Photoshop CS3 Channels & Masks: Advanced Techniques
Illustration by John Hersey

The Subtract mode in action


Photoshop CS3 Channels & Masks: Advanced Techniques

with Deke McClelland

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Video: The Subtract mode in action

So in the last exercise we used the Add mode combined with the Calculations command in order to generate an Alpha Channel, this one right here, for our superhero hair dude from photographer Chris Schmidt. And this is a very serviceable Alpha Channel, at least a base channel on which to build a mask. However, maybe we'd be able to do better and certainly we would like to gain some experience with the Subtract mode. So let's give it a shot, let's see if subtract might give us something better than Add. I want you to return to the RGB composite view of the image. I am going to press the F key -- oh, by the way before I press F however I am working with a catch up document, in case you want to open it up, it's called Hero with Add.tif, meaning it's our Hero guy with of course an Alpha channel created using the Add Blend mode.
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  1. 2h 12m
    1. The Odyssey Continues
      2m 39s
    2. Mapping one image onto another
      7m 12s
    3. Making a custom contrast mode
      7m 9s
    4. Luminance blending
      8m 39s
    5. Forcing the visibility of underlying layers
      4m 3s
    6. Adjusting the appearance of clipped layers
      4m 33s
    7. Selecting a Blend If channel
      6m 11s
    8. Enhancing highlights by hiding them
      5m 8s
    9. Smart Object first, layer mask second
      7m 22s
    10. The Fill Opacity Eight
      4m 30s
    11. Blending Smart Filters
      7m 24s
    12. Cleaning up edges
      7m 38s
    13. More fun with luminance blending
      6m 22s
    14. A first peek at the Calculations command
      12m 11s
    15. Masking a softly focused model
      11m 46s
    16. Moving layers and masks between images
      7m 34s
    17. Matching colors
      9m 12s
    18. Building transitional blended layers
      6m 33s
    19. Restoring normal colors
      6m 49s
  2. 2h 32m
    1. Layer masks, clipping masks, and knockouts
      1m 20s
    2. Rotating and stretching
      7m 54s
    3. A jet of motion blur
      9m 17s
    4. Keyboard tricks
      5m 4s
    5. Merging sky and landscape
      6m 2s
    6. Masking an adjustment layer
      6m 37s
    7. Creating two windows into an image
      7m 42s
    8. Whitening teeth and adding other highlights
      3m 46s
    9. Mapping a texture onto an image
      4m 0s
    10. Isolating a texture with a layer mask
      6m 44s
    11. Welcome to the glass composition
      3m 18s
    12. Balancing shadows and highlights
      5m 51s
    13. Masking the glass
      7m 24s
    14. Masking the text
      9m 23s
    15. Adding and blending the goldfish
      8m 44s
    16. Assembling the perfect group photo
      5m 12s
    17. Aligning photographs automatically
      5m 26s
    18. Masking in each person's best shot
      5m 18s
    19. Masking densely packed people
      6m 17s
    20. Crafting the perfect final poster
      5m 15s
    21. From the improbable to the impossible
      1m 56s
    22. The fantastical "world of clones" effect
      10m 0s
    23. Upsampling and blurring a background
      8m 39s
    24. Adding a knockout mask
      8m 3s
    25. Choking edges with Gaussian Blur and Levels
      3m 46s
  3. 2h 26m
    1. The corrective power of masking
      1m 6s
    2. The amazing luminance mask
      7m 21s
    3. Brightening and neutralizing the eyes
      8m 21s
    4. Adjusting a nondestructive composition
      5m 52s
    5. Creating a corrective mask
      6m 3s
