# Understanding Blending Models

I decided to write this manual because I read several books about Blending Modes, a lot of blogs in Internet and many magazines, and I found all of them unsatisfactory when not incorrect or misleading. Most of them describe Blending Modes by examples, by the use of recipes on ad-hoc images without actually explaining what is happening behind the scenes. I found myself following a lot of beautiful retouching techniques only to find out that the same steps cannot be apply to my own pictures and unable to change them in order to fit my needs.

Why do we apply the Overlay Blending Mode between an image and a negative blurred copy of the same image? Why after applying a Linear Burn, a Multiply, a Screen Blending Mode on a color portrait we need to apply some sort of correction filters to fix the skin tones that inevitably changes? I found a lot of these tricky steps in many retouching recipes and I always found them very puzzling. So I decided to dig deeper.

This manual aims to solve most of these problems. You will acquire a deep understanding of Blending Modes, you will be able to use them to develope your own style, you will be able to fully understand and manipulate most of the retouching recipes that you find around and you will be even able to reverse engineer an image and extrapolate the steps to get the same result.

You will understand Blending Modes in terms on two simple concepts that will be fully explain later: Curves and the Image Frequency.

What is in simple words a Blending Mode? In a retouching software like Gimp or Photoshop, a Blending Mode is a fusion between two different layers obtained by applying a mathematical formula between every single pixel of one image and the corresponding pixel of another image. Matematically speaking, if x is the color of a pixel in one image, y is a color of a pixel in another image we apply a function f to obtain the color z in the resulting image

$$z = f(x,y)$$

Because an image is usually a composition of three different channels or colors (Red, Green, and Blue), the same mathematical formula is applied independently on the different channels. This means that the Red channel of the first image is merged with the Red channel of the second image, the Green one with the Green and the Blue one with the Blue.

This manual describes Blending Modes first on Black and White images where each shade of gray of a pixel will be considered having a value of 0 for pure Black and 100 for pure White. It will then explain how to apply the same techniques and results on color images using concept like Luminosity and Cromacity.