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Comparing two channels with Difference

From: Photoshop CS3 Channels & Masks: Advanced Techniques

Video: Comparing two channels with Difference

In the last two exercises we used both the Add and the Subtract mode combined along with the Calculations command in order to create a couple of base alpha channels. Now if you are just joining me, I am working inside of a catch up document called ANS in action.tif. ANS standing of course for the Add and Subtract modes, and this image is found inside the 14 Calculations folder. If you go down to the bottom of the Channels palette, you'll see that we have one alpha channel that's called Add, -90, 1.4. -90 being the Offset value, 1.4 being the Scale value. And we have another one called Subtract, 90, 1.4. 90 being the Offset value, 1. 4 being the scale value.

Comparing two channels with Difference

In the last two exercises we used both the Add and the Subtract mode combined along with the Calculations command in order to create a couple of base alpha channels. Now if you are just joining me, I am working inside of a catch up document called ANS in action.tif. ANS standing of course for the Add and Subtract modes, and this image is found inside the 14 Calculations folder. If you go down to the bottom of the Channels palette, you'll see that we have one alpha channel that's called Add, -90, 1.4. -90 being the Offset value, 1.4 being the Scale value. And we have another one called Subtract, 90, 1.4. 90 being the Offset value, 1. 4 being the scale value.

Bear in mind that these two variations on the Alpha channel were created using slightly different settings for the base channels, for the Source 1 and the Source 2 channels. So in the case of Add we were taking an Inverted version of the Blue channel and adding an Inverted version of the Green channel to it. In the case of Subtract we were taking an Inverted version of the Blue channel and subtracting a standard version of the Green channel from it. Now I was telling you that we can get similar though, not quite identical effects out of Add and Subtract to which you might have responded, really? Because these look totally identical to me. Well, we can test that, can't we? You may recall way back when we used the Blend mode in order to compare a JPEG file to the non JPEG, the uncompressed original. And we did that using the Difference mode. We can also use the Difference mode to find out the differences between different alpha channels such as the Add and Subtract variations. Why don't we go ahead and do that.

Go ahead and click on Add, the Add channel here, inside the Channels palette in order to make it active. Then let's go up to the Image menu and choose the Calculations command. And right now it's saying that it wants to take Source 1 set to Channel Add and Source 2 set to the exact same channel and subtracting from each other. Well let's go ahead and change Source 2 to subtract. It doesn't really matter who's Source 1 and who's Source 2. But one of them needs to be subtract and the other needs to be add. And then we'll change the blending mode to Difference. Now before I choose Difference let's review what it's going to do. Where we light pixels that means we have significant differences between the two channels. Where we see black pixels, it means we have no differences between the two channels, and other variations, Gray of course either means slight differences in the case of dark grays and big differences in the case of light grays.

All right, so if we see Black there's not much difference, if we see white there's a lot of difference. Let's choose difference, it turns Black meaning that there's not a heck of a lot of difference between these two channels. However, even though it looks like the entire darn thing is black now which would mean that the two channels are utterly and completely identical to each other, they are not. They just happen to be very similar, but there are some differences. All right, so we have gone ahead and created this new Alpha Channel using the settings that you see here. Once you have done that click OK and there's your new Alpha channel. Let's go ahead and call it, What's the diff? Because it's going to tell us the differences between these two channels.

Now we need to elaborate those differences using the Levels commands. So press Ctrl+L or Command+L on the Mac to bring up Levels. Notice that the histogram is shoved all the over to left hand side. So we don't have much histogram to work with but we do have a few bars meaning that the two channels are not identical, they are just very, very similar. Let's go ahead and take this White value down to 2 or 3, actually 2, I think, works out better for it. We could take it two 3, or 4, or one of these higher values because we will see that there are shades of difference going on but I say 2, is going to give us the most significant reading of the image. And then click OK.

So we have yanked this histogram all the way apart so that we only have really black, white and most -- couple of shades of are gray. I think really we just have one shade of gray. So now seeing the differences between the two channels you may still wonder, okay, I understand the effect at this point but I am still left wondering what's the diff. Deke, you know the big question still goes unanswered. Well, there's three reasons we perform this check here. One, is I want you to see that it's possible. I want you to see how you can use the difference mode in order to compare any two Alpha channels inside of your images, or any two channels in general inside of your image. Two, it's a darn cool effect, don't you think. There's something you might be able to use for whatever reason. Not for masking necessarily but for black and white posters maybe. And then finally three, it does demonstrate to us that there is one channel that's going to be even slightly better than the other.

So let's go back and check them out. Look at them very closely. Here's Add, here's Subtract, what did you see happen? Did you see a change occur on screen? Subtract got darker, the background darkened up a bit. I would say from my reading of the image and my experience with working with this image that subtract is going to be closer to our final goal because the background is slightly darker and there's a higher degree of contrast between the hair and background. So I am going to suggest we start working from that subtract channel in order to generate our final mask, and that is what we are going to be doing of course, starting in the next exercise.

