Channels. Channels are a very important feature of Photoshop and are. of special interest to photographers. The book primarily deals

Transcription

1 PDF Channels What Channels Are All About Channels are a very important feature of Photoshop and are of special interest to photographers. The book primarily deals with the concept of painting with Photoshop. However, I feel it is necessary to address this topic in some detail from the photographic side because it will be helpful for the painterly use of Photoshop as well. The color channels are an important topic for the photographer in you. But another aspect of channels is also very important to all Photoshop users the Alpha channel. pdf : Channels photoshop studio with bert monroy : digital painting

2 Color Channels Let s start with the color channels. Every image is made up of channels that contain the tonal and color information for that image. Opening an RGB file in Photoshop will display each of the individual color channels in the Channels panel. RGB displays three channels: Red, Green, and Blue, plus the RGB composite, as shown in Figure 1. Figure 2 shows the Channels panel for a CMYK file. Here you see four channels present: Cyan, Magenta, Yellow, and Black, plus their composite. A grayscale image (Figure 3) will contain a single channel. Let s not forget the LAB color space, which consists of a Luminance channel and two color channels (Figure 4). My good friend Dan Margulis, who was the technical editor for the book, wrote the best book on the planet about LAB: Photoshop LAB Color: The Canyon Conundrum and Other Adventures in the Most Powerful Colorspace (Peachpit Press, 2005). If you need to know about this color space or are just curious, buy Dan s book! Figure 1 The Channels panel displaying an RGB image. Figure 2 The Channels panel displaying a CMYK image. Figure 3 The Channels panel displaying a grayscale image. Figure 4 The Channels panel displaying a LAB image. pdf : Channels photoshop studio with bert monroy : digital painting

3 Many people like to keep track of the individual channels with the Channels panel (Figure 1). In a color file, the channels can be displayed on the monitor by using the keyboard shortcut Command-1 (Ctrl-1) to display the first channel (Red or Cyan). Pressing Command-2 (Ctrl-2) displays the second (Green or Magenta) and so on. Pressing Command-~ (Ctrl-~) makes the composite of all the color channels visible. These shortcuts are clearly displayed in the Channels panel, as shown in Figures 1 through 4. The channels can be seen in their corresponding color or as a grayscale representation. The Interface section of the Preferences dialog box (Figure 5) allows you to set the view you want. When Show Channels in Color is on, the channels can be seen in the color for that channel. Figure 6 shows what the color channels of an RGB image look like. This is a nice feature but can make it difficult to make alterations accurately. This is especially true when viewing CMYK files. In Figure 7 you see a CMYK file where the Yellow is currently visible. When the channel is displayed in yellow, it s so light that it s hard to study the nuances of tone. With Show Channels in Color turned off, it is much easier to see what is going on in the channel, as shown in Figure 8. Figure 5 The Preferences dialog box showing the color viewing choice on. Figure 6 The composite channels and the RGB channels of an image. Figure 7 When displaying channels in color, it is sometimes very difficult to see certain values the Yellow channel for example. Figure 8 When the channel is viewed in grayscale mode, subtle variations are easy to see. pdf : Channels photoshop studio with bert monroy : digital painting

4 Choosing a Channel I tend to work in an RGB color space the majority of the time, mainly because red, green, and blue are the primary colors of light. Working on a computer monitor that uses light to display color makes perfect sense to me. Another reason is that some of the features, such as certain filters, will not work in CMYK. CMYK is primarily for the print industry. An image is broken down into the standard four-color process for printing. When it comes to fine art printing, the CMYK color space is far too limited. Color printers on the market today offer as many as 12 colors, making the reproduction of RGB imagery much richer and truer to the original image for the photographer or the artist. When we eventually need a CMYK for print, my personal preference is to work in RGB as long as possible and convert to CMYK only at the last minute. Others prefer to work in CMYK from the start and that also makes perfect sense. That way they will be choosing colors that work and print properly, especially if Pantone colors are used. Figure 9 The Color Picker displaying the warning that the color chosen is out of gamut. Figure 10 Clicking the alert icon automatically chooses a color that closely matches the original selected color but is within gamut range. I do not find working in RGB a problem because Photoshop warns you when you are picking colors that will not translate properly into CMYK. The small symbol that appears in the Color Picker (Figure 9) alerts you when you are picking colors that are out of gamut. Clicking that symbol automatically changes your choice of color to one that will print properly in CMYK. Figure 10 shows the Color Picker after the alert symbol has been clicked. Note that not only did the alert disappear but all the values changed with the exception of the CMYK values. When the out of gamut color was chosen, Photoshop created the CMYK values that would approximate the chosen color as closely as possible. pdf : Channels photoshop studio with bert monroy : digital painting

