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The quick de-grunge trick
Posted by: byRo on 12-31-1969.

"A thought came to me while planning this tutorial. The 3-D modeling folks who try to mimic real skin have their work cut out trying to introduce random imperfections, blotches and general grunge into their perfect skin – all to get it more human. We are going the other way, and this technique is actually an inversion of one of the 3-D techniques."


This tutorial will show a very quick way to acheive smooth skin, but without losing all of the texture (bottom-right, if you're in doubt!).
Many thanks to NancyJ who found this image at Stock Exchange.
Copyright stuff. This image is not liberated for commercial use.

The Basics

If you had the patience to read through the other tutorials in this little sequence (“High-pass / Gauss” and “The Radius”) and, even better, if you worked through the examples, you will remember that any image can be thought of as a combination images at various frequencies.

You (should) have seen that combining the Gaussian blur of an image with the High-pass of the same image, at the same radius, will get you back to where you started.
No, that’s not a waste of time because you can control the separate parts.

For the technique here we are going to divide the image in three parts:
1) A blurred part;
2) A part with the fine details;
3) A third part in between the others

This third part is the interesting bit because if we can separate some aspect of the image here we can control its appearance independently of the rest of the image.


Have you ever contemplated skin? Really looked at it? If not, you should do – just be careful in public, OK?

Skin will follow the general forms of whatever it is covering; there are some tiny pores and maybe some hairs; and then there are blotches and grunge.

In retouching we must retain the general form, we want to retain the pores and hairs but we don’t want the blotches.

Yeah, you’ve already seen where we're going, let’s make:

1)    A blurred part with the general form;
2)    A part with the details of pores and hairs;
3)    An in between part with the grunge.

… and then kill the grunge!

Separating the parts

Let’s work through this together. First make three duplicate layers of your original.

1)    Blurred part

Run a Gaussian blur changing the radius until the grunge just becomes invisible. Be careful here, getting the radius right is very important.

For the image here I used a radius of 5.1, but remember every image will be different.

Remember this number, you’re going to need it.

2)    Detail part

On the second duplicate run the High-pass filter and look for a radius where the details are visible but the grunge has disappeared.

(The image has been auto-levelled to be more visible)

This isn’t always easy and often I’ll fall back on “Plan B” which is to just divide the Gauss radius by 3. Here a radius of 1.7 looks about right.

3)    The Grunge

On the third duplicate run the High-pass filter at the first radius (5.1) and do a Gaussian blur at the second (1.7)

(The image has been auto-levelled to be more visible)

Now let’s organize the palette. Put the blurred part just above the background layer, next put the Grunge and put the Details on top.

For the Grunge and the Details layer set the blending to Linear Light and the opacity to 50%.

And what do you have – the original again! Duh…..

No wait!

Degrunging the skin

Turn off the Grunge layer and you have some nice pretty skin, and some very ugly outlines.

OK, turn the Grunge back on.

Add a white (see-all) mask to the Grunge layer and paint with a soft black brush where you want to eliminate the grunge – but keep away from the outlines!

Yeah, that’s cool but do I have to do all this every time?

No, that was just to learn why this works. Let’s do it the quick way now.

The Quick Degrunge Technique

(at long last)

1)    Make a duplicate layer;

2)    Open the Gaussian blur filter and change the radius until the grunge just becomes invisible. Be careful here, getting the radius right is very important. Note the radius and cancel the filter;

3)    Apply the High-pass filter at the radius you just noted down;

4)    Apply the Gaussian blur on this layer at 1/3 of the radius;

5)    Invert the layer (<Ctrl><I>), set the blending to Linear Light and the opacity to 50%;

6)    Apply a Hide-all mask and paint white where you want to degrunge.

Why is this the same as the 3-layer technique?

If you’re afraid of math(s) look away, now!

Let’s call the three layers Blur, Grunge and Detail.

We saw that:
B + G + D = Original

When we took away some grunge we did this:
B + (G – bit of G) + D = Nice Skin

Rearranging the terms:
B + G + D – bit of G = Nice Skin

Original – bit of G = Nice Skin

So we don’t need the Blur and Details layer at all, just subtract a bit of Grunge from the original.

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