Noise Factors

Photoshop offers four loosely associated filters in its Filter Noise submenu. One filter adds random pixels ” known as noise ” to an image. The other three ” Despeckle, Dust and Scratches, and Median ” average the colors of neighboring pixels in ways that theoretically remove noise from poorly scanned images. But in fact, they function nearly as well at removing essential detail as they do at removing extraneous noise. In the following sections, you'll learn how the Noise filters work and see a few effective demonstrations , and then you can apply what you've learned to your own images.

Adding noise

Noise adds grit and texture to an image. Noise makes an image look like you shot it while someone was throwing sand in your face as you were hammering away at the shutter release. Noise makes a photo look like there's a story behind it, and when it's added to some photos, it can make you look cool, even if you aren't.

You add noise by choosing Filter Noise Add Noise. Shown in Figure 8-31, the Add Noise dialog box features the following options:

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Figure 8-31: The Add Noise dialog box asks you to specify the amount and variety of noise you want to add to the selection.
  • Amount: This value determines how far pixels in the image can stray from their current colors. The value represents a color range rather than a brightness range and is expressed as a percentage. You can enter a value as high as 400 percent. The percentage is based on 256 brightness values per channel if you're working with a 24-bit image and 32,768 brightness values for 16-bit-per- channel images. So with a 24-bit image (8-bit channels), the default value of12.5 percent is equivalent to 32 brightness levels, which is 12.5 percent of 256.

    For example, if you enter a value of 12.5 percent for a 24-bit image, Photoshop can apply any color that is 32 shades more or less red, more or less green, and more or less blue than the current color. If you enter 400 percent, Photoshop theoretically can go 1,024 brightness values lighter or darker . But that results in colors that are out of range; therefore, they get clipped to black or white. The result is higher contrast inside the noise pixels.

  • Uniform: Select this option to apply colors absolutely randomly within the specified range. Photoshop is no more likely to apply one color within the range than another, thus resulting in an even color distribution.

  • Gaussian: When you select this option, you instruct Photoshop to prioritize colors along the Gaussian distribution curve. The effect is that most colors added by the filter either closely resemble the original colors or push the boundaries of the specified range. In other words, this option results in more light and dark pixels, thus producing a more pronounced effect.

  • Monochromatic: When working on a color image, the Add Noise filter distributes pixels randomly throughout the different color channels. However, when you select the Monochrome check box, Photoshop distributes the noise in the same manner in all channels. The result is grayscale noise. (This option has no effect on grayscale images, because the noise can't get any more grayscale than it already is.)

Figure 8-32 compares three applications of Uniform noise to identical amounts of Gaussian noise, the latter of which produces higher contrast. The final row in the figure demonstrates the generally more visually appealing Monochromatic-type noise. In each case, the Distribution was set to Uniform.

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Figure 8-32: The Gaussian option (middle row) produces more pronounced effects than the Uniform option (top) at identical Amount values. Select the Monochromatic check box to apply the noise evenly in all channels (bottom).

Noise variations

Normally, the Add Noise filter adds both lighter and darker pixels to an image. If you prefer, however, you can limit the effect of the filter to strictly lighter or darker pixels. To do so, apply the Add Noise filter, and then apply the Fade command(Ctrl+Shift+F on the PC or z +Shift+F on the Mac) and select the Lighten or Darken blend mode. Or you can copy the image to a new layer, apply the filter, and merge the filtered image with the underlying original.

Figure 8-33 shows sample applications of lighter and darker noise. After copying the image to a separate layer, the Add Noise filter was applied with an Amount value of 40 percent and Gaussian selected. To create the upper left example in the figure, Lighten was selected from the blend mode pop-up menu. To create the upper right example, the Darken mode was applied. In each case, a layer of strictly lighter or darker noise was added while at the same time retaining the clarity of the original image.

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Figure 8-33: You can limit the Add Noise filter to strictly lighter (left) or darker (right) noise by applying the filter to a layered clone. To create the rainy and scraped effects (bottom examples), applyMotion Blur and UnsharpMask to the noise layers .

To achieve the streaked noise effects in the bottom examples of Figure 8-33, Motion Blur and Unsharp Mask were applied to the layered images. Inside the Motion Blur dialog box, the Angle value was set to “30 degrees and the Distance to 30 pixels. Then the Unsharp Mask filter was applied with an Amount value of 200 percent and a Radius of 1. Naturally, the Threshold value was 0.

Chunky noise

Your biggest frustration with the Add Noise filter may be that you can't specify the size of individual specks of noise. No matter how you cut it, noise only comes in 1-pixel squares. It may occur to you that you can enlarge the noise dots in a layer by applying the Maximum or Minimum filter. But in practice, doing so simply fills in the image because there isn't sufficient space between the noise pixels to accommodate the larger dot sizes.

