How Blending Modes Work

For many readers, it may be enough to work through the book, looking at the various blending modes and how they can be used effectively for a multitude of recipe combinations. That's absolutely fine. However, if you would like to know something more about how the blending modes work, this is the section for you. It is a survey of how blending modes are made and calculated and, unsurprisingly, it involves a little bit of math. But don't let that put you off I

Base and blend colors

Photoshop's documentation usually describes blending modes in terms of base and blend colors For instance, this is Adobe's definition of Color Dodge.

"Looks at the color information in each channel and brightens the base color to reflect the blend color by decreasing the contrast Blending with black produces no change "

Let's clarify what this actually means Imagine a two-layer image The bottom layer is the base, and the top layer (also known as the blend layer) contains the blend color Changing the blend layer's blending mode affects how Photoshop renders colors that have come from the underlying image In Color Dodge's case, if the top layer is black, the underlying image is unchanged This is what is meant by "blending with black "

This is a single pixel. Color Dodge makes Red and Green values brighter, but because the Blue channel's blend value is Black, the Blue remains unchanged.

Layers

Channels Paths

Color Dodge

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Master opacity and fill opacity

The Layers palette has two opacity sliders The top one is labeled "opacity" and is called the master (or layer) opacity It controls the percentage opacity of the entire layer, including any blending modes or layer styles The lower slider is the fill (or interior) opacity and affects pixels painted on a layer without affecting the opacity of any layer effects

Adjusting these sliders affects some blending modes differently The effect of the Dissolve blending mode isn't visible at all until you reduce either of the opacity percentages below 100% On the other hand, when the fill opacity is reduced, the Hard Mix blending mode behaves differently than when you reduce the layer opacity Most recipes use one or the other of the opacity sliders to fine-tune the result

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interior opacity for the layer

Changing the master and interior opacities affects each blending mode differently.

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Blending modes and math

Adobe doesn't publish precisely how blending modes are calculated, but if you are interested, it is possible to work out what each blending mode is doing Some websites document the mathematical formulas

The Color Dodge calculation is a simple example For each RGB channel, Photoshop calculates an output color by dividing the base color by the inverse of the blend color To check this for yourself it's best to work with color values on a o-i scale where o is black and white is equal to 1

In Color Dodge's case, if the blend color is black or o, then the base color will be divided by 1, so Photoshop outputs it unchanged. You can prove all the other blending modes in this way.

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Create a simple two-layer RGB image and fill the base layer with one color, the top layer with another, and change the top layer's blending mode. Then use the Color Picker tool to sample the resulting color.

How Hue works

While Color Dodge is based on dividing one color value by another, Hue belongs to another group of blending modes where Photoshop calculates the output color by maintaining the base layer's "luminosity " Based on the eye's sensitivity to the colors, Photoshop calculates the base color's Luminosity as 30% Red, 59% Green, and 11% Blue and then creates an output color with the same overall Luminosity

0.3*111 0.59*105 0.11*32 Luminosity 99

RGB: 111 105 32

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RGB: 251 233 30

0.3*150 0.59* 71 0.11*108 Luminosity 99

RGB: 150 71 105

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