Introduction to Frequency Separation

October 15, 2020  •  Leave a Comment

Frequency separation for touching up faces has gotten quite a buzz over the past several years.  I still think it's a great way to retouch portraits, particularly if there are some abnormalities that need to be removed.  This article looks at the method and the way I use it for that purpose.

We’ll first look at what the technique actually does. The entire premise of Frequency Separation (FS) is to separate the texture and color into two separate layers. By doing this you can work on each layer separately without affecting the other.

After I have done my global adjustments and any local tonal adjustments I am ready to work on the skin of the subject. It’s important to not do your dodging and burning on the skin until after applying FS. Otherwise, the adjustments you make to the color layer may affect those changes.

We’re going to use the image of the basketball player below. His complexion isn’t what he’d like so we’re going to smooth it out so something more pleasing.

Before Frequency Separation

The first thing you should do is to make sure all of the adjustments are consolidated into a single layer. To do this you should create a Merge Stamp layer by pressing CMD-Option-Shift-E, or Ctrl-Alt-Shift-E on a windows version. This will put a consolidated layer at the top of the layers palette. Now make a copy of that layer. Your layers palette should look something like the image below.

Layers After Merge Stamp and Copy

You should now have two additional layers at the top of the layers palette. To make it easier for me to follow I like to rename the layers and then group the FS layers together. Double click on the label of the top layer and it will be highlighted in blue. Then change the name to “Texture”. Now go to the second layer and do the same thing but change the name to “Color”. After that highlight both the top and middle layer and press CMD-G, or Ctrl-G in Windows. This will place the layers in a group. Double click on the group name and change it to “Frequency Separation”. Click on the “>” symbol next to your group to expand it. Your layer palette should now look something like the below image.

Grouping the Layers

Now that we have the layers ready, we need to be sure the Texture layer only has texture of the image and the Color layer has everything except texture.

We’re going to start with the Color layer. First, turn off the Texture layer. It’s above the Color layer, so if you don’t turn it off you won’t see any affect on the Color layer. Turn the Texture layer off by clicking on the “eyeball” next to it in the Layer palette. Now highlight the Color layer and select Filter->Blur->Gaussian Blur from the menu. Move the Radius slider all the way left to 0. Now slowly move it to the right until you no longer see texture in the skin. Remember, the purpose of our Color layer is to have everything except for texture. In our case I moved the slider to 2.7. See the image below.

Removing Texture from the Color Layer

Now we need to remove everything but texture from the Texture layer. The way we do this is to subtract the Color layer from the Texture layer. How? By using the Apply Image function.  There are other methods to separating these two, but using the Apply Image function is how I learned so that's how I will show you.

Turn the Texture layer back on by clicking on the eyeball next to it in the Layers palette. Now click on the layer to make it active. From the main menu select Image->Apply Image to open the dialog box. The parameters in the dialog box depend on whether your image is 8-bit, or 16-bit. I normally process in 16-bit, but I will show you the dialog box with parameters for each just in case your image is 8-bit. See images below.

8-bit Setup


16-bit Setup

It’s important to note that the Layer selected in the dialog box is the “Color” layer because it’s the layer you are comparing to. Also, for the 16-bit be sure to click on the Invert checkbox.

When you apply this, by clicking on the OK button, your image should be turned to grayscale. If not, you have done something wrong. See image below.

Grayscale After Apply Image

Finally, go back to the layers palette and change the blending mode of the Texture layer to “Linear Light”. Now you should see the image as it was before doing anything with FS. That’s because the Texture layer is only texture and the Color layer is everything else.

Now we are ready to begin retouching.

The first thing to do is to remove the blemishes. We will do this on the texture layer by using one of four tools, the Clone Stamp tool, the Healing Brush tool, the Spot Healing Brush tool, or the Patch tool. Any of these will work, but my preference is the Healing Brush tool. With it we select a good texture and blend it on top of the blemish. The Clone Stamp tool completely replaces the blemish with the texture, where the Healing Brush tool blends it. That’s why I like it best. But, as I said, any of these tools will work.

Click on the Texture layer to make sure it is active. Select your tool of preference from the tool bar for removing the blemishes. Be careful to go over them individually and sample an area close by. Your sample should be about the same size as the blemish. Don’t try to remove the red blotchiness because we are only working with the texture. The red will be removed in a moment. Once you are done the face should look something like this.

After Blemish Removal

You can see that we still have red spots where the blemishes were. To remove those we will go to the Color layer. The objective here is to go around the red spots and blur them out using the Gaussian Blur filter. You can use any selection method, but I like to use the Lasso tool. With it I just draw a circle around the red spot and use the filter to blur it. With whatever selection method we choose, we should also look to feather the edges of the selection. This will blend the blur (color correction) into the skin more evenly. I usually use a feather of 15 pixels. Be aware that when you feather the selection it will shrink from what you actually draw around the blemish, so you may want to draw a little bigger that you actually want. See below how I make my selection.

Selecting Blemishes with the Lasso Tool

Now I want to remove the blemish by removing the red color. To remove the red we use the Gaussian Blur filter. With the selection made around the blemish just go to the main menu and choose Filter->Blur->Gaussian Blur. Now move the radius slider to 0 and then slowly to the right until the red in the blemish disappears. Be careful, if you go too far the skin will begin to turn dark. See below how I removed the blemish with the Gaussian Blur filter.

Red Color of Blemish Removed with Gaussian Blur

Look at the selection and see how the red of the blemish is now gone. I slowly moved the Radius slider to the right until the color was gone. In this case I used a Radius of 9.5. Also, notice that inside the selection the texture remained even though we used a blur filter. That is because the Texture layer above includes the texture and is unaffected.

Now just go around the face and make selections on each blemish and blur it to remove the red color as indicated above. You can use larger selections for larger areas, but I would encourage you to be fairly specific. There is no reason to blur regions that don’t need it. However, with females you may want to blur a little extra and in areas that don’t really need it to soften the skin. Just be careful not to go too far and give it dark splotches.

Once you have completed with this step you should have something that looks something like the below image.

Final Image After Frequency Separation

We focused here on removing blemishes, but you could also use this tool remove other imperfections such as scars or rashes. The idea is to get the subject to look normal as if the “unusual occurrence” of this skin abnormality hadn’t happened.

The technique looks fairly complicated, but once you have done it a couple of times it will become fairly easy. Below is a comparison of the before and after so you can more easily see the change that was made.

                                                                     Before Any Adjustments                                                     After Frequency Separation

I truly hope this has been helpful with understanding how this technique works. Please don’t hesitate to contact me if you have any questions.

Happy Shooting
 

 

 


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