5 The RGB Channels Now let s look at how the RGB channels usually stack up and the way they interact in an image. The lighter the values in a channel, the more that color will hit your eye. Figure 11 shows some vegetables in a strainer. The area for the tomatoes and red peppers in the Red channel (Figure 12) appears very light. The darker the same areas are in the other two channels, the more saturated the dominating color will be, in this case red. The red vegetables are a strong red color, so you would expect the Green and Blue channels to be rather dark in the area of the red vegetables. Looking at the same areas of the image in Figures 13 and 14 proves this point. The green areas in this image aren t nearly as saturated as the reds are. The peppers are definitely what most people would call a dull green. For such a color, the dominating channel here the green will only be a little lighter than the other two. There aren t any blue areas in this image, so there aren t any areas where blue is light and the other two dark. However, there are two prominent intermediate colors: the purple eggplant and the yellow pear tomatoes. Such colors have two light channels and one dark channel. In purple, the green is dark and the red and blue are light; in yellow, the red and green are light and the blue is dark. Figure 11 An RGB image with strong, contrasting colors. Figure 13 The Green channel shows the red areas as darker. Figure 12 The Red channel shows the red values as very light. Figure 14 The Blue channel shows the red areas as almost black, indicating the absence of blue in that area. pdf : Channels photoshop studio with bert monroy : digital painting

6 What you perceive as contrast may just be drama, not detail. That s what s happening in the Red channel (Figure 11). The difference between the very light tomatoes and the dark peppers is stark. It almost hides the fact that the Green channel is generally better detailed. The Green channel (Figure 13) has more definition in both the peppers and the tomatoes. In fact, the only area in which the red is better is in the light green lettuce behind the bowl on the left. This is par for the course. The best definition in any object is usually found in the channel that hugs the middle one that s neither too light nor too dark. In the tomatoes, the Red channel is too light and the Blue channel is too dark, so the Green channel has the best tonal range. The green peppers are so dark that the Green channel is still the best, even though it s the lightest of the three channels. The Blue channel is the darkest of the three in almost every image. It also can be home to a good deal of noise an objectionable, irregular pattern of stray pixels. It has little impact on the contrast of the image. We aren t as sensitive to blue light. We are more sensitive to red light and even more so to green. Professional retouchers rarely try to get contrast out of the Blue channel and instead concentrate on the Red and Green channels. Each channel can be accessed independently. This can be very advantageous in certain situations. Understanding the functions of these color channels is crucial when making color corrections or other modifications to an image. Knowing these little tidbits about the RGB color space makes correcting images an easy task. Take for example the Despeckle filter (Filter > Noise > Despeckle). Most people apply it or some substitute, such as Gaussian Blur or Dust & Scratches, to remove film grain or other noise. The downside is that the image tends to become blurred. If you apply one of these filters to the Blue channel only, you will get a much bigger bang for the buck. You will get much less degradation because the noise or grain appears disproportionately in the Blue channel as opposed to the other two. Similarly, with experience you will learn which channel is your best bet for enhancing certain types of images. If you re working with a picture of a face, the Green channel will usually be the best detailed channel because the Red will be too light and the Blue too dark. But if you re trying to emphasize the cloud pattern in a blue sky, the Red is your best option because the Green and Blue will be too light to hold much detail. pdf : Channels photoshop studio with bert monroy : digital painting

