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Using the Calculations command

From: Photoshop CS5 One-on-One: Mastery

Video: Using the Calculations command

In this exercise I'm going to introduce you to the Calculations command, which allows you to merge two channels together to create a new alpha channel that will serve as the basis for your mask. I'm still working inside Hair in flight.tif, found inside the 26_masking folder. And the first trick is to figure out which two channels you want to merge together. And here is the way I reason it, you start with a base channel and that would be the channel that provides the best overall contrast, and then you look for a support channel that is as different as possible from that base channel.

Using the Calculations command

In this exercise I'm going to introduce you to the Calculations command, which allows you to merge two channels together to create a new alpha channel that will serve as the basis for your mask. I'm still working inside Hair in flight.tif, found inside the 26_masking folder. And the first trick is to figure out which two channels you want to merge together. And here is the way I reason it, you start with a base channel and that would be the channel that provides the best overall contrast, and then you look for a support channel that is as different as possible from that base channel.

So, for example, in our case, we already know which channel provides the best contrast. that's the red channel inside of this image. We discovered that in a previous exercise because we have these bright skin tones set against the darkish background and the hair is darker still. So now that we know that it's the base channel, we need to figure out whether the Green or Blue channel is going to be the best support channel. So if I click on Green, I can see that the skin tones darken up slightly and the background gets brighter. And then if I click on the Blue channel, slightly darker flesh tones once again and even a brighter background.

Now, it's a little bit difficult to figure out who is the most different than red because really the differences are very subtle throughout the channels inside of this image. So when in doubt, if you're starting things off with the red channel, the best support channel is probably going to be blue. And the reason is that red and blue come to closest to being color complements. So that's the way we're going to work inside of this image. I'm going to click on RGB in order to make the composite view of the image active, and I'll go to the Image menu and choose the Calculations command. That brings up an old school, remarkably, unfriendly dialog box.

It's been this way for years and years now. And it's talks to you in this, kind of, engineering language where we've got Source 1, and Source 2. Well, those here two channels by the way. And if you want to think of one channel is being stacked on top of the other, then Source 2 is in the background and Source 1 is on top. So just like we're seeing them here and then we're going to take those two channels and blend them together using a Blend mode plus an Opacity value. Now, 99% of the time you're going to leave the Opacity value set to 100% just as we are now.

And you're going to let the Blend mode do the heavy lifting, but first we need to establish the channels. Now, notice that we have three options inside Source 1 and Source 2. First, we're able to specify the image which most of the time is going to be the image you're working on. On rare occasions, you might bring a channel in from a different image and I could do that in this case. I could choose Elaborate Composition, which I still have opened, because it's the exact same size. The two images, if you're going to merge them together here, the two images have to be the exact same pixel dimensions, not one pixel off.

Next, you can blend the contents of different layers if you want to. Now, this is a single layer image, so we just have a Background layer to work from and then you decide the channels. So most of the time you just make sure that the images are set to the image you're working on, that the Layer is either set to Background or if you're working on a larger composition, then you might set it to merged so you can see a merged view of all the layers and then you select the channels you want to use. I typically put the base channel, that is the high contrast channel, at the bottom of the stack. And then I layer the support channel on top of it.

So I'm going to leave the Source 2 Channel is set to Red and then I'm going to change the Source 1 Channel to Blue. And by default, we're merging the two channels together using the Multiply mode. I rarely use that mode for creating a base alpha channel, by the way. I'll usually select one of the others and I'll show you how that works in just a moment, but if you want to get a sense of how the channels look at any moment in time, then go ahead and change the Blend mode to Normal for a moment. And then you assuming that the Opacity value is a 100% you will see the top channel by itself. So the Source 1 channel is now visible by itself because it's opaque and it's on top and that would be the Blue channel.

So this is what the Blue channel looks like. If you want to compare it to the Red channel, then you would take the Opacity value down to 0% so that you're seeing through the top channel down to the bottom channel which in our case is red. So that's how you compare the two. Just FYI. All right, I'm going to reinstate that Opacity value to a 100%. Now, comes to the tough part, choosing the proper Blend mode and, of course, one way is to just sort of cycle through the Blend modes and see what works. I could choose the Screen mode and go, hmm, no that's not going to work at all. And then I could try, for example, the Multiply mode that we just saw a moment ago and say nope, that's really wrong.