    6. Averaging away irregular flesh tones
      3m 52s
    7. Modifying specific colors
      7m 46s
    8. Initiating the color mask
      5m 59s
    9. Refining the color mask
      6m 39s
    10. Adjusting the edges around fabric
      7m 55s
    11. Perfecting hair
      9m 34s
    12. Sharpening with a High Pass layer
      10m 12s
    13. The also-amazing density mask
      4m 47s
    14. Adjusting the knockout depth
      8m 48s
    15. Fashioning a depth map
      6m 12s
    16. Invoking a depth mask from Lens Blur
      6m 38s
    17. The perfect depth-of-field effect
      6m 25s
    18. Sharpening an archival photograph
      7m 7s
    19. Creating an edge mask
      8m 29s
    20. Making a High Pass sandwich
      7m 45s
    21. Applying the edge mask
      6m 1s
    22. Customizing your sharpening effect
      4m 6s
  4. 2h 3m
    1. Channel Mixer, I am your father!
      1m 39s
    2. Three ways to gray
      7m 48s
    3. Meet the Channel Mixer
      8m 26s
    4. Mixing a custom black-and-white image
      7m 9s
    5. Creating a professional-level sepia tone
      5m 36s
    6. Employing the Black & White command
      8m 0s
    7. Extreme channel mixing
      4m 50s
    8. The infrared photography effect
      6m 42s
    9. Taking shadows to the brink of black
      3m 56s
    10. Elevating highlights, leeching saturation
      5m 58s
    11. Deepening a black-and-white sky
      5m 49s
    12. Infusing luminance levels with color
      5m 43s
    13. Creating an opposing colorization scheme
      4m 58s
    14. Bolstering contrast with the Green channel
      5m 37s
    15. A tiny improvement to a terrific technique
      7m 38s
    16. The simple (but wrong) approach to red-eye correction
      6m 39s
    17. Channel-mixing red pupils
      9m 17s
    18. The expert approach to red-eye correction
      5m 20s
    19. Fixing problem coronas (pupil edges)
      8m 8s
    20. Making pupils match
      4m 7s
  5. 2h 33m
    1. Chops are dead; long live maskops
      1m 36s
    2. The Calculations command
      8m 16s
    3. Blue Screen blending
      7m 40s
    4. Refining the Blue Screen mask
      5m 53s
    5. Brushing away color fringing
      7m 24s
    6. Locking the transparency of a layer
      6m 21s
    7. Nondestructive layer painting
      7m 36s
    8. How the Add blend mode works
      8m 40s
    9. How the Subtract blend mode works
      6m 43s
    10. Focus, noise, and other masking challenges
      5m 32s
    11. The Add mode in action
      7m 51s
    12. The Subtract mode in action
      8m 24s
    13. Comparing two channels with Difference
      5m 23s
    14. Enhancing the contrast with Curves
      9m 10s
    15. Gathering details with Apply Image
      9m 43s
    16. Dodge highlights, burn shadows
      6m 6s
    17. Dodge and Burn in action
      8m 24s
    18. Painting in the scalp
      10m 0s
    19. Painting away the face and chin
      4m 52s
    20. Compositing complementary images
      4m 13s
    21. Multiply, Minimum, Blur, and Apply Image
      6m 40s
    22. Crafting the final composition
      7m 7s
  6. 1h 57m
    1. Mark of the Pen tool
      1m 34s
    2. The big paths project overview
      6m 51s
    3. How to make a path
      8m 24s
    4. Corner points and freeform polygons
      8m 6s
    5. Editing paths with the arrow tools
      5m 2s
    6. Adding and deleting endpoints
      5m 14s
    7. Adding and deleting interior points
      6m 6s
    8. Converting a path to a selection
      3m 35s
    9. Converting a path to a mask
      6m 38s
    10. Smooth points and control handles
      8m 57s
    11. Making cusp points
      5m 59s
    12. Combining paths in a vector mask
      7m 54s
