subject). I have found using Gaussian blur and the clone tool to be too invasive of the artwork. I wondered if the high pass filter could be utilised or ideally Color Range in the Select menu or a gradient map for just the background colors. If you know the answer to this question, please guide me through it as much as possible as I am a bit of an ignoramus on the more complex sides to photoshop. I tried a technique used for removing skin blemishes but it didn't have much of an effect. Many thanks

Conrad_C · Accepted Answer

One way to reduce blotchiness is to reduce the variance of colors there. For that, you might also want to look into the range-adjustable color-targeted corrections in Adobe Camera Raw. For example, the Point Color adjustment lets you sample a color and shift its hue/saturation/luminance, but it also lets you customize the HSL range of colors affected, and the new Variance option lets you make the affected range of colors more or less similar. It’s definitely complicated, but you can watch the video by Julieanne Kost that walks you through how you would use the Variance option to make the targeted colors more similar or less similar.

Not sure how off-white the papers actually are, but if you need to reduce or remove the overall color cast, one quick and easy technique used in video editing is to add a Solid Color Fill layer, set it to the color you want it to removed (for example, by sampling it), then set the blending mode of that layer to Divide, as shown in the demo below. This won’t necessarily remove all the blotchiness if the range of blotchy colors is wider, but at least it can cut down on the manual retouching/correction you have to do.

In the demo, the reason I have to hide the Solid Color Fill layer right after creating it is so I can sample the yellowed paper color in the layer behind it.

davescm · Answer

You might want to consider using Frequency Separation to separate the low frequency colours and the high frequency detail onto separate layers. This would allow you to treat each separately.

The detailed steps are set out below and, of course, they can be built into actions:

1. Make 2 duplicates of your image layer

Name the lower - "Low frequency"

Name the Higher - "High Frequency"

2. Select the Low frequency layer and use Filter Gaussian Blur (around 6.0)

3. Select the High frequency layer
Go to menu Image > Apply Image
Set the source layer to Low Frequency
Set the blending mode to Subtract
Set Scale to 2 *
Set offset to 128 *
Click OK

4. Change the blending mode of the high frequency layer to Linear Light

5. Now you can clone out blemishes on the high frequency layer and lighten or darken areas and smooth out variations on the low frequency layer. Don't forget though to set tools such as colour sampler, or spot healing, to sample 'current layer'.

* Note - the above steps are for 8 bit/channel images. For 16 bit/channel step 3 changes slightly

3. Select the High frequency layer
Go to menu Image > Apply Image
Set the source layer to Low Frequency
Check "Invert" next to channel

Set the blending mode to Add
Set Scale to 2
Set Offset to 0
Click OK

Now carry on with step 4 above

Dave

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