That's not the best approach because you're going to be at it for an awfully longtime. What you want to do instead is remember that you have three really great Blend modes to work with where developing a mask are concerned and that's Add, Subtract, and Difference. And so I tend to start with those and see what I come up with. I'm going to start with Difference right out of the gate here so that we can see the differences inside of the image. Because what Different does is it subtracts one image from another and then it finds the absolute value. That is, if a luminance level goes negative it switches back to positive.

You may not find that description very helpful. So here is another way to think of it. You're using one channel to invert the contents of the other and wherever that channel is white, you're totally inverting, wherever it's black, you're not inverting at all. You might also want to go ahead and invert the channel while you're at it because the idea is you want to create as many differences as possible between Source 1 and Source 2. So for example I might turn on the Invert checkbox for blue and see what I come up with and now I'm seeing a lot of differences where the hair is concerned.

And just for larfs, I might try the opposite effect. I might Invert the Red Channel as well and then turn off the inversion of the Blue Channel and see what I come up with. Pretty similar effect, although slightly different. So that's one way to work. That is the Difference mode. Another way, I'm going to go ahead and turn the Invert checkbox off for both of these options, another way is to try Add. Now, the Add mode is identical to Linear Dodge (Add). That's why Add appears in parentheses next to Linear Dodge, with one difference, you can control Add to a larger extent.

So it does the exact same math as Linear Dodge, but it adds some options as well. So, for example, if I just chose Linear Dodge (Add), then I would go ahead now the luminance levels from the two images and I would end up blowing out luminance levels all over the place, leaving black in the background as well. I could turn on the Invert checkbox for both of these channels here. And I could get an opposite effect where the hair is very bright and the background is somewhat dark, but it's not dark enough because as I say, we've blown out all those highlights like crazy.

Well with Add, if I go ahead and choose that mode, notice initially I get the exact same effect, not a single pixel on the screen has changed, but I have two additional options, Offset and Scale, and I'm going to explain exactly how those work in the next exercise.