7 To better see the difference between the three channels, take a close look at the image in Figure 15 a simple shot of a building at dusk. If you zoom into the window and look at the individual channels, you ll see the difference. Figure 16 is the Red channel. This channel holds the contrast information for the image. The lights and darks are very pronounced. Figure 17 is the Green channel. Note the sharpness of the detail. Applying the Sharpen filters to this channel exclusively will give you better results in smaller increments than applying the filter to the overall image. If you choose to sharpen the image this way, it should be done on a separate layer set to Luminosity mode. That way, the sharpening effect will be retained, while eliminating the possibility that any artifacts of the process become green. Figure 15 The photo of a building at dusk. Figure 18 is the Blue channel. It is clear to see where the film grain lives. Figure 16 The Red channel shows strong contrast. Figure 17 The Green channel shows the fine details. Figure 18 The Blue channel shows the film grain. pdf : Channels photoshop studio with bert monroy : digital painting

8 The Alpha Channel An RGB file has three channels, CMYK has four, and so on, but users can add additional channels called Alpha channels. These channels can be used for different colors such as spot colors. More commonly they house masks needed for retouching. Though Layer Styles have simplified many of the processes that earlier required the use of Alpha channels, the channels still have very powerful functions. With the exception of spot color channels, Alpha channels contain no color information; in fact, they are purely grayscale channels. They are used as specialized, reusable selections or masks. Reusable because they reside within the file and will be there every time the file is opened, provided it is still a PSD file. Some formats will eliminate the Alpha channel. For example, JPEG images do not support alpha channels. If you ask Photoshop to save such an image in JPEG format, you will be warned that it must be saved as a copy and that the alpha channel(s) will be discarded. Selections and Alpha Channels Selecting part of an image with any of the selection tools, such as the Lasso Tool, is a temporary state. Once you deselect it, the selection is gone. But if that selection is saved to an Alpha channel, it can be recalled at any time. When you have selected an area of an image, you have isolated that area for some action that will affect that area exclusively. Once the area is selected you can colorize it, filter it, scale it, or perhaps duplicate it. Anything you want to do will be done to only that area, leaving the unselected areas of the image untouched. pdf : Channels photoshop studio with bert monroy : digital painting

9 Selecting can be done with any of the selection tools including the Quick Selection Tool, Lasso Tool, Rectangular Marquee Tool, and Magic Wand. Color Range, found in the Select menu, allows you to make selections based on sampled colors, specific colors, highlights, mid-tones, shadows, or colors that are out of gamut. As previously stated, these selections are temporary. Once the selection is made, choosing Save Selection from the Select menu will give you the choice of storing the selection to an Alpha channel (Figure 19) or as a separate grayscale document. When you choose Save Selection, a dialog box appears (Figure 20), allowing you to name the channel. You also have the ability to save the channel in an entirely different document. Figure 21 shows the drop-down menu in the dialog box where New has been chosen. This option sends the contents of the selection into an Alpha channel in another file. This is very important if you have more channels saved than a Photoshop document can store. Although you might think that the limit of 56 channels might be enough, a complex image might require more than that. Damen has over 500 Alpha channels. Figure 19 The Save Selection option in the Select menu. Figure 20 The Save Selection dialog box. So what is so important about an Alpha channel? Yes, you can save it; therefore, it is a reusable selection. But the really cool feature of the Alpha channel is that it creates a controlled selection. You can control where something is going to happen and how much will happen. If you read the explanation of layer masks in the Layers PDF, you know that the mask goes from black to white with 254 levels of gray in between. In the layer mask, white allowed you to see the layer and black made it invisible. The level of gray matched the level of visibility. Figure 21 The New option is selected to send the Alpha channel to another document. pdf : Channels photoshop studio with bert monroy : digital painting

10 The Alpha channel uses the same concept but to a different end. Where the Alpha channel is white, the image is exposed to an effect. Basically white is the selected area. Black protects the image. The Alpha channel also has 254 levels of gray the level of gray equals the level of exposure. The Alpha channel is a selection; therefore, anything that you can do in Photoshop can be done through the selection created by an Alpha channel. If you want to apply a filter to an image, you simply click the Filter menu and choose a filter. For example, Figure 22 is an image I want to apply a Find Edges filter to, so I select Filter > Stylize > Find Edges. The result is shown in Figure 23. Let s say I want the effect to happen only in the top portion of the image and gradually fade as it goes down. This is where the Alpha channel comes in. Figure 22 The image to be filtered. Figure 23 The Find Edges filter has been applied to the entire image. pdf : Channels photoshop studio with bert monroy : digital painting 10

