Today’s Question: On several occasions you have provided guidance on specific uses for blend modes in Photoshop. But could you explain exactly what a blend mode is to being with?
Tim’s Quick Answer: Blend modes are a feature of Photoshop that enable you to combine two or more layers, with specific math being applied to the pixel values of those layers. The blend modes are divided into categories based on their effect, such as lightening, darkening, and adding contrast.
More Detail: I think a couple of examples could be helpful to better understand how blend modes work in Photoshop. Let’s assume a simple scenario where there are two grayscale (black and white) image layers in a Photoshop document, and the top layer is set to a given blend mode. I’ll also present the details here using luminance values for a grayscale image based on a bit depth of 8 bits per channel. In other words, my numbers will be based on a range from 0 for black to 255 for white.
One of the simpler blend modes is Multiply. As the name implies, with the Multiply blend mode each pixel value in the “top” layer is multiplied with the corresponding pixel in the “bottom” layer. The product of that multiplication is then divided by 256 to determine the final value. So, for example, a pixel value of 81 on the top layer and 128 on the bottom layer would produce a product of 10,368, which divided by 256 gives a final result of 41. As a result of this math, using the Multiply blend mode will always result in a darker pixel value, except in the case of white and black, which would remain unchanged.
The Screen blend mode is essentially the opposite of the Multiply blend mode in terms of the net effect. The math involves multiplying the inverse value for each pixel. That means subtracting each value from 256 to produce the inverse values, multiplying the result, dividing by 256, and inverting again (subtracting from 256). So, with the same values as above (81 and 128), the inverse values would be 175 and 128. The product of those would be 22,400, which divided by 256 yields 87.5. Subtracting that value from 256 produces a final result of 168 (based on the closest whole number. The result is that pixel values will become brighter when the Screen blend mode is applied.
Of course, as a photographer working in Photoshop there is generally not much benefit to understanding the math behind each of the blend modes. My point in sharing this info was more to demonstrate that blend modes are relatively simple in concept, applying math equations between pixel values on two (or more) layers. In some cases that math can certainly get quite complicated, so I think in general it is more helpful to understand the basic concepts behind the blend modes.
The first set of blend modes includes the “normal” blend modes. Then come the darkening blend modes, the lightening blend modes, the contrast blend modes, the inversion and cancelation blend modes, and the color component blend modes.
For a little more info on blend modes, you might be interested in the article “6 Favorite Blend Modes” from the March 2013 issue of Pixology magazine.