Show transcript

This video is part of

Image for Photoshop CS5 One-on-One: Mastery
Photoshop CS5 One-on-One: Mastery

192 video lessons · 43604 viewers

Deke McClelland
Author

 
Expand all | Collapse all
  1. 40m 45s
    1. Welcome
      2m 45s
    2. Making Photoshop your default image editor
      7m 43s
    3. Installing the dekeKeys keyboard shortcuts
      8m 10s
    4. Remapping OS shortcuts
      7m 37s
    5. Installing the Best Workflow color settings
      4m 31s
    6. The color settings explained
      6m 54s
    7. Loading the CS5 color settings in Bridge
      3m 5s
  2. 1h 11m
    1. Your creative range continues to expand
      1m 46s
    2. The Avatar project so far
      2m 38s
    3. Painting on a photograph
      7m 50s
    4. Adding texture and depth
      6m 14s
    5. Simulating chalky white paint
      7m 23s
    6. Masking and placing an image
      7m 20s
    7. Upsampling and Lens Blur
      5m 9s
    8. Blending blurry elements
      3m 48s
    9. Making a Smart Object
      6m 46s
    10. Placing an image as a Smart Object
      3m 22s
    11. Blending away a background
      5m 56s
    12. Applying Smart Filters
      4m 34s
    13. Creating a glow with Lens Flare
      3m 45s
    14. Blending and masking a glow
      5m 3s
  3. 1h 26m
    1. Using the image to select itself
      1m 53s
    2. Introducing masking
      6m 32s
    3. Making an alpha channel
      6m 54s
    4. Using the Calculations command
      6m 48s
    5. Add, Subtract, Offset, and Scale
      5m 54s
    6. Prepping an image with the Dodge tool
      6m 55s
    7. Fixing mistakes before they get too big
      6m 32s
    8. Painting in the Overlay mode
      5m 51s
    9. Exaggerating and selecting flesh tones
      7m 39s
    10. Smudge, Median, and the Blur tool
      6m 59s
    11. Masking low-contrast details
      6m 7s
    12. Creating a flesh-and-clothing mask
      5m 45s
    13. Masking and compositing the foreground
      5m 27s
    14. Finessing the final composition
      7m 39s
  4. 2h 24m
    1. Connecting the dots
      1m 40s
    2. The Pen tool and the Paths panel
      6m 32s
    3. Drawing a straight-sided outline
      6m 25s
    4. Editing a path outline
      6m 36s
    5. Adding and editing smooth points
      5m 35s
    6. Creating vector masks with the shape tools
      4m 59s
    7. Building a complex outline from shapes
      4m 26s
    8. Subtracting and transforming shapes
      6m 45s
    9. Cloning, flipping, and combining shapes
      8m 58s
    10. Roughing in non-symmetrical paths
      7m 41s
    11. Finessing a complex outline
      9m 15s
    12. Masking a layer effect
      8m 26s
    13. Isolating an image element
      6m 8s
    14. Smooth points and control handles
      9m 3s
    15. Stretching curved segments
      7m 49s
    16. Using the Rubber Band option
      9m 33s
    17. Drawing smooth points with the Pen tool
      6m 59s
    18. Shading an isolated object
      3m 45s
    19. Drawing cusp points
      7m 14s
    20. Setting points in the pasteboard
      9m 57s
    21. Using the Convert Point tool
      6m 42s
  5. 2h 57m
    1. Everything you need to know about blending
      1m 45s
    2. Photoshop CS5's blend modes
      7m 21s
    3. Cycling between blend modes
      6m 15s
    4. Darken and Lighten and their derivatives
      6m 3s
    5. The blend mode shortcuts
      8m 6s
    6. The Multiply and Burn modes
      4m 28s
    7. The Screen and Dodge modes
      6m 0s
    8. How opposite blend modes work
      8m 24s
    9. Why Multiply darkens and Divide lightens
      5m 23s
    10. Cleaning up a client's bad art
      5m 3s
    11. Dropping out a white background
      5m 56s
    12. Blending inside blend modes
      8m 3s
    13. Overlay, Soft Light, and Hard Light
      6m 26s
    14. Vivid, Linear, and Pin Light (and Hard Mix)
      6m 35s
    15. Difference, Exclusion, Subtract, and Divide
      7m 34s
    16. Great uses for the Difference mode
      6m 18s
    17. Promising uses for the Divide mode
      9m 6s
    18. Hue, Saturation, Color, and Luminosity
      7m 0s
    19. Blending an inverted layer
      3m 32s
    20. The "Fill Opacity Eight"
      7m 25s
    21. Making bad blend modes good
      5m 16s
    22. Making a knockout layer
      6m 53s
    23. Blending in the CMYK mode
      8m 3s
    24. Overprinting black text
      8m 29s
    25. Using the Luminance slider
      5m 24s
    26. Parametric luminance masking
      6m 21s
    27. Adjusting the behavior of luminance effects
      10m 8s
  6. 2h 2m
    1. Smart Objects = protective containers
      1m 35s
    2. Placing an Illustrator graphic
      6m 30s
    3. Vector copy and paste options
      6m 56s
    4. Applying Puppet Warp to vectors
      8m 9s
    5. "Gluing" vector art for Puppet Warp
      5m 50s
    6. Warping art onto the surface of an image
      8m 7s
    7. Blending a Smart Object
      4m 30s
    8. Blurring and blending a Smart Object
      6m 8s
    9. Making changes in Illustrator
      5m 57s
    10. Creating "true clones"
      7m 18s
    11. Double-flipping text
      4m 44s
    12. Applying effects to multiple layers
      3m 24s
    13. Updating true clones in one operation
      7m 36s
