I am excited to share a relatively simple and effective technique that I use on a regular basis. This wonderful retouching technique is called Frequency Separation and can be mastered in Adobe Photoshop. Interestingly, Frequency Separation has a lot in common with Fourier Transform.
Fourier Transform (symbol ℱ) is an operation that associates one function with a real perennial function. This function is a new function of coefficients ("amplitudes") when the original functions are decomposed into component elements - harmonic vibrations with different frequencies (just as a musical chord can be expressed as a sum of musical sounds that make it up). (Wikipedia)
In this case, our image will be considered the initial function. In the end, the result of applying this technique should be two layers of the image. A high frequency layer which consists of the textures and fine details of the image, and a low frequency layer which consists of the color and light spots.
When is this technique useful you might ask? and in what areas of application should it be used?
In the most obvious situation, Frequency Separation is super effective when we need to smooth out shadows “under” the texture. I have used it incredibly often when retouching clothes for various online stores. In the case that we have sharp and/or strict textures, as you can see in the example below.
This technique is also very effective when only texture imperfections need to be corrected, at the edge of a light or color gradient. For example, in the specific spots on the images below.
and last but not least! When working with noisy images, retouching is much more convenient with this technique, and the healing brush and stamp tools are much easier to use when used to edit split frequencies.
Noise is actually a small texture, and when we use the healing brush or patch tools on it, we may experience inconsistencies and all sorts of artifacts at the edges of the brush, due to the fact that the algorithm cannot calculate how to match the texture. But when we separate the image into frequencies, all the noise remains at the upper frequency and we can correct color and light spots without any problems. And in my opinion, working with texture is also much easier in this case for Photoshop, perhaps because Photoshop needs to process less information. Below are two examples, I applied 15 noise to the picture. And I made the same movements on the base picture and picture that was decomposed into frequencies.
Try to find the remaining differences yourself! Let's consider the simplest and at the same time most effective method of implementing this technique.
Step by Step
- Open your image in Photoshop. Duplicate the layer with the original image 2 times. (ctrl+j)
2. On the first copy layer, apply the filter “Dust & Scratches ...” with this filter we remove all textures, saving only the shape line of the objects.
This is the setting I used most often in my work.
However, "Radius" value can still change from image to image.
3. For the second copy layer, apply the "Apply Image" function with our settings.
In the "Layer" parameter, specify the layer to which we applied "Dust & Scratches ..." Be sure to tick the "Invert" checkbox.
After applying “Apply Image” you need to change the layer blending mode to “Linear Light”
You have successfully separated your picture into high and low frequencies. I'll tell you a little about how to retouch an already separated image.
When working with high frequencies (the texture layer), I use 2 tools.
Patch tool (J) and Clone Stamp tool (S) - but for it to work correctly, you need to switch the sample parameter to Current Layer.
But with a layer of low frequencies, everything is somewhat more interesting. Since there are no textures and it (for example) looks like this:
We can use a wider range of tools but the same tools Patch (J) and Clone Stamp (S) work quite well as well as a regular Brush (B). I personally, like to create an extra blank layer between the high and low frequencies and work inside of that. This way, I can make as many mistakes as I want.
I pick the color with the Eyedropper (hold down the alt when Brush is selected) and paint with a soft brush on an empty layer. If you make a mistake, you can simply erase this part, but if you make the same mistake on the low pass layer, you only need to roll back the history steps.
Additionally, the higher you set the radius of the "dust & scratches" filter, the less detail remains in the low frequency. Some elements can be located at two frequencies at once after decomposition, as in the example below.
In the upper frequency, in this case we have the texture of the skin, but the color of the red spots will be at the lower frequency. This gives us the opportunity for some extraordinary solutions. Here's a visual example below with a moustache and forehead shadow.
But personally, I do not always use both layers when applying "frequency separation." Often, I use this technique only in order to make color / light transitions smoother. In such cases, you can set radius of the "dust & scratches" filter higher. The higher the value, the less detail will be on this layer. In the example below, the value is set to 24.
Implementing the Frequency Separation technique can be a powerful tool in any designers work. The best way to master this technique is to try it! Get retouching and let us know what you found out in the comments!