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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."
In this exercise I am going to introduce you to the world of 16-bit per channel color using a couple of gray scale scans of black and white artwork, which may seem like a strange application of 16-bit color but it's actually a great application of 16-bit color as you will learn over the course of these first couple of exercises. The images that you see before you are on screen are Pen & ink(8bit).jpg and the other one is called Pen&ink(16bit).tiff and they are both found inside of get this, the 17_16bitHDR folder. That's going to sound strange too that I have a folder called 17_16 right in a row, but it's because the topic of this chapter is 16-bit per channel in HDR and, of course, this is chapter 17. So we get 17_16 because of the folder naming structure there you go.
Anyway, so that folder, once again, is called 17_16bitHDR. Pen & ink(8bit).jpg, Pen&ink(16bit).tiff, so I took this artwork that I created years and years ago, back when I was struggling freelance artist in addition to my normal job. All these drawings are pen and ink drawings and I got paid $25 a piece but you have to bear in mind this top drawing that's actually just one drawing and so far as my fee was concerned because the client wanted a change made and I did that for free, of course, and then this one earned me another $25, so for the entire page I got $50 and then in order to backup the artwork I made photostats of each of my pieces because I worked at a newspaper at the time and we did not have things like floppy disks, back then we couldn't actually scan the artwork back then so photostats were the way to go and, of course, I was poor, so I bought the worst possible photo album.
One of those things that has diagonal sticky stuff on it and then you put down a piece of acetate and supposedly all your photographs, you know, erode over time but I didn't know any better and I have never tried to take these things off of their background. I removed the acetate which basically, sort of, flops around anymore, it's not sticky but I just left these guys on there to scan them. So I went ahead and saved one scan in the 8-bit per channel space so that I have 28 of possible luminance variations which is 256 possible variations and you can save such images in the JPEG file format so 8-bit per channel images can be saved as JPEGs but if you scan at 16 bit per channel which I did in the case of this image here then I had to either the image as a TIFF image or in the native PSD file format and this image contains as many as 215 power.
For reasons I won't go in to that 16th bit just gets dropped off so far as luminance levels are concerned. So 215 different luminance variations, which is upward of 32,000 different variations. So a lot more variations available to us on right then on left and if you want a method for checking that indeed one is 8 and the other is 16 you only have to look at the title bar, notice that it's Gray/8. It's telling me I am working in the 8-bit per channel space and Gray/16 tells me that I am working in the 16-bit per channel space.
You can also see down here at the bottom that the files sizes are different, the image on left is 4.29M and the image on right which is the exact same dimensions is 8.57M. So it's twice as large and also by the way, because I can apply JPEG compression to the image on left and I can't to the image on right, the file size is very different as well. The size of the file on left is a mere 1.3M and the size of the file on right is 10.4M, so about eight times bigger over here on the right hand side.
Now you might wonder, Well, Gosh! Then there must be some terrific benefits to the 16-bit per channel space that it's so much bigger, but if you were to actually examine these two images, side-by-side, and you could visually accurately measure the images here on screen pixel-for-pixel they would appear to be exactly the same. Now when they are viewed side-by-side they may look a little different because the luminance levels on your monitor tend to vary across the span of the monitor. So that's strictly an appearance issue, but really they should look exactly the same on a pixel-by-pixel basis because your monitor is essentially an 8-bit per channel device, so it's capable of displaying the image on left in total but it's not capable of displaying all the luminance levels inside of the image on right.
So how in the world do we know that the image on right is so much better? Well, we are going to see how much better it is, when we go to correct these images; we need to correct both of the images because they should be black on white not dark gray on light gray. So we are going to need to apply a big contrast enhancement and we will see that the image on the right survives that contrast enhancement with flying colors as it were, whereas the image on left starts to get beaten up a little bit and you will see that, sort of, because we are not going to see it on screen we are going to measure it using some Photoshop tools in the very next exercise.
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