    14. Editing JPEGs as Camera Raw objects
      5m 49s
    15. Creating a double-exposure effect
      7m 15s
    16. Masking and shading transitions
      7m 47s
    17. Applying and repeating Camera Raw edits
      6m 9s
    18. Copying vs. cloning a Smart Object
      5m 18s
    19. Flipping a Smart Object and its mask
      3m 42s
    20. Adjusting multiple Camera Raw clones
      3m 53s
    21. Text that inverts everything behind it
      5m 34s
  7. 1h 59m
    1. This time, "smart" means dynamic
      1m 37s
    2. Introducing Smart Filters
      6m 28s
    3. Traditional High Pass sharpening
      5m 17s
    4. Smart High Pass in the Lab mode
      7m 57s
    5. Sharpening a high-frequency image
      7m 46s
    6. Retroactively reducing noise
      7m 31s
    7. Which filters are Smart Filters?
      6m 20s
    8. Shadows/Highlights as a Smart Filter
      4m 37s
    9. Nesting one Smart Object inside another
      7m 11s
    10. Drawing a mask from a nested Smart Object
      8m 7s
    11. Better Shadows/Highlights inside Lab
      9m 16s
    12. Tempering saturation values in Lab
      7m 0s
    13. Filtering live, editable text
      9m 2s
    14. Enhancing filters with layer effects
      4m 33s
    15. Applying a filter multiple times
      5m 0s
    16. Creating a synthetic star field
      7m 7s
    17. Making a stucco or drywall pattern
      6m 28s
    18. Land, sea, and clouds
      8m 30s
  8. 2h 50m
    1. Photoshop's advanced painting tools
      2m 3s
    2. Canvas texture and brush libraries
      6m 40s
    3. Painting with a predefined custom brush
      9m 21s
    4. Dissecting a custom brush
      11m 9s
    5. Designing and using a custom brush
      4m 54s
    6. Saving and loading brush presets
      5m 27s
    7. The ten styles of bristle brushes
      9m 47s
    8. Size, Spacing, and Angle
      7m 2s
    9. Using the Bristle Brush preview
      7m 53s
    10. Bristles, Length, Thickness, and Stiffness
      6m 53s
    11. Stylus tilt and mouse behavior
      5m 25s
    12. Stroking a path outline with a brush
      4m 0s
    13. Troubleshooting a stylus
      5m 49s
    14. Introducing the Mixer Brush
      7m 22s
    15. The Load, Mix, and Wet values
      5m 1s
    16. Cleaning and loading a brush
      6m 26s
    17. Shading a piece of graphic art
      6m 34s
    18. Shading with color
      7m 53s
    19. Mixing a photographic portrait
      6m 11s
    20. Tracing the fine details in an image
      5m 52s
    21. Crosshatching and brush size
      5m 53s
    22. Covering up and augmenting details
      7m 36s
    23. Painting in hair and fabric
      5m 54s
    24. Painting and scaling very fine hairs
      8m 7s
    25. Adding texture with the Emboss filter
      8m 31s
    26. Exploiting a "happy accident"
      2m 46s
  9. 1h 40m
    1. Artificial intelligence that works
      1m 22s
    2. The Auto-Align Layers command
      7m 25s
    3. The Auto-Blend Layers command
      3m 54s
    4. Masking auto-aligned layers
      4m 50s
    5. The Geometric Distortion setting
      6m 44s
    6. The Seamless Tones and Colors checkbox
      4m 8s
    7. Creating the best possible layer mask
      9m 18s
    8. Auto-blending depths of field
      5m 54s
    9. Finessing masks, accepting imperfections
      6m 29s
    10. Shooting and downsampling panorama images
      5m 54s
    11. Introducing the Photomerge command
      6m 40s
    12. Evaluating the Layout settings
      6m 47s
    13. Loading, aligning, and blending with Photomerge
      5m 36s
    14. Tracing and extracting seams
      7m 18s
    15. Adding a masked element into a panorama
      5m 55s
    16. Simplifying and correcting a panorama
      5m 58s
    17. Smart Filters and nondestructive cropping
      6m 43s
  10. 1h 18m
    1. The most mysterious of mysterious topics
      2m 29s
    2. Introducing HDR Toning
      6m 43s
    3. Reigning in clipped highlights
      5m 54s
    4. The Local Adaptation options
      9m 5s
    5. Nondestructive editing with HDR Toning
      8m 22s
    6. Using the HDR Toning Curve
      7m 2s
    7. HDR Toning vs. Shadows/Highlights
      6m 0s
    8. Merging multiple exposures
      7m 14s
    9. A first look at HDR Pro
      6m 24s
    10. Removing ghosts, correcting backlighting
      7m 11s
    11. Generating and editing an HDR comp
      7m 0s
    12. HDR rendered to completion
      5m 19s
  11. 1h 27m
    1. Processing hundreds of files in no time
      1m 43s
    2. Creating an action set
      6m 37s
    3. Making an action
      7m 7s
    4. Stop, Delete, and Record
      7m 12s
    5. Add, Undo, and Rerecord
      6m 40s
    6. Playing and testing an action
      6m 31s
    7. Playing and editing a specific operation
      6m 39s
    8. Permitting the user to change settings
      4m 58s
    9. Explaining an action with a custom stop
      5m 0s
    10. Batch-processing multiple images
      7m 22s
    11. Adding a Save As operation
      6m 34s
    12. Creating an action to save web graphics
      7m 59s
    13. Batching two actions into one
      7m 15s
    14. Saving and loading actions
      5m 30s
  12. 1m 19s
    1. See ya
      1m 19s

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