11 Nothing is selected in the image, so Save Selection is not active. Clicking on the Create new channel icon in the Channels panel gives me a black channel. Using the Gradient Tool, I create a gradient from top to bottom (Figure 24). When I load the channel, the marching ants of the selection appear from the 50% gray to the white, as shown in Figure 25. If I apply the Find Edges filter through the Alpha channel selection, the result is quite different. Figure 26 shows the door again, but this time the effect of the filter is segregated to the top portion of the image and gradually fades as it goes down. Figure 24 An Alpha channel containing a gradient from white to black. Figure 25 The image is selected through the Alpha channel gradient. Figure 26 The Find Edges filter is applied through the gradient of the Alpha channel, causing the effect to happen gradually. pdf : Channels photoshop studio with bert monroy : digital painting 11

12 Effects and Alpha Channels Figure 27 demonstrates the effect of a colorization of an image done through the mask of an Alpha channel. You can see how the effect is gradually applied through the gray levels to white. Figure 28 shows a series of filters applied through a radial gradient in an Alpha channel. Again you can see that the level of gray affects the strength of the effect. Figure 27 Colorization is applied through a linear gradient. The colorization is 100% through the white of the Alpha channel and lessens as it passes through the grays. Figure 28 A series of filters are applied through a radial gradient in an Alpha channel. pdf : Channels photoshop studio with bert monroy : digital painting 12

13 So what is so useful about applying effects through Alpha channel selections? Figure 29 shows a file that has a tree in a layer. The layer was duplicated, filled with black, and scaled to make it look like a long shadow cast by the setting sun. It looks pretty good except in real life when a shadow of this type is cast, it softens as it gets farther from the object casting it. A filter will correct the problem, but you need the shadow to occur only on the far left and gradually lessen as it gets closer to the tree. In Figure 30 you can see the Alpha channel through which this filter will be applied. Figure 29 A shadow stretches across the wall in this document. The filter is loaded by either choosing Select >Load Selection or by simply clicking on it in the panel while pressing the Command (Ctrl) key. Figure 31 shows the marching ants of the selection. Figure 30 The Alpha channel through which the shadow will be altered. Figure 31 The selection is made with the Alpha channel. pdf : Channels photoshop studio with bert monroy : digital painting 13

14 In Figure 32 the Motion Blur filter is applied (Filter > Blur > Motion Blur). The result is shown in Figure 33 where the shadow now has an appropriate look. A similar situation appears on the shadow for the glass on the lower right of the painting Lunch in Tiburon (Figure 34). The shadow softens as it gets farther from the glass. Figure 35 shows the Alpha channel. In Figure 36 you can see the layer containing the shadow after the Gaussian Blur filter has been applied through the Alpha channel. Figure 32 The selected area is processed through the Motion Blur filter. Figure 35 The Alpha channel used to filter the shadow. Figure 33 The final shadow. Figure 34 The shadow cast by the glass. Figure 36 The filtered shadow in its layer. pdf : Channels photoshop studio with bert monroy : digital painting 14

15 Complex Alpha Channels Until now you have seen fairly simple Alpha channels. Gradients were all that were needed to achieve the desired effects. Alpha channels can become very complex. But no matter how difficult the task, a proper mask will make it easy. Let s look at a complex Alpha channel in the painting Oakland (Figure 37). Figure 38 shows a close-up view of the neon tubes at the top of the marquee. Look closely at the different tones that give the neon tubes their three-dimensional appearance. Every one of the tones was added through an Alpha channel mask. Why not just use a layer style? A layer style wouldn t work because layer styles affect the entire shape evenly. I wanted to control where the tones would appear, thus enhancing the effect of lights and shadows on uneven surfaces. Figure 37 The painting Oakland. Figure 38 A close-up showing the details of the neon tubes. pdf : Channels photoshop studio with bert monroy : digital painting 15

