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Photoshop Top 40
Illustration by John Hersey

33. Calculations


From:

Photoshop Top 40

with Deke McClelland

Video: 33. Calculations

(Music playing) Deke's Photoshop? Deke's Photoshop? Top 40! Deke's Photoshop, Deke's Photoshop Top 40. All right friends, as glad fortune would have it over the course of the next three features we're going to be generating a Mask. We'll Mask this woman and her crazy hair away from the background. This image comes to us from photographer Stas Perov and it presents us with an interesting challenge, not only do we have all this hair all over the place. That's actually the least of our concerns as it turns out but we also have very low color information. So we've got a lot of Flesh Tones here inside of her arms, her chest and her face.

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Photoshop Top 40
7h 13m Intermediate Dec 21, 2009

Viewers: in countries Watching now:

There's nothing people love more than lists, and Photoshop Top 40 offers a great one, highlighting the best features in Photoshop. Deke McClelland counts down to #1 with a new video each week, detailing one great feature after another in this popular digital imaging application. The videos cover tools, commands, and concepts, emphasizing what's really important in Photoshop.

This is an ongoing course that will be updated monthly.

For the newest updates please go to our blog entry for Deke's Photoshop Top 40.

Topics include:
  • Assembling multiple pieces of artwork with layer comps
  • Creating a black-and-white image from a color photograph
  • Merging multiple channels to create an alpha channel with calculations
  • Selecting images with the Pen tool
  • Masking images using the Brush tool
Subjects:
Design Photography
Software:
Photoshop
Author:
Deke McClelland

33. Calculations

(Music playing) Deke's Photoshop? Deke's Photoshop? Top 40! Deke's Photoshop, Deke's Photoshop Top 40. All right friends, as glad fortune would have it over the course of the next three features we're going to be generating a Mask. We'll Mask this woman and her crazy hair away from the background. This image comes to us from photographer Stas Perov and it presents us with an interesting challenge, not only do we have all this hair all over the place. That's actually the least of our concerns as it turns out but we also have very low color information. So we've got a lot of Flesh Tones here inside of her arms, her chest and her face.

And then we've copious low saturation blues in the background, in the hair and in the dress, which makes for a challenging masking scenario. So we're going to bring up the Big Guns and the biggest Gun of them all is feature #33 the Calculations command, one of the most daunting and powerful functions inside a Photoshop. And what it allows you to do is merge two channels inside of an image to create a new alpha channel that will serve as the basis for a Mask. So let's go ahead and take a look at the channels inside of this RGB image.

I'm looking at the Channels palette. I'm going to switch over to the Red channel. You can see that she's very bright, her hair is dark and the background is relatively dark as well. If we switch to the Green channel, her skin tones darken up just a little bit. The background starts to lighten up. And then if we switch to the Blue channel she gets just slightly darker because she's so very very brightly lit. And then her background is the brightest of them all and so is the hair incidentally. So we've the most contrast between the Blue and the Red channels where this image is concerned, which is not surprising because we've Flesh Tones which live largely inside the Red channel and then we've a Blue background which of course is going to live mostly inside that Blue channel.

And so we're going to merge those two channels together Red and Blue to create our new alpha channel. So I'll switch to the RGB image, I'll go up to the Image menu and I'll choose the Calculations command in order to bring up the Calculations dialog box. Now you start off with a couple of Source channels, Source 1 and Source 2, these are going to be the two channels you merge together and here is your blend mode that's how you specify how the channels blend together and an opacity value as well. Now nine times out of ten you're going to leave that opacity value set to 100 percent and that's what we're going to do in this case as well.

Think of the Source 1 channel as being set on top of the Source 2 channel, so the Source 2 channel is ultimately your base layer, if you want to think it with that way and then you have Source 1 on top of it. And we want Red on top of Blue in our case so I'm going to stick with the Red channel for Source 1 right here. We don't have to worry about the layer because this is a flat image and I only have one file open. So we don't have to worry about that either. We're just worried about the channel Option. I'm going to go ahead and set Channel 2 to Blue and right now we're not getting much of a good mix, what we want to see by the way is white hair against the black background.

That's what we're going for in this case. So what we need is we need a big huge high degree of contrast between the Red and the Blue channels and in order to achieve that I'm going to go ahead and Invert the Blue channel so that we're comparing two very different images. So if we're to set this mode right here to Normal, this is what the Red channel looks like because it's on top and then if I were to reduce the opacity value to zero percent this is what the Inverted Blue channel looks like because it's on bottom, just so that you have a sense of the degree of contrast that we're working with here.

All right I'm going to set that opacity value to 100 percent. Now there's a lot of blend modes that you can choose from. The blend mode is ultimately going to decide the fate of this Mask right up front here. So you want to choose it wisely that you can play with every single one of these blend modes if you want to, just to get a sense of how they work. But the ones that are most likely to afford you the best results are these two guys right down there, Add and Subtract and what I am going to recommend you do is you to go ahead and start with the Add function and see what result it delivers. Now in our case, it's making the image far too bright as you can see here.

Now I can Offset that and darken up those colors by reducing this Offset values so let's say I take that value down to negative 100, luminous levels incidentally so we're Subtracting 100 luminous levels from every single pixel inside of this image and we end up getting this effect here which is not going to do us any good. Notice that we have these sort of light grey hairs against a slightly darker background but really we just have dark edges. This is not going to provide the effect we're looking for. So I' going to go ahead and take that Offset value back to zero and I'm going to switch from Add to Subtract and see what we get that way and this is a much better effect as you can see.

Now in our case, it really is doing everything we needed to do where the hair is concerned. Now where the arms are concerned, it's not working out very well at all. But it is the best result we're going to achieve inside of this image. So I'm sticking with the Subtract mode if I wanted to darken up the colors, I could go ahead and reduce this Offset value to something like negative 30. I don't want to do that though because I'm losing the bright values inside of the hair. So I'm just going to go ahead and leave that Offset value set to zero. Then I'll go ahead and click OK and notice that the result by default is going to appear in a new channel.

So click OK and we have a new alpha channel down here at the bottom of the Channels palette, I recommend you go ahead and rename this channel something like MASK and then pressing Enter key and you now have a base Mask for your future modifications and we're going to be applying those future modifications using the next two features, features #32 and #31. I don't want to blow it for you don't want to tell you what they're. So just stay tuned. But in the mean time you've witnessed one of the powerful Masking commands in all of Photoshop, Calculations.

Find answers to the most frequently asked questions about Photoshop Top 40.


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Q: Is there a way to batch convert an entire folder of photos from the RBG color mode to the CMYK color mode without having to open and convert each individual image?
A: In the Actions panel in Photoshop, create an action that converts an image from RGB to CMYK. Then link to that action from File > Automate > Batch inside Photoshop.
Next, in the Bridge, select a folder of images. Choose Tools > Photoshop > Batch. Select the action inside the ensuing dialog box.
Or, in Photoshop, select File > Automate > Batch, and select the action and the folder inside the dialog box.
See also: Photoshop CS2 Actions & Automation, Chapter 2 “Action Essentials.”
 
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