    13. Turning a path into a shape layer
      8m 57s
    14. Combining paths to make a layer mask
      7m 52s
    15. Mixing layer and vector masks
      10m 14s
    16. Editing character outlines as paths
      8m 39s
    17. Using the Convert Point tool
      7m 8s
  7. 3h 17m
    1. Where there's a will, there's a way
      1m 18s
    2. Masking natural cast shadows
      4m 9s
    3. Applying the cast show
      4m 2s
    4. Creating a difference mask
      3m 7s
    5. Applying an arbitrary map
      3m 50s
    6. Making the flesh mask
      7m 16s
    7. Roughing in an object mask
      6m 48s
    8. Drawing missing details with the Lasso tool
      4m 7s
    9. Combining flesh and object masks
      3m 52s
    10. Amplifying the cast shadow
      4m 10s
    11. Selectively choking edges
      3m 58s
    12. Power duplication in Photoshop
      7m 8s
    13. Masking blond hair
      5m 48s
    14. Using Levels to mask iterations
      3m 14s
    15. Drawing an iteration boundary
      4m 54s
    16. Merging the best of two Levels iterations
      4m 4s
    17. More fun with Dodge and Burn
      6m 14s
    18. Fixing edges with the Pen and Stamp tools
      7m 28s
    19. Pulling from another file with Apply Image
      4m 52s
    20. Blending clipped layers independently
      5m 42s
    21. Building the flame mask
      9m 21s
    22. Amplifying the flame
      3m 52s
    23. Masking an image against a busy background
      5m 15s
    24. The Freeform and Magnetic Pen tools
      6m 52s
    25. Masking with arbitrary maps
      9m 32s
    26. A more deliberate approach to arb maps
      10m 51s
    27. Combining arb maps with paths
      9m 28s
    28. Masking with the help of the History brush
      11m 38s
    29. Creating a High Pass mask
      7m 24s
    30. Coloring in the outlines
      8m 31s
    31. Mastering Calculations
      7m 28s
    32. Subtracting and merging the beak
      11m 5s
  8. 1h 33m
    1. The meaning of bit depth (and why you care)
      2m 49s
    2. Scanning line art in 8-bit and 16-bit
      5m 8s
    3. Measuring the 16-bit difference
      8m 8s
    4. Correcting 8-bit images in the 16-bit space
      9m 31s
    5. Opening a raw image directly in 16-bit
      6m 12s
    6. Editing in Camera Raw, opening in 16-bit
      8m 21s
    7. 16-Bit/channel vs. 32-bit/channel (HDR)
      8m 17s
    8. Working with auto-bracketed photographs
      5m 6s
    9. Using the Merge to HDR command
      5m 59s
    10. Adjusting the HDR preview
      6m 0s
    11. Building a 32-bit sky mask
      6m 29s
    12. Properly exposing land and sky
      4m 24s
    13. Modifying a layer mask in 32-bit
      4m 56s
    14. Converting to and correcting in 16-bit Lab
      12m 7s
  9. 2h 8m
    1. Photoshop flirts with the third dimension
      1m 13s
    2. The displacement map
      8m 24s
    3. Making custom waves
      7m 14s
    4. Creating a Gaussian distribution
      4m 31s
    5. Using a two-channel displacement map
      6m 27s
    6. Creating a rustic edge effect
      8m 21s
    7. Distorting and shading with a DMap
      6m 33s
    8. Moonlight reflecting off water
      8m 48s
    9. Mapping the reflection onto the water
      7m 3s
    10. Dipping the moon into the water
      6m 18s
    11. Turning flesh into stone
      7m 55s
    12. Wrapping the stone around the face
      7m 27s
    13. Softening a displacement map
      8m 4s
    14. Making a repeating watermark pattern
      9m 21s
    15. 3D embossing with Lighting Effects
      10m 47s
    16. The amazing credit card type effect
      6m 56s
    17. Lightening the credit card letters
      6m 16s
    18. Wrapping the background around the text
      6m 27s
  10. 1m 43s
    1. Goodbye
      1m 43s

