لطفا منتظر باشید ...
Quick Color Effects
Before we get into all the high-end gunk, however, I take a moment to explain the first category of commands, all of which happen to occupy one of the lower sections in the Image Adjustments submenu. These commands — Invert, Equalize, Threshold, and Posterize — produce immediate effects that are difficult or require too much effort to duplicate with the more full-featured commands.
Invert
When you choose Image Adjustments Invert or press Ctrl+I (z -I on the Mac), Photoshop converts every color in your image to its exact opposite, as in a photographic negative. As demonstrated in Figure 17-2, black becomes white, white becomes black, fire becomes water, good becomes evil, dogs romance cats, and the brightness value of every primary color component changes to 255 minus the original brightness value.
Figure 17-2: An image before the advent of the Invert command (left) and after (right).
Image Adjustments Invert is just about the only color-mapping command that retains the rich diversity of color in an image. (The Hue/Saturation command also retains color diversity under specific conditions.) For example, if you apply the Invert command twice in a row, you arrive at your original image without any loss in quality.When you’re working on a full-color image, the Invert command simply inverts the contents of each color channel. This means the command produces very different results when applied to RGB, Lab, and especially CMYK images. Typically, the Invert command changes most pixels in a CMYK image to black. Except in rare instances — such as in night scenes — the black channel contains lots of light shades and few dark shades. So when you invert the channel, it becomes extremely dark.
| Note | Just so you know, when I refer to applying color corrections in the CMYK mode, I mean applying them after choosing Image Mode CMYK Color. Applying corrections in the RGB mode when View Proof Setup Working CMYK is active produces the same effect as when Working CMYK is not selected. The only difference is that the on-screen colors are curtailed slightly to fit inside the CMYK color space. You’re still editing inside the same old red, green, and blue color channels, so the effects are the same. |
| Tip | As I mentioned back in Chapter 12, inverting the contents of the mask channel is the same as applying Select Inverse to a selection outline in the marching ants mode. In fact, this is one of the most useful applications of the filter. |
Equalize
Equalize is the smartest and at the same time least useful of the Image Adjustments pack. When you invoke this command, Photoshop searches for the lightest and darkest color values in a selection. Then it maps the lightest color in all the color channels to white, maps the darkest color in the channels to black, and distributes the remaining colors to other brightness levels in an effort to evenly distribute pixels over the entire brightness spectrum. This doesn’t mean that any one pixel will actually appear white or black after you apply Equalize. Rather, one pixel in at least one channel will be white and another pixel in at least one channel will be black. In an RGB image, for example, the red, green, or blue component of one pixel would be white, but the other two components of that same pixel might be black. The result is a higher contrast image with white and black pixels scattered throughout the color channels.If no portion of the image is selected when you choose Image AdjustmentsEqualize, Photoshop automatically maps the entire image across the brightness spectrum, as shown in the upper-right example of Figure 17-3. If you select a portion of the image before choosing the Equalize command, however, Photoshop displays a dialog box containing the following two radio buttons:
Figure 17-3: An image before (top left) and after (top right) applying the Equalize command when no portion of the image is selected. You can also use the brightness values in a selected region as the basis for equalizing an entire image (bottom left and right).
Equalize Selected Area Only: Select this option to apply the Equalize command strictly within the confines of the selection. The lightest pixel in the selection becomes white, the darkest pixel becomes black, and the others remap to shades in between.
Equalize Entire Image Based on Selected Area: If you select the second radio button, which is the default setting, Photoshop applies the Equalize command to the entire image based on the lightest and darkest colors in the selection. All colors in the image that are lighter than the lightest color in the selection become white and all colors darker than the darkest color in the selection become black.
The bottom two examples in Figure 17-3 show the effects of selecting different parts of the image when the Equalize Entire Image Based on Selected Area option is in force. In the left example, I selected a portion of the image with both light and dark values, which boosted the amount of contrast between highlights and shadows in the image. In many ways, the result is better than the overall adjustment in the upper-right example. In the bottom-right example, I selected a predominantly lighter area, which resulted in an over-darkening of the entire image.The problem with the Equalize command is that it relies too heavily on some bizarre automation to be of much use as a color correction tool. Certainly, you can create some interesting special effects. But if you’d prefer to automatically adjust the colors in an image from black to white regardless of the color mode and composition of the individual channels, choose Image Adjustments Auto Levels or press Ctrl+Shift+L (z -Shift-L on the Mac). If you want to adjust the tonal balance manually and therefore with a higher degree of accuracy, the Levels and Curves commands are tops. I explain all these commands at length later in this chapter.
Threshold
I touched on the Threshold command a few times in previous chapters, including the discussion of the new Hard Mix blend mode in Chapter 13. As you may recall, Threshold converts all colors to either black or white, based on their brightness values. When you choose Image Adjustments Threshold, Photoshop displays the Threshold dialog box shown in Figure 17-4. The dialog box offers a single option box and a slider bar, either of which you can use to specify the medium brightness value in the image. Photoshop changes any color lighter than the value in the Threshold option box to white and changes any color darker than the value to black.
