Alpha channels are collections of luminance data that control the transparency of images, and they influence just about everything in Photoshop. Coming to terms with alpha channels, also known as masks, is a surefire way to maximize results. Omni Award-winning Photoshop expert Deke McClelland leaves no pixel unturned as he explores Refine Edge, Color Range, the Channels palette, the Quick Mask mode, channel masking, blend modes, and more. After watching Photoshop CS3 Channel and Masks: The Essentials, even the most complex techniques will seem like child's play. Exercise files accompany the tutorials.
Download Deke's customized keyboard layouts for Channels and Masks from the Exercise Files tab."
In this exercise we're going to have a very basic discussion about what's going on with blend mode math. Now I'm not going to be talking about every single one of the equations and by the way, some of the blend modes have two equations actually, all of the contrast modes do, but we're not going to go into that, that was just a little FYI in case you're curious. We're going to be talking about why it is, specifically that the Linear Dodge mode, which adds luminance levels brightens the image whereas the Multiply mode, which multiplies luminance levels ends up darkening the image. last I checked, when you add two numbers they get bigger, but when you multiply two numbers, they even get bigger than that. So why doesn't multiply end up brightening colors as well. Well, let's go ahead and switch to this image right here, which is called Mode math.psd and I'm going to go ahead and switch to the full screen mode so that we can see the slide all by itself.
So here it is, 'Why add lightens, but multiply darkens.' When working with values greater than one which is what we're normally used to, right, division usually delivers the smallest results, then subtraction and we're just talking about basic arithmetic here, we're not talking about square roots and logarithmic operations and exponents and bla, bla, bla, just divide, subtract, add and multiply. Division is the smallest, subtraction is next, then addition and then we end up with multiplication as the top doc, so in other words, you multiply two numbers, you're going to get the biggest result. For example, let's say we have a couple of numbers, let's call them 204 and 76. Where did I get those numbers, you'll see, but anyway, 204 and 76. If we were to divide them, 204 divided by 76, we get approximately 3, it's actually 2.9 something, but let's call it three. Subtract, if we were to take 204 and subtract 76 we get 128, so medium gray, oh, so we're talking about luminance levels in this case. Fine, so that would be the equation right there. If we were to add them, 204 plus 76 we get 280, 255 is white, so that would go beyond 280 right and then if we were to multiply, 204 times 76 we get 15,504 which last I checked, is quite a bit higher than 255 and as a result, we would be blowing the heck out of the image so any time we multiply two images by each other, they just turn white, just except for the darkest shadows, the entire image would turn white. So what good would multiply be. It would be the most ridiculous radical operation on earth, but here's the deal, blend modes are standardized and that means that black is zero, White is 1 and 50% gray is 0.5, so everything happens in this tiny realm from zero to one.
In this tiny realm multiplication and subtraction decrease, division and addition increase. So things switch on us. For example our standardize levels are 0.8 and 0.3. So those are the same. If we took luminance level of 204, it becomes 0.8 in this new world of ours and 76 becomes 0.3. Fair enough, so if we were to multiply 0.8 times 0.3 we get 0.24, that's an even smaller number. It's much darker than 0.8, but it's even darker than 0.3. So we end up getting something that's more like 60 or something along those lines in terms of a luminous level. Subtract, we get 0.8 minus 0.3, we get 0.5, so that's bigger, but wouldn't necessarily be that way, it depends what numbers we're working with. Subtract could end up being smaller than multiply, but they're both going to be darker. Add, we get 0.8 plus 0.3, so 1.1, is white, so that means we just blew out that color, just went beyond white, so and of course anything that's lighter than white is going to become white, anything that's darker than black is going to be black, and then divide 0.8 by 0.3 and you get 2.67, which is way the hack white and that's why there really isn't a devide mode. There is a couple of subtract modes actually inside of Photoshop, but there is no straight divide mode instead what you have is more complex blending equations that involv some addition as well as some other operators. So basically what you got is the Multiply blend mode does this operation right here, goes ahead and multiplies it to luminance levels.
The Linear Dodge mode does this, it goes ahead and adds the two luminous levels together. So what's up with something like Screen. Well Screen goes ahead and adds the two colors together just like Linear Dodge and then it subtracts the two colors multiplied by each other. So it's really a combination of Linear Dodge and Multiply working together in order to create an effect that is in opposition to Multiply, just so as you know, you know, if you're curious. All right, in the next exercise we're going to get away from all this math stuff. I hope you enjoyed it, I hope you found it useful and we're going to take a look at the seven contrast modes.
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