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This video is part of

Image for Photoshop CS3 Channels & Masks: Advanced Techniques
Photoshop CS3 Channels & Masks: Advanced Techniques

190 video lessons · 26391 viewers

Deke McClelland
Author

 
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  1. 2h 13m
    1. The Odyssey Continues
      2m 39s
    2. Mapping one image onto another
      7m 12s
    3. Making a custom contrast mode
      7m 10s
    4. Luminance blending
      8m 40s
    5. Forcing the visibility of underlying layers
      4m 4s
    6. Adjusting the appearance of clipped layers
      4m 34s
    7. Selecting a Blend If channel
      6m 12s
    8. Enhancing highlights by hiding them
      5m 9s
    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 39s
    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 35s
    17. Matching colors
      9m 13s
    18. Building transitional blended layers
      6m 33s
    19. Restoring normal colors
      6m 50s
  2. 2h 33m
    1. Layer masks, clipping masks, and knockouts
      1m 20s
    2. Rotating and stretching
      7m 54s
    3. A jet of motion blur
      9m 18s
    4. Keyboard tricks
      5m 4s
    5. Merging sky and landscape
      6m 3s
    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 1s
    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 45s
    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 16s
    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 27m
    1. The corrective power of masking
      1m 6s
    2. The amazing luminance mask
      7m 22s
    3. Brightening and neutralizing the eyes
      8m 22s
    4. Adjusting a nondestructive composition
      5m 52s
    5. Creating a corrective mask
      6m 4s
    6. Averaging away irregular flesh tones
      3m 52s
    7. Modifying specific colors
      7m 46s
    8. Initiating the color mask
      6m 0s
    9. Refining the color mask
      6m 40s
    10. Adjusting the edges around fabric
      7m 56s
    11. Perfecting hair
      9m 35s
    12. Sharpening with a High Pass layer
      10m 12s
    13. The also-amazing density mask
      4m 48s
    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 46s
    21. Applying the edge mask
      6m 2s
    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 49s
    3. Meet the Channel Mixer
      8m 26s
    4. Mixing a custom black-and-white image
      7m 10s
    5. Creating a professional-level sepia tone
      5m 36s
    6. Employing the Black & White command
      8m 1s
    7. Extreme channel mixing
      4m 50s
    8. The infrared photography effect
      6m 43s
    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 44s
    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 39s
    16. The simple (but wrong) approach to red-eye correction
      6m 39s
    17. Channel-mixing red pupils
      9m 18s
    18. The expert approach to red-eye correction
      5m 20s
    19. Fixing problem coronas (pupil edges)
      8m 9s
    20. Making pupils match
      4m 8s
  5. 2h 33m
    1. Chops are dead; long live maskops
      1m 37s
    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 22s
    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 33s
    11. The Add mode in action
      7m 51s
    12. The Subtract mode in action
      8m 25s
    13. Comparing two channels with Difference
      5m 24s
    14. Enhancing the contrast with Curves
      9m 11s
    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 1s
    19. Painting away the face and chin
      4m 53s
    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 35s
    2. The big paths project overview
      6m 51s
    3. How to make a path
      8m 25s
    4. Corner points and freeform polygons
      8m 6s
    5. Editing paths with the arrow tools
      5m 2s
    6. Adding and deleting endpoints
      5m 15s
    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
      6m 0s
    12. Combining paths in a vector mask
      7m 55s
    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 10s
    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 17s
    7. Roughing in an object mask
      6m 49s
    8. Drawing missing details with the Lasso tool
      4m 7s
    9. Combining flesh and object masks
      3m 53s
    10. Amplifying the cast shadow
      4m 10s
    11. Selectively choking edges
      3m 58s
    12. Power duplication in Photoshop
      7m 9s
    13. Masking blond hair
      5m 48s
    14. Using Levels to mask iterations
      3m 14s
    15. Drawing an iteration boundary
      4m 55s
    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 29s
    19. Pulling from another file with Apply Image
      4m 52s
    20. Blending clipped layers independently
      5m 43s
    21. Building the flame mask
      9m 22s
    22. Amplifying the flame
      3m 53s
    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 25s
    30. Coloring in the outlines
      8m 31s
    31. Mastering Calculations
      7m 29s
    32. Subtracting and merging the beak
      11m 6s
  8. 1h 33m
    1. The meaning of bit depth (and why you care)
      2m 50s
    2. Scanning line art in 8-bit and 16-bit
      5m 9s
    3. Measuring the 16-bit difference
      8m 9s
    4. Correcting 8-bit images in the 16-bit space
      9m 31s
    5. Opening a raw image directly in 16-bit
      6m 13s
    6. Editing in Camera Raw, opening in 16-bit
      8m 22s
    7. 16-Bit/channel vs. 32-bit/channel (HDR)
      8m 18s
    8. Working with auto-bracketed photographs
      5m 6s
    9. Using the Merge to HDR command
      6m 0s
    10. Adjusting the HDR preview
      6m 0s
    11. Building a 32-bit sky mask
      6m 29s
    12. Properly exposing land and sky
      4m 25s
    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 32s
    5. Using a two-channel displacement map
      6m 28s
    6. Creating a rustic edge effect
      8m 21s
    7. Distorting and shading with a DMap
      6m 34s
    8. Moonlight reflecting off water
      8m 48s
    9. Mapping the reflection onto the water
      7m 7s
    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 5s
    14. Making a repeating watermark pattern
      9m 22s
    15. 3D embossing with Lighting Effects
      10m 48s
    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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