Figure 17-4: The histogram in the Threshold dialog box shows the distribution of brightness values in the selection.
The dialog box also includes a graph of all the colors in the image — even if only a portion of the image is selected. This graph is called a histogram. The width of the histogram represents all 256 possible brightness values, starting at black on the left and progressing through white on the right. The height of each vertical line in the graph demonstrates the number of pixels currently associated with that brightness value. Therefore, you can use the histogram to gauge the distribution of lights and darks in your image. It may seem weird at first, but with enough experience, the histogram becomes an invaluable tool, permitting you to greatly improve the colors that you see on screen. In fact, histograms are so useful that Photoshop CS has added a new palette devoted to displaying a histogram of your image. For complete details, see “The Histogram palette” section later in this chapter.
| Tip | Generally speaking, you achieve the best effects if you change an equal number of pixels to black as you change to white (and vice versa). So rather than moving the slider bar to 128, which is the medium brightness value, move it to the point at which the area of the vertical lines to the left of the slider triangle looks roughly equivalent to the area of the vertical lines to the right of the slider triangle. |
The upper-left example in Figure 17-5 shows the result of applying the Threshold command with a Threshold Level value of 122. Although this value provides a fairly even distribution of black and white pixels, I lost a lot of detail in the dark areas.
Figure 17-5: By itself, the Threshold command tends to deliver flat results (top left). But with judicious application of filters, and a fair amount of manual labor, Threshold can ultimately be the cornerstone in turning a photograph into a line drawing (bottom right).
As you may recall from my “Using the High Pass filter” discussion in Chapter 10, you can use Filter Other High Pass before you use the Threshold command to retain areas of contrast. In the upper-right image in Figure 17-5, I applied the High Pass filter with a Radius of 5.0 pixels, followed by the Threshold command with a value of 122. To get thicker edges in the bottom-left image, I first applied Median and then followed that up with High Pass and Threshold. Next I took all three of these images, put them on separate layers in the same image, used the Multiply blend mode to mix them together, and alternately erased portions of the layers. When I had a satisfactory rough blend, I merged the three layers and used the pencil tool for cleanup. The final image in 17-5 shows the result. It’s not an automatic process, by any means — about an hour of manual labor, in my case — but it’s the most expedient way to convert an image into a line drawing.Figure 17-6 shows combinations of various filters on the same image, always topped off with an application of Threshold. It’s not much as a standalone effect, but Threshold definitely plays well with others.
Figure 17-6: You can create some fun and interesting effects by combining Threshold with other filters.
If you want to achieve a colorful Threshold effect, try applying the Threshold command independently to each color channel. In an RGB image, for example, press Ctrl+1 (z -1 on the Mac) and then apply Image Adjustments Threshold. Then press Ctrl+2 (z -2 on the Mac) and repeat the command, and press Ctrl+3 (z -3 on the Mac) and do it again. Alternatively, you can make Photoshop perform this process for you by using the new Hard Mix blend mode, which I discuss in Chapter 13. Don’t forget that you can also apply the same effect to two channels at once by selecting one channel and Shift-clicking the second. Color Plate 17-1 shows examples of what happens when I apply Threshold along with a handful of filters to independent color channels in the RGB, Lab, and CMYK modes. From the end results on the right of each row, I think it’s clear that Andy Warhol would have loved the Threshold command.
Posterize
The Posterize command is Threshold’s rich cousin. Whereas Threshold boils down an image into only two colors, Posterize can retain as many colors as you like. However, you can’t control how colors are mapped, as you can when you use Threshold. The Posterize dialog box does not provide a histogram or slider bar. Instead, Posterize automatically divides the full range of 256 brightness values into a specified number of equal increments.
To use this command, choose Image Adjustments Posterize and enter a value in the Levels option box. The Levels value represents the number of brightness values that the Posterize command retains. Higher values result in subtle color adjustments; lower values produce more dramatic effects. The first example in Figure 17-7 shows an image subject to a Levels value of 4.
Figure 17-7: Posterize on its own (upper left) and as a follow-up to Add Noise (upper right), Median (lower left), and a combination of Median and Unsharp Mask (lower right).
By now, you may be thinking, “By golly, if Posterize is so similar to Threshold, I wonder how it works when applied as a capper with other filters?” Well, you’re in luck, because this is exactly what I did in the other examples of Figure 17-7. First I applied Add Noise and then Posterize. Interesting how Median increases the impact of Posterize by melding the pixels together, and Add Noise diminishes the impact by dithering the pixels, huh? This Add Noise/Posterize technique is great for creating images with very small color palettes, ideal for the Web.