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Watch the Online Video Course Photoshop CS3 Channels & Masks: Advanced Techniques
20h 48m Advanced Nov 21, 2007

Viewers: in countries Watching now:

The elusive alpha channel remains one of the most misunderstood yet powerful tools in Photoshop. Alpha channels are collections of luminance data that control the transparency of an image, and they inform just about every aspect of Photoshop. As he builds transitional blended layers, fashions a depth map, makes edge adjustments, and takes on extreme channel mixing, Omni Award-winning expert Deke McClelland teaches Photoshop users that where there's a will, there's a way. Photoshop CS3 Channels and Masks: Advanced Techniques covers mapping texture on an image, turning flesh into stone, using vector masks, working with all different channels, creating a rustic edge effect, and much more. Exercise files accompany the tutorials.

Download Deke's customized keyboard layouts for Channels and Masks from the Exercise Files tab."

Topics include:
  • Distorting and shading with a DMap
  • Understanding bits and channels
  • Creating paths with the Pen tool
  • Using blend modes and the Dodge and Burn feature
  • Understanding channel mixing
  • Using layer masks, clipping masks, and knockouts
  • Applying Smart Filters
Design Photography
Deke McClelland

The Subtract mode in action

So in the last exercise we used the Add mode combined with the Calculations command in order to generate an Alpha Channel, this one right here, for our superhero hair dude from photographer Chris Schmidt. And this is a very serviceable Alpha Channel, at least a base channel on which to build a mask. However, maybe we'd be able to do better and certainly we would like to gain some experience with the Subtract mode. So let's give it a shot, let's see if subtract might give us something better than Add. I want you to return to the RGB composite view of the image. I am going to press the F key -- oh, by the way before I press F however I am working with a catch up document, in case you want to open it up, it's called Hero with Add.tif, meaning it's our Hero guy with of course an Alpha channel created using the Add Blend mode.

All right, now I am going to switch back to the Full Screen mode here, so that I can keep an eye on as much of the vertical aspect of the image as I can inside of this fairly small screen view here. Then I am going to go out of the Image menu and I am going to choose the Calculations command. Now Calculation should come up with a last calculation we applied, and it certainly does for me, the Source 1 channel is set to Green, Invert; and the Source 2 channel is set to Blue, Invert. Fine! Blend is set to Add because that's the last thing we applied. This time let's try Subtract, so I am going to ahead and change the Blending from Add to Subtract.

And just to get a sense of what's going on, notice that this doesn't deliver anything like what we had a moment ago. We are not seeing any of the hairs. This mask would be pretty useless, I think, unless we were trying to select some sort of ghoulish version of the face. That's definitely not what we are going for. We are trying to separate the hairs from the background. So I am going to suggest to you that our channels are messed up at this point. We are not going to just go in and start adjusting the Offset and Scale value because we are near where we need to be. We should at least be in the range before we start fooling around with Offset and Scale. So our channels are indeed messed up, and let me show you what's going on. I am going to go ahead and change the Opacity value to 0 for a moment, so we can focus on the Source 2 by itself. And remember from a couple of exercises ago that the subtract equation is Source 2 minus Source 1.

All right, so we are going to start with whatever the Source 2 channel is and we are going to subtract Source 1 from it. So let's take a look at the Source 2 channel. It's Blue Invert so it looks like this, light hair against a darkish background. That's a good thing. Let's just see what our green channel looks like when Inverted as well. We'll just go and change Source 2 to Green since we can see the Source 2 channel right now thanks to that Opacity value of 0%. And it's also white against dark. Ding, ding, ding! That's the warning signal going off. It's what I am suggesting here, because if we are trying to make the hair turn white which we are against the black background then we can't be subtracting white from white.

Because white has a Luminance Level of 255, so if we take 255 minus 255 so the white herein the Blue Invert channel and we subtract the white here in the Green Invert what are we going to get? We are going to get 0 which is black. So we are going to send the hair black, which is exactly what's happening. So if I change this back to blue and I set the Opacity value to 100% you can see that we are sending the hair to black, we are sending the background to black, we are sending everything but a little bit of his face to black. That's why we need to subtract from the Blue Invert channel we need to subtract Green with Invert turned off, and we are going to get this effect right here instead.