Luckily, Photoshop provides several alternatives. One is the Pointillize filter, which adds variable- sized dots and then colors those dots in keeping with the original colors in the image. Though Pointillize lacks the random quality of the Add Noise filter, you can use it to add texture to an image.

To create the top left image in Figure 8-34, Filter Pixelate Pointillize was chosen and 5 entered into the Cell Size option box. After pressing Enter or Return to apply the filter, Ctrl+Shift+F ( z +Shift+F on the Mac) was pressed to fade the filter, changing the Opacity value to 50 percent. The effect is rather like applying chunky bits of noise. The top right image is similar, but with the Opacity value at 100 percent and with the Pin Light blend mode applied. (For more on blend modes, see Chapter 6.)

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Figure 8-34: The results of applying several different Add Noise-like filters, including Pointillize, Halftone Pattern, and Grain.

The Filter Gallery filters provide a few noise alternatives. Sketch Halftone Pattern adds your choice of dot patterns, as shown in the two middle examples in Figure8-34 (Size and Contrast settings were the same for both examples). But like all filters in the Sketch submenu, it replaces the colors in your image with the foreground and background colors. Texture Grain is a regular noise smorgasbord, enabling you to select from 10 different Grain Type options, each of which produces a different kind of noise. The bottom examples in Figure 8-34 show off two of the Grain options, Clumped and Stipple. The Intensity and Contrast remained the same in each; the Soft Light blend mode was added in the Stipple example.

Removing noise with Despeckle

Now for the noise removal filters. Strictly speaking, the Despeckle command probably belongs in the Filter Blur submenu. It blurs a selection while at the same time preserving its edges ” the idea being that unwanted noise is most noticeable in the continuous regions of an image. In practice, this filter is nearly the exact opposite of the Sharpen Edges filter.

The Despeckle command searches an image for edges using the equivalent of an Unsharp Mask Threshold value of 5. It then ignores the edges in the image and blurs everything else with the force of Filter Blur Blur More, as shown in the upper left image in Figure 8-35.

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Figure 8-35: The effects of the Despeckle filter (upper left) and Median filter, the latter of which averages the colors of pixels in an image according to a Radius value.

Averaging pixels with Median

Another command in the Filter Noise submenu, Median removes noise by averaging the colors in an image, one pixel at a time. When you choose Filter Noise Median, Photoshop produces a Radius option box. For every pixel in a selection, the filter averages the colors of the neighboring pixels that fall inside the specified radius ” ignoring any pixels that are so different that they might skew the average ” and applies the average color to the central pixel. You can enter any value between 1 and 100. However, even at relatively low settings like 12 pixels, significant blurring occurs, as is evident in the bottom right image in Figure 8-35. At the maximum Radius value, the face would decline into a blurry, gummy goo, with all image detail obliterated.

As with Gaussian Blur, you can achieve some very interesting and useful effects by backing off the Median filter with the Fade command. But rather than creating a romantic glow (as shown earlier in this chapter), Median clumps up details, giving an image a plastic, molded quality, as demonstrated by the examples in Figure 8-36. To create every one of these images, the Median filter was applied with a Radius of 5 pixels. For the second example, Ctrl+Shift+F ( z Shift-F) was pressed to display the Fade dialog box and the Opacity value lowered to 65 percent. For the bottom-left image, the Opacity was raised to 100 percent in the Fade dialog box and the Darken blend mode applied. For the final example, the Opacity was taken back down to 80 percent and the Linear Dodge mode was used.

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Figure 8-36: After applying the Median filter, reverse the effect slightly using Edit Fade Median. Then vary the blend modes and Opacity values, as labeled beneath the images.

Another difference between Gaussian Blur and Median is that Gaussian Blur destroys edges and Median invents new ones. This means you can follow up the Median filter with Unsharp Mask to achieve even more pronounced sculptural effects.

Sharpening a compressed image

Digital cameras are some of the hottest and most commonplace gadgets around. And it's hardly surprising. Your typical midrange device is not only fun to use, but it also shoots three million or more pixels and lets you download the images to your computer in a matter of seconds. Yet despite the $200 to $500 entry price, even a halfway decent 4-megapixel camera captures at best half the resolution theoretically attainable with run-of-the-mill 35mm film.

To give you an idea, a 3 million-pixel photograph measures 5.5 x 7.5 inches when printed at the glossy prepress standard of 267 pixels per inch. That's not bad, but bear in mind, most midrange digital photographs receive a heaping helping of JPEG compression, and all are upsampled from the mere 8 bits of data captured by the CCD to the 24 bits that we see on-screen. As a result, no pixel is altogether accurate in its representation of the real world.

