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In this exercise, I am going to try to give you a better sense of what's going on with blend modes under the hood. Because now that we've seen a few practical applications of the darkening and lightening modes, I want you to feel comfortable sort of perusing the blend modes in general. Now, I am going to show you every single one of them before we're done here. But as you apply blend modes to your own work here inside of Photoshop I want you to be able to do so with a fair amount of authority. So, I've gone ahead and saved my changes as Opposite effects.psd found inside the 28_blending folder.
Let's go ahead and switch my layer Comp. I am going to bring up the layer Comps panel and I am going to click in front of Darken in order to reinstate that version of the composition that's created using the Linear Burn mode. I'll close the layer Comps panel for a moment and I want you to see that four of the modes are named exactly after their equations and they are the most basic equations possible. So, we have Subtract down here which subtracts one pixel from another, we've got Divide and we have Add which is a Linear Dodge mode and then we have Multiply.
So, all your basic arithmetic computations are represented here. The thing is if you just go and apply the Divide mode. So, you are just curious, you know you just got CS5, you're playing around with it, you choose Divide and you go, wholly molly, what in the world is that. I'd never use that mode in the million years as it really giving me brighter results; it's Divide for crying out loud. When you divide one luminance level by another you get a lower number, don't you? In which case it should darken things up and of course we've already seen how Multiply which you would think would give you brighter luminance levels; actually darkens the image and this is typical behavior for both of those modes.
Whereas Add and Subtract behave the way you might think. If you're adding luminance levels you are going to brighten the heck out of the image like so and if you're subtracting one luminance level from another then you are going to darkening it up, fairly considerably in many cases. So, what's going on? Why in the world is Photoshop behaving logically with Add and Subtract and illogically with Multiply and Divide, including by the way, this pervades all the other blend modes as well, because they also include Add and Subtract and Multiply and Divide. Well, let me show you if I may, I've got this what seems to be a terrifying image right here but it actually helps us make sense of things.
It's called mode math.psd again, found inside that 28_blending folder and I am going to go ahead and press Shift+F to fill the screen with the image. So, the name of the slide is Why Add Lightens But Multiply Darkens, and it could just as easily be called Why Subtract Darkens But Divide Lightens. When working with values greater than 1 which is the way we think of luminance levels, right? We think of luminance level of black being 0 and what of white being 255. So, they're all above one except for black and its next door neighbor.
Division Usually Delivers The Smallest Results, Then Subtraction, Then Addition, Ending With Multiplication As Top Dog. For example, let's say we have a couple of luminance levels here 204 and 76. So 76 would be the pixel on the active layer by the way and 204 would be the pixel below, the composite pixel blow. So, we have a very bright pixel below, a darker pixel above. If we divide the two we'll get 3 that's going to be incredibly dark. So, what's with that really bright image we saw just a moment ago, subtract 204 from 76 that will give us is 128, which is medium gray; that make sense.
Add, 204+76 it's going to give us 280, that's beyond 255 so it's going to be clipped to white and then Multiply. Let's go ahead and multiply 204 x 76, we'll get 15,504. I think that safely given that white is 255, it going to get clipped to white. Well, that's not the way it works because blend modes are standardized. Even though Photoshop talks to us in terms of zero as black and 255 is white, it's actually performing its computations in the background. Generally speaking in this much smaller range where Black is 0, so black is still 0, White is 1 however and 50% Gray is 0.5.
So, everything is compressed in this range from 0 to 1. In the tiny realm of 0 to 1, Multiplication and Subtraction Decrease and Division and Addition Increase. For example, think about that. With multiplication, if you're taking a 0.5 x 0.5 you're taking half of half and you're getting a quarter or 0.25. So, our standardized luminance levels would be 0.8 in the case of 204 and 0.3 in the case of 76. So, the active layer has a pixel with the luminance of 0.3 and layers behind have a pixel with a composite brightness of 0.8.
As a result, Multiply produces 0.8 x 0 .3 and you get 0.24 which is darker. Subtract gives you 0.8-0.3 that's 0.5, that's the same. By the way that's exactly the same as we would have gotten with standard luminance levels. Add is going to give 0.8+0.3 which is 1.1, that's also the same as 280 just standardized. So, in other words the standardization doesn't affect Add and Subtract but it does affect Divide, 0.8?0.3 is 2.67 which is nearly 3 times as bright as white, and therefore gets clipped to white.
So that's what's going on under the hood. For a practical application of this information, stick with me.
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