Additive Mode In Photoshop
By Neil Blevins
Created On: Sept 25th 2005
Updated On: Mar 13th 2008

One thing that appears to be missing in Photoshop layers is an additive mode. The idea behind additive is that whatever color is in layer 1 will be added to the color in layer 2 to get your final color. So for example, if you have a 25% grey color on layer 1, and a 50% grey color on layer 2, then the final color will be 75% grey, because 25% + 50% = 75%. The is useful for bump maps, if say you're doing a lizard, and you have the scales of the lizard, and then on top of the scales you want a rough texture, you can take your scale bump map, and then add to it the rough bump map to achieve your final bump map.

Anyways, Photoshop does in fact have an additive mode, sortof, it's called "Linear Dodge" (In CS3, they have actually renamed this mode to "Linear Dodge (Additive)").

Why is it called "Linear Dodge" instead of just "Additive"? Well, because it's only additive as long as you don't bring Opacity into the equation.

Look at the following diagrams made in photoshop.

The first diagram is 2 layers, the first is 25% grey, the second is 12% grey (these are colors, not Opacity). The second is set to linear dodge. The color in the middle is 37% grey, which is what you'd expect. The same with the second diagram.

However, look what happens if you bring opacity into the equation. A white circle that's 50% opaque returns a color of 50% grey. Then you add that to a white circle with a opacity of 12% (which returns a color of 12% grey), and you get 56% grey? It should equal 62%? Basically, if you use opacity on your layers (including the antialiased edge around anything on your layer), these return values that do not mathematically follow the additive rule. And that's why its called "Linear Dodge" instead of "Additive".

You may notice that as well as Opacity, a layer has something called a Fill value. According to the photoshop manual, when you reduce the opacity of a layer, it also changes the amount of whatever blend mode (multiply, linear dodge, etc) is applied to the layer (this claim is not verified, if I try the same experiment with Multiply, it seems to respond correctly to Opacity). Fill does indeed work as expected. So in the example above (3rd set of circles), if you reduced the fill value to 50% and 12%, you will indeed get a value of 62%.

So that's great, all we need to do is use "Fill" instead of "Opacity", right? Well, not exactly. One of the problems with Fill is it only affects the original object on the layer. So, if you use say a Layer Style like Outer Glow on your layer, fill will not affect the Layer Style. Whereas Opacity does.

Adobe AfterFx on the other hand has a true additive mode, which does a real mathematical add, even when dealing with opacity.

So in practical terms, if you're not using opacity, "Linear Dodge" is an "Additive" mode. If you are using opacity, you won't get a mathematical add, but you'll get something close. You can also choose to use Fill to get true additive, but then you can't use stuff like Layer Styles. So I can still recommend using "Linear Dodge" if you want to add two colors together in photoshop, just note you may not getting mathematically perfect results. Thanks to Tuomo Kulomaa for helping me with the opacity experiments above.

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