16 Stroking paths in a layer created the basic tubes. The Pen Tool and Patterns PDF file explains this technique. The layer was turned into a selection by Command-clicking (Ctrl-clicking) on the preview icon for the layer in the Layers panel and then saved to an Alpha channel, as shown in Figure 39. The channel was automatically named Alpha 1. Any subsequent channels were named in numerical sequence, but you also have the option to name them. I duplicated the Alpha channel and blurred it with the Gaussian Blur filter (Filter > Blur > Gaussian Blur) to create the soft edges. The blurred Alpha channel, Alpha 2 is shown in Figure 40. Figure 39 The Alpha channel for the neon tubes. I turned on the eye for both channels (Figure 41) to see their interaction. The default viewing color is red. I wanted to see where one channel was in relation to the other. To make this visualization easier, I double-clicked on the channel in the Channels panel to bring up the Channel Options dialog box (Figure 42). Figure 40 The duplicate of the Alpha channel that has been blurred. Figure 42 The Channel Options dialog box. Figure 41 The two Alpha channels visible at the same time. pdf : Channels photoshop studio with bert monroy : digital painting 16

17 In the Channel Options dialog box I set the color to blue. This box has nothing to do with how a channel will operate. The settings deal only with how you see the channel when other channels are visible. The result was a mixing of the red for Alpha 1 and the blue of Alpha 2, which gave me a clearer depiction of the two channels, as shown in Figure 43. A closer view is shown in Figure 44. Then I needed to create a single Alpha channel that was composed of where the two channels intersected. To do this, I relied on one of the most overlooked features of Photoshop Calculations. I chose Calculations from the Image menu (Figure 45). Figure 43 The changed color for the Alpha channel makes it easy to see it in contrast to the other visible Alpha channel. Figure 44 A close-up of the two Alpha channels. Figure 45 Calculations in the Image menu. pdf : Channels photoshop studio with bert monroy : digital painting 17

18 I placed the blurred Alpha 2 in Source 1 and the original Alpha 1 in Source 2. The position of the channels does make a difference. I set the Blending to Subtract. I sent the Results to a New Channel (Figure 46). The ensuing channel exposed the outer edges of the tubes (Figure 47). Making this Alpha channel a selection allowed me to introduce colors only where I wanted them limited to the outer edges of the tubes. Moving the blurred channel up and down with the Move Tool, I followed the same procedure and created Alpha channels that exposed the top and bottom of the tubes independently, as shown in Figures 48 and 49. Figure 46 The Calculations dialog box. You just saw the use of Calculations, which I mentioned as being one of the most overlooked features. I urge you to experiment with this feature because it will become a vital player in your arsenal of tools. Figure 47 The result of Calculations on the two Alpha channels. Figure 48 The result of Calculations on the two Alpha channels with the blurred channel repositioned. Figure 49 The result of Calculations on the two Alpha channels with the blurred channel repositioned in the other direction. pdf : Channels photoshop studio with bert monroy : digital painting 18

19 Calculations Calculations has been part of Photoshop since the beginning. It went through one change in all the versions of Photoshop it was split into three commands. The three commands live in their own section of the Image menu (Figure 50). Duplicate Duplicate is the first of the three commands. Back in the days before layers, I used to do a bunch of demos where I used the Duplicate command within Calculations for all sorts of special effects. Mark Hamberg, then the Product Manager, told me that it was my demos that inspired him to make Duplicate a separate command. The downside was that as a separate command it only did one thing make a duplicate of the file. That wasn t really a problem since other features, not to mention the introduction of layer styles, had simplified a lot of what I was doing. However, making a duplicate of a file was, and still is, a major function. If you are working on a two gigabyte file and want to create a second version of it on the screen, try selecting all and then copying it. Open a new file and paste. You ll find lots of RAM being used there! Try saving it with a new name and then reopening the original so both are open. You ll find it s a good time to go for a walk! Figure 50 Duplicate, Apply Image, and Calculations in the Image menu. Figure 51 The Duplicate Image dialog box. Figure 52 The Duplicate Image dialog box with Duplicate Merged Layers Only turned on. Duplicate makes this process a snap. Duplicate uses an internal pipeline to stream data that does not tax the RAM of your machine or throw massive amounts of data into your scratch disk. When you choose Duplicate, a dialog box appears (Figure 51). Here you can give the new file a name. If the file being duplicated contains one or more layers, Duplicate Merged Layers Only will be active. Turning it on, as shown in Figure 52, will duplicate the file as a flattened file. In other words, all the layers will be placed into a single layer in the duplicate file. pdf : Channels photoshop studio with bert monroy : digital painting 19

