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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, let's take a look at how we change the preview of our 32 Bit HDR file that we created from three auto-bracketed exposures using the Merge to HDR function here inside Photoshop CS3. But before we do I should go ahead and save off this file so that you can open it up if you need to, if something went wrong on your end. So let's save it. You might want to save it along with me if everything went right on your end. Go up to the File menu and choose the Save command, I bet you knew that part.
But here is the interesting news which is the file formats that are available to you. Typically you are going to work with one of two file formate where HDR images are concerned. You can use TIFF if you want to so I guess one of three formate. But most likely you are going to use a native PSD format especially if the image has layers. If you have a 32 Bits/HDR file that has layers associated with it or Alpha Channels that kind of thing, then you are probably going to go with the Photoshop format. If you are looking for a wider compatibility with other applications that support HDR out there and there are some amazing HDR applications, then you are probably going to want to go with the Radiance format which ends with a .HDR extension. And so that's the way I am going to save this particular file, because it doesn't have any layers. And I am going to go ahead and call this, Houseboat 32-bit.hdr. So if you want to go ahead and open the file along with me here then you can. That's going to be the name of the file. It will appear in the 17_16bit_HDR Folder. I'll go ahead and click on the Save button.
Now this is going to be a big file incidentally. I am going to go down here to the right pointing arrowhead and I am going to switch over to Document Sizes, and you can see that is 90 Megs in memory. It's about 23 Megs on disk incidentally this HDR file is. All right, so having saved it which is of course a good idea, now we are going to switch this option back to where it was a moment ago, 32-bit Exposure. And notice that you have this little slider. It's only applicable to 32-bit HDR files. And you can actually change the Exposure of the photograph. If you drag to the right you are going to elevate the Exposure, you are going to make the image lighter, and if you drag to the left you are going to reduce the Exposure and this is strictly a preview function down here. And so you can see how your preview is changing on-screen.
The problem with this function, one of the rather confusing things about it is it's actually applying this Exposure function, this additional Exposure function that works on top of the preview. And so it doesn't update the thumbnail inside of the Layers palette or inside of the Channels palette. So those thumbnails and I actually think that Navigator palette let me check that out also the Navigator palette suffers. So all of these thumbnails are showing the original preview setting. And if you want to change that then here is what you do. You might want to go ahead and reset your Exposure to the middle. It doesn't matter where you set this Exposure by the way. You aren't actually modifying the pixels for real, because bear in mind as they are moving through these walls to other chairs it's really pretty much impossible to mess up the image using this Exposure slider incidentally.
Go up to the View menu, this one is strictly a preview setting so it can't harm the file. And you choose 32-bit Preview Options. And this will update all previews including the thumbnails. Notice that the Exposure is really cranked +6.67 that's a huge Exposure modification. I am going to go ahead and change the Exposure value to 0, and I am going to reset the Gamma to 1 as well, just so that we have just baseline settings for this particular image. Notice that the thumbnails did update, Navigator palette, Layers palette in the Channels palette as well, and I'll click OK in order to accept this new preview.
Now we can use this Exposure slider down here to test the highlights and shadows once again to see whether you really have as good the highlights and shadows as you thought you did in the previous dialog box back in the last exercise? Or whether in our case things are even better than we thought. So I am going to increase the Exposure so that I can see the shadows disappear and see exactly how much we have in a way of shadows and we've got perfect shadows look at those just disappear off there, and I might as well go ahead and zoom in on this image just so that we can really check it out in detail.
So I am not suggesting for a second that we leave this slider in this position, we are just using it to see where our shadows are. And you can see that things get pretty noisy in the darkest shadows there. But that's fairly to be expected by the way. All right, now let's go check out those highlights in the upper left corner of the image. As long as I've got my Navigator palette open, I am going to click in the upper left corner of that Navigator. I am also going to get myself a little more vertical room for this palette so I can move around a little more easily there. Here is the sky, right now it's very hot. Let's go ahead and move that Exposure value down. And yeah this looks pretty -- we are missing some highlights right in this location you can see that we are going from having white hot highlights there to the highlights now sort of disappearing into the light grays.
But we do have some banding right there so we have -- but essentially these light gray clouds in this case flattening out in this region right here. So there is not really any sculptural detail in this portion of the clouds. But we do have some nice gradients going after this point right there. So that's pretty good. I think things are working pretty well in our favor. If we have a little bit of hotness then we have a hotness, what can we do? All right, so that's just that's all about previewing the image, the HDR image here inside of Photoshop. In the next exercise we are going to start to correct the image so that we have some nice warm wonderful detail down here in the boat area so the boat is reasonable light and we still manage to retain this wonderful sky action up here in the upper left-hand portion of the image.
Please join me.
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