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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 show you how to create a Depth Mask, which will allow us to establish a depth of field effect inside of a photograph. Take by way of example this image right here, it's called In the grass.tif and it's found inside of the 12_Specialty_Mask folder, and it features this model, the same model we have been seeing in the previous exercises. So no surprise, this image also comes to us from the same photographer Bobby Osborn of iStockphoto.com and the image features this model in high focus, meaning that she is sharply focused.
The grass which represents her background is at times in high focus, and another time in low focus, meaning blurry. So towards the bottom of the photograph the grass is in high focus, toward the top of the photograph the grass is in low focus. We want to exaggerate that effect using a depth mask as I say, in combination with the Lens Blur Filter inside of Photoshop. So we are going to need to mask her away, so we are protecting her from any modification, and we will create a gradient across the grass, so that portions of the grass are protected, and portions of the grass are selected. So here is how we are going to do it. I am going to start things off by pressing Ctrl+D or Command+D on the Mac to deselect everything inside the image. Then let's go up to the Select menu and choose the Color Range command, because all we want to select inside of this image is the grass, and the grass is so consistently green, and she is so consistently not green, Color Range is our best solution.
So I will go ahead and choose the command. Then I am going to start things off by switching the Selection Preview option to None, so that I can see the full color version of the image, and I am going to click on the grass down here at the base of the image, and then I will Shift+Drag over this region of grass. Now I want you to take care to make sure that you don't Shift+Click or drag over any of these beige leaves in the background, or over any of the yellow or white dandelions. Go ahead and miss those as well, because if you do, Shift+Click or Shift+Drag over those areas, you are going to select a big region of the foreground image being the model of course.
So I am now going to switch the Selection Preview back to Grayscale, so that I can see what I have selected and what I have deselected. And notice that even though I have been pretty careful just to lift greens inside of the image, I have still gone ahead and encroached upon her. I have selected a goodly amount of the model. That maybe just because I have the Fuzziness value set so high. So let's go ahead and take that Fuzziness down to something along the lines of 60. I think that looks a lot better. If you feel like you have still selected too much of the image, you can try again. You could just Shift+Click in the grass, and then Shift+Drag over a little bit of grass down here like so. Then actually it looks pretty good to me. I might Shift+Drag just a little more just to see if I can add tiny bits more grass inside of the image. Once again, without selecting too much of her, and if you feel like you got too far, remember that you can press Ctrl+Z or Command+Z on the Mac to undo that last beige color addition.
From my part I think this looks pretty good. So I will go ahead and click OK in order to accept that selection. Then I am going to Shift+Tab to bring my palettes up on screen, so that I can see my channels palettes. I am going to add a new alpha channel to this image by Alt+Clicking or Option+Clicking this icon down here at the bottom of the channels palette, that little circle in the square icon. I am going to call this channel rough, because it represents my rough base channel. I will go ahead and click OK. Then I will press Ctrl+D or Command+D on the Mac to get rid of that selection. So now I am going to move rough up the stack here a little bit so that it appears before the field alpha channel. The field alpha channel is the one that I have given you in advance here. So here is the rough alpha channel. I will go ahead and move this over so that we can see it, and here is the filed alpha channel.
You can see that there is not all that much difference between two channels. This rough represents that selection that I just created with the Color Range command. Field represents the modified version, the finished version of the rough alpha channel. And the only difference is that I went in there with my Paint/Brush tool set to the Overlay mode, and I painted in white and black in order to firm up the edges a little bit. But notice that I didn't do any work toward the bottom of the image here. So I have done a great job of fixing up all the edges up here toward the top of the image, because I want to absolutely select that top grass, and deselect the model. However, down here, and I don't want to waste that much time, because I am going to deselect the grass in the foreground, right? So we've got the rough step that we've gone ahead and saved off. We've got the field step that we've gone ahead and saved off. Let's create one more variation of this mask. I will go ahead and drag the field alpha channel down here onto the little page icon and drop in order to create a duplicate of that channel.
And I am going to name it Depth, because it will serve as our final depth mask. Then I am going to go grab my Gradient tool right here, and I am going to go ahead and make sure that my foreground color is set to black as it right now. If it's not for you; you would want to press D and then X for default, and then switch. Then go up to your Option bar, and notice that I have made some changes. We will be drawing a radial gradient at this point, and I have got the Reverse checkbox turned on. So just in order to reset things, I am going to right-click on this down pointing arrow and choose Reset tool, to reset the default settings, and then I will switch from the foreground to the background gradient, to the Foreground to Transparent gradient right there. I will be creating a linear gradient of course. Now I am going drag from about this position just above the white dandelion right there, and I am going to drag upward like so.
I have got the Shift key down, so that I am constraining the direction of my gradient to perfectly vertical, and then I will release. Now you can see what I have done, is I have protected, I have gradually protected the foreground grass, the grass toward the bottom of the image, and I have revealed the background grass, the grass toward the top of the image. So that's how you go about creating a Depth Mask. I will show you how we can employ the Depth Mask by using it inside of the Lens Blur Filter, inside of the next exercise.
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