20 Apply Image and Calculations Apply Image and Calculations pack the true power for doing the interchannel math necessary for controlling many of the functions within Photoshop. Math? Did he say math? Don t let that word scare you. No, you don t have to brush up on the multiplication tables you learned back in grammar school. Apply Image and Calculations compare the pixels of two images, layers, or channels and generate new pixels based on the operations you choose in the dialog box. Apply Image and Calculations allow you to process data from one place to another. The only requirement is that both sources need to be the exact same size and resolution. Color spaces can differ. For example, you can apply a channel from an RGB image into a CMYK channel or document. There are two exceptions: Bitmap and Indexed Color color spaces do not work. Apply Image and Calculations perform similar functions but with one major difference: Apply Image allows you to work with the composite of all the color channels of an image as the source, the target, or both. The result is applied to the target. On the other hand, Calculations only uses a single channel or layer as a source that works with a second source. It cannot be applied to composite channels. The result, unlike the fixed target of Apply Image, is chosen within the Calculations dialog box. pdf : Channels photoshop studio with bert monroy : digital painting 20

21 Apply Image Apply Image does not give you a choice of targets you must apply the result to whatever channel or channels are currently active. Any processing done with Apply Image is applied to the target. The target consists of the currently selected image, channel, or layer. The source of the blend, however, can be any channel, from any layer, of any document in any colorspace, provided only that it is the same size as the target. Figure 53 shows the image Lunch in Tiburon. In Figure 54 you see the Apply Image dialog box. The Source is set to be the RGB composite channel of the painting Lunch in Tiburon. The Target is set for the same data as the Source. The Blending is set to Multiply. Figure 55 shows the result of the Apply Image of the original. The result is exactly the same as when you duplicate the image into a layer and set the layer to Multiply. In Figure 56 the background has been duplicated and put in Multiply mode. The results shown in Figures 55 and 56 are exactly the same. The difference is that there is no second layer in Figure 55; therefore, the file size remains the same. If you look at the memory usage on the lower left of each window for the two files, you ll see that the file with a second layer (Figure 56) is larger. Figure 53 The painting Lunch in Tiburon. Figure 54 The Apply Image dialog box. Figure 55 The result of Apply Image. Figure 56 The result of a layer duplicated and put in Multiply mode. pdf : Channels photoshop studio with bert monroy : digital painting 21

22 Calculations Calculations is the feature I use the most in my paintings to create specific masks to apply effects. Earlier in this chapter I discussed Alpha channels. It is through these Alpha channel masks that I apply most of my filters and other effects. With Calculations I manipulate those Alpha channels. Calculations, unlike Apply Image works with two sources at one time. These two sources can be in the same file or in separate files. The sources can be component color channels or Alpha channels. An active selection can also be used as a source. Photoshop will treat the selection as an Alpha channel with the selected area becoming white and the unselected area black. If a Layer is selected as a source, the transparency of that layer will be used as a mask. Figure 57 The Calculations dialog box. The result of Calculations is always a separate channel, whereas Apply Image modifies an existing channel or channels. For example, Figure 57 shows the Calculations dialog box. Source 1 is set to a different file than Source 2. As mentioned earlier, the only requirement for using two different files is that both files must have the exact same dimensions and resolution. The Channel for Source 1 is the Red component channel, whereas an Alpha channel that exists in Source 2 is being used for the Channel in that Source. The Blending can be set to any of the modes. The modes are outlined in the Layers PDF file. You ll find two modes here and in Apply Image that do not exist anywhere else: Add and Subtract. The Opacity deals with Source 1. Mask uses a channel as a mask for Source 1. It works exactly as a layer mask; it masks parts of Source 1 so that parts of Source 2 can show through. pdf : Channels photoshop studio with bert monroy : digital painting 22

