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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."
All right, glad to see you back for more here because this is kind of a tough topic to wrap your mind around. So we have got an 8 bit per channel image open here on the left and a 16 bit per channel image open here on the right. The images in question are Pen&ink (8bit). jpg and Pen&ink(16bit).tif and they are both found inside of the 17_16bitHDR folder, just to keep things simple. To go ahead and throw them in the folder called 17_16bit_HDR that will simplify things. Anyway it's our folder naming convention so there you go but I have got these two images open there and functionally identical scans. Well, at least they are scans of the exact same artwork. And they are both scanned using the same model scanner the only difference is I saved one of the scans in the 8 bit per channel space and I saved the other scan in the 16 bit per channel. So we have up to 256 different brightness values over here on the left and we have as many as upwards of 32000 brightness values over here on the right.
We probably don't actually have that many brightness values but we could have that many, put it that way. But it's safe to say, we don't have any more than 256 over here on the left and we have probably a couple of thousand really over here on the right and that's going to make a big difference when we go to edit the image and the edit that we need to apply, we need to increase the contrast of these images so that the dark lines are black, absolutely black and the white areas are absolutely white instead of gray as they are right now, these light gray areas.
So, let's go ahead and use the Levels command, I am going to first make sure that the image on left is active then I am going to press Ctrl+L or Command+L on the Mac to bring up the Levels dialog box and you can see this histogram right there and notice that there is essentially three different lumps in the histogram. This lump over here, this guy right there, that represents the dark lines inside of my artwork so that's the actual Pen&ink work and then this big hump over here on the right represents page white, this area right there and then this hump that represents all of those gray diagonal lines that are holding the photos into place on the photo album page.
So here is what we are going to do, I am going to increase the black point value beyond that first hump, well beyond it, in fact, I am going to take that black point value up to a 115 so anything with the luminance level of a 155 or darker becomes black and then I am going to drag the white slider not just beyond page white. Notice that leaves the photo album down here at the bottom of the screen. Let's go ahead and take it beyond the next hump as well which makes the photo album lines disappear. Good, and I am going to take the White Point Value down to a 150 so anything with a luminance level of a 150 or lighter becomes white. So all we have left is this little range of luminance levels right there that's going to get stretched across the entire histogram.
All right, I will go ahead and click OK to apply that modification. Now let's go over to Pen&ink(16bit).tif and I want you to press Ctrl+Alt+L or Command+Option+L on the Mac in order to replay that exact same levels modification. So notice it's taking to me using the exact same values as before as if zero is black, which is true in 16 bit as well but 255 for white that is strictly an 8 bit per channel convention it doesn't apply to 16 bit, it would be 32000 something for white and Levels goes ahead and talks to me in 8 bit per channel language no matter what bit depth I am working with. And that's true for a lot of commands inside of Photoshop.
They talk to you in 8 bit language even if you are working on a 16 bit per channel image but rest assured you are working on a 16 bit per channel image as you will see very shortly. So anyway exact same modification being applied and the two images as a result, I will go ahead and click on the OK button in order to accept that modification, look the same. They look the same before the modification, they look the same after the modification. The reason being of course, that we are viewing the images on a 8 bit per channel display device and so the device really can't show us the difference between these two images but we can see the difference between the two images using a histogram.
So check this out, I will go back to the original image and we might as well use the histogram in the Levels dialog box so I will press Ctrl+L once again and check that out. Notice these little whiskers down here at the bottom of the screen. Those represent all the gray values that are available to us and then this big line over here represents black and this big line over here represents white. Now the problem is that Photoshop goes ahead and scales everything in between so they just appear as tiny little whiskers even though there is tons of gray pixels inside of this image.
So what we are better off doing is selecting only the gray pixels and measuring them. And here is how we are going to do that, go ahead and cancel out of this dialog box. I want you to go over to the Magic Wand tool, grab the Magic Wand tool her from the toolbox and make sure these settings are enforced; The Tolerance value should be zero, Anti- Alias should be turned off, Contiguous off, Sample All Layers off, so all the check boxes off this time around Tolerance zero. And now click in the white pixel and that will select all of the white pixels inside of the image. Now I want you to press the Shift key and I want you to click on a black pixel like, let's click inside of the guys eye, sort of towards the middle of the eye. So Shift+Click on a Black pixel and that selects all of the Black pixels as well. So now all of the black pixels are selected and all of the white pixels are selected. None of the Gray pixels are selected.
Now I want you to go up to the select menu an chose Inverse. Now you don't have any White pixels or Black pixels selected, you only have gray pixels selected. And you can see the marching ants, go ahead and press Ctrl+H or Command+H on the Mac so that the marching ants stop animating on screen there. Then press Ctrl+L or Command+L on a Mac and those are your remaining gray values. Notice that they look like columns or at least longer hairs separated by big areas of space and those big areas of space represent gaps in the histogram areas where there are no continuous colors so we do not have continuous colors anymore. We have posterization, pretty bad posterization in fact as well. Go ahead and click Cancel in order to cancel out that dialog box, let's run through those same steps here in the 16 bit per channel image.
So I have gone ahead and selected all the white pixels, I am going to Shift+Click on the black pixels to select all of the black pixels as well. And then I am going to go up to the Select menu and choose the Inverse command and now only the gray pixels are selected inside the image, neither black nor white just the gray pixels in between. I am going to press Ctrl+H or Command+H on the Mac to stop animating the marching ants and then I am going to press Ctrl+L and get a load to this. Ctrl or Command+L, a nice full rich histogram. So we still have more than 256 luminance levels available inside of this image, despite our drastic modification.
As a result this image on right, even though it doesn't look any better on screen, is going to hold up better to further editing because at some point we would start noticing the difference and it's going to print better as well and it's going to up sample better and down sample better and everything else. Anything else you care to do it inside of Photoshop is going to go better, thanks to the fact that we are working with a 16 bit per channel image. So the practical up shot in this case is when you are scanning black and white line art always scan to 16 bit per channel, then any modifications that you make inside of Photoshop are essentially, really, non-destructive modifications, you are going to have a hard time, harming this image where you are going to have a very easy time harming the one over here on the left.
All right, so that's just an introduction to 16 bit. We are going to see more 16 bit color this time applied to a continuous tone full color photograph in the next exercise.
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