And that's because we are subtracting black hair because Green Channel has black hair in it. Let's go ahead and look at the Green Channel thumbnail here inside the Layers palette. It has got black here. So we are taking the White here inside the Blue Invert channel we are subtracting Black from it, 255 minus zero gives you 255 so the hair remains white. Now it's going to take you sometime to get used to this kind of stuff and come to terms with what's going on with subtract but that gives you an early sense of what to look for. You should also just bear in mind that you can have a little bit of fun, irritating fun but still you can have some fun with turning the Invert checkboxes On and Off and see what you come up with.

For example, how did I know that I wanted to subtract Green normally from Blue Invert as opposed to Green Invert from Blue Normal? Well, just try it out. Turn one on and turn the other off, see what you come up with. Gosh! That's exactly the opposite of what I want, I don't have another Invert checkbox that I can select. So I need to turn these guys on and then back off like so. All right, so experiment, that's going to give you a clue as to what's going on as well. I am now going to take the offset value up, because we are roughly in the right neighborhood here. Because I have applied subtract I need to raise the offset value, so I am going to press Shift+Up Arrow, and I held on that Up Arrow key there and I raise that offset value to 160. Now another thing I should be able to do with Scale is telling you how slowly the Scale value reacts. I should be able to Shift+Drag on the word Scale in order to scrub it. Notice what happens when I Shift+Drag on the scale, I start scrubbing the offset value instead.

That's nice. Thank you for that, Calculations dialog box. Here's another thing you can thank the Calculations dialog box for. I was telling you that you can change the Scale value from 1 to 2. So this is the Scale value of 1, and this is the Scale Value of 2. Fine! What if I decide to change the Scale value to 4, which is out of range, by the way you can't use a Scale value of 4. If I were to press the Tab key, I'll get a warning. That's wrong buddy it's got to be between 1.000 and 2.000. But I can change it to four. It's not going to nag me immediately; it's just going to ignore me. But here's what I love. 5, I get ignored, 6, I get ignored, 7 I get ignored. And notice that I am getting ignored to the tune of it; it's just treating it like I have applied the maximum value of 2.

If I take it up to 9, I am still just getting ignored. If I take it up to 10, I get ignored in a different way. It now treats it as if I have a Scale value 1, because it's paying attention to the initial 1, is what it seems to be doing, and then ignoring the second value. So like if I change it to 16, same thing. You got to remember because otherwise you know you got to press Tab and it complaints at you that kind of thing. You just got to remember, you are going between 1 and 2. I am going to change this value to 1.4 again, that's the same scale value we used a moment ago for the Add mode and that is going to be consistent. Whatever you come up with for Add is going to work for subtract as well, is what I am trying to say. And then I'll go back to Offset, now that the image is too bright of course because I ended up brightening it additionally with a Scale value in addition to softening the edges. So I'll take that Offset value down to 90, and last time Blending was set to Add and the Offset was -90 and Scale was 1.4. This time it's Subtract, Offset is 90 as opposed to -90, and scale is 1.4. So scale is the same. And the only difference we have applied up here is to turn the Invert checkbox off for Green. It gives us the similar result.

All right, so once you have established these values, go ahead and click Okay for me, would you and you will create a second Alpha channel, actually it's a fourth alpha channel, and we'll call it subtract of course, and we'll call it 90 and then 1.4 like so. And then press the Return key or the Enter key in order to accept that modification. So this is the Add version of the mask and this is the subtract version of the mask. They are almost identical to each other, thanks to the modifications that we applied. So you can do the job using Add or Subtract, it's entirely up to you which approach you take. Sometimes you are getting slight differences here and there. Other times you are going to get almost identical result. And the approach you take as I say, is entirely up to you. You may find that one mode makes a lot more sense to you than the other for example.

All right, so we now have practical experience with both Add and Subtract, very useful modes for creating image calculations to create a base Alpha channel for a complex mask. In the next exercise we are going to set about modifying this mask using some traditional techniques in order to create an accurate selection outline that conveys all the different degrees of focus associated with this fellow's hair.

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