23 In Result you decide the destination for the result of your Calculation parameters. The result can be sent to a New Channel, a New Document, or a Selection in the current file. The different Blending modes coupled with masks and specific channel information give you unlimited possibilities to the kinds of masks you can create. There may be times when you want to have the area where two masks intersect be a mask. Figure 58 shows an Alpha channel that might be used to create a light streaming through an open door. In Figure 59 is another light source from a window on the opposite side of the door. You might want to create an effect within the area where the two lights meet. Figure 60 shows the Calculations for the two channels with Blending set to Multiply. The result is the area where the two light sources intersect. Figure 58 An Alpha channel to create a light source from a doorway on the ground. Figure 59 An Alpha channel to create a light source from a window on the ground. Figure 60 In the Calculations dialog box, a new channel is created that consists of the area where the two light sources intersect. pdf : Channels photoshop studio with bert monroy : digital painting 23

24 Sometimes a typical selection technique will not work to select complex areas. Often, an Alpha channel derived from the image data will do a better job. In Figure 61 you see a redwood tree with tons of needles covering the sky. The sky area needs some clouds. It is the tricky sections in between all the needles that make this selection difficult. Your best course of action in this case would be to study the existing color channels and pull a mask based on the data in the channel. You need to look for a channel that gives you the best contrast. The Red channel (Figure 62) has a lot of detail visible on the tree trunk as well as the needles on the left. As you learned earlier, these areas are light in the channel because there is more red in that area due to the reddish light of the setting sun. Figure 61 An image of a redwood tree. The Green channel in Figure 63 has better contrast. But the Blue channel in Figure 64 has the strongest contrast, making it the best candidate for the mask. Figure 62 The Red channel for the image of the tree. Figure 63 The Green channel for the image of the tree. Figure 64 The Blue channel for the image of the tree. This channel shows the most contrast for the sky against the limbs and needles of the tree. pdf : Channels photoshop studio with bert monroy : digital painting 24

25 The Blue channel is duplicated by dragging it over the Create new channel icon (Figure 65). This does not double the amount of blue in the image, in fact it makes no change in the image at all. It is now an Alpha channel based on the information in the Blue channel. Remember that Alpha channels have nothing to do with the actual image. They are masks through which you alter the image. As you see in Figure 66, there are still some areas of the tree trunk where you can see detail. These areas need to be forced to black to protect them when you alter the sky. In this case Curves does the trick (Figure 67). Figure 65 The Blue channel is duplicated to become an Alpha channel. Figure 66 Areas of the tree are still showing detail. Figure 67 Curves is chosen from the Image menu. pdf : Channels photoshop studio with bert monroy : digital painting 25

26 Using the Pencil option, as shown in Figure 68, the cursor is dragged over the area of the tree trunk where you see detail. Notice that as the cursor is dragged over an image the cursor becomes the Eyedropper Tool. Clicking on the area of the image will denote on the scale where those particular values exist. In Figure 67 you can see that the values are on the lower left, as indicated by a small circle. Since black protects, those values must be forced to black. This is what the Pencil Tool in Curves can do. By clicking and dragging the Pencil along the bottom edge of the box (Figure 69), the values of that area are forced to black. Figure 68 In the Curves dialog box, the cursor is dragged over the area of the tree to determine where the values for the trunk exist. The Smooth button is clicked (Figure 70) to soften the transition from the black to the first gray values to eliminate any harsh edges that might have resulted from the alteration. Figure 69 In the Curves dialog box, the Pencil cursor is dragged along the bottom, forcing those values to black. Figure 70 In the Curves dialog box, click the Smooth button. pdf : Channels photoshop studio with bert monroy : digital painting 26

27 In Figure 71 the Alpha channel has been made into a selection. Once you have a selected area, you can alter it in any way you want. Figure 72 shows that I then simply chose New Layer Via Copy for the selected area. I used it to create a Clipping Group with a layer containing a cloud-filled sky. Figure 71 The Alpha channel is made into a selection. Figure 72 The dull blue sky is replaced with a sky full of clouds. pdf : Channels photoshop studio with bert monroy : digital painting 27