Naturally, the brunt of this dilemma falls to graphics and design professionals. Our mission is to take these imperfect images and render them suitable for printing. Fortunately, armed with color adjustment commands such as Levels and Hue/Saturation and the trusty corrective filters Median, Gaussian Blur, and Unsharp Mask, you have everything you need to smooth over extreme imperfections and enhance the most fragile details. In the following steps, you'll learn how to save an irreplaceable photograph from near extinction . Hopefully, you won't encounter anything as dramatic as the image pictured in Figure 8-37, but even a reasonably sound photograph may benefit from this approach. And if you ever find yourself with an image that appears beyond repair, you'll know what to do.

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Figure 8-37: Captured with a 1-megapixel digital camera, this photograph suffers all the woes of many low-end and midrange digital photographs ” dark color, low contrast, random compression artifacts, and limited detail.

STEPS: Fixing a Digital Photograph

  1. Fix the brightness and contrast. Choose the command Image Adjustments Levels or press Ctrl+L ( z +L on the Mac). Then balance the brightness, contrast, and color of the image. The results appear in Figure 8-38.

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    Figure 8-38: Brightness, contrast, and color are adjusted through the Levels dialog box.

    Cross-Reference  

    You can learn how to balance the brightness, contrast, and color of an image in Chapter 12.

  2. Copy the image to a new layer. The best tool for making colors more vivid is Hue/Saturation. Unfortunately, this command has the side effect of enhancing JPEG compression artifacts, often radically . So before applying the command, duplicate the image to a new layer by pressing Ctrl+J ( z +J).

  3. Increase the color saturation. Choose Image Adjustments Hue/Saturation or press Ctrl+U ( z +U). Then increase the Saturation value well beyond what seems sensible . For example, the Saturation of the example image was raised to 60 percent. Don't worry that the image looks really grainy (see Figure 8-39); you'll need an extreme effect when it comes time to blend this layer with the original image in Step 6.

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    Figure 8-39: Boosting the Saturation value in the Hue/Saturation dialog box by 60 percent made the photograph much more vivid. However, it also brought out JPEG compression artifacts that interrupt the natural edges and color transitions.

  4. Choose the Median filter. The next step is to reduce the undesirable grain and enhance the highly desirable edge detail. You achieve the first using the Median command. Choose Filter Noise Median and then enter a Radius value of 4 pixels or higher until the compression artifacts appear almost entirely smoothed away. (In Figure 8-40, a Radius value of 6 was used). Don't worry that the Median command makes the photograph appear doughy or indistinct; you'll recover the focus of the image shortly.

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    Figure 8-40: With the Radius values for the Median and Gaussian Blur filters set to 6 and 2 pixels, respectively, the compression artifacts are blurred away and all you have is smooth color.

  5. Choose Filter Blur Gaussian Blur. Now that you've gummed up the detail a bit and rubbed out most of the compression, use the Gaussian Blur filter with a Radius of 2.0 or thereabouts to blur the gummy detail slightly. This softens the edges that the Median filter creates. (You don't want any fake edges, after all.)

    Note  

    The result will by no means be a perfect image. The photograph should look oversaturated and gummy, as if it were molded out of brightly colored plastic. You have created an extreme correction ” a radical combination of saturation, averaging, and blurring that you simply cannot reproduce in more delicate applications.

  6. Lower the layer's opacity. Blend the exaggerated layer with the original image behind it using the Opacity value. If your image had few faults to begin with, you may want to go as low as a few percent ” say, 5 percent or even lower.(Bear in mind, a low Opacity value favors the original image over the filtered layer.) Just a little dab of detail enhancement can make a perceptible difference. For more objectionable images such as the one shown in this procedure, higher Opacity values are warranted. In Figure 8-41, a value of 50 percent was applied, which weighted the original and filtered images equally. This is about as high as you'll want to go ” placing more weight on the extreme filtered layer than the original image generally results in a surreal effect. Bear in mind, what you're trying to achieve is not an image that looks radically corrected, but rather one that looks as if it didn't require correction in the first place.

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    Figure 8-41: The Opacity value of the filtered layer was lowered to 50 percent, evenly mixing the oversaturated filtered image with the undersaturated original.

  7. Merge the image. Press Ctrl+E ( z +E on the Mac) to fuse the two layers into one. The result is a base image that will better respond to standard enhancements. Now you can apply color and focus adjustments as if the image were scanned from a high-quality film source.

  8. Continue to correct the image as you normally would. In the example, the saturation of the colors was boosted using Hue/Saturation ” albeit far more subtly than before. Then the Unsharp Mask filter was applied with an Amount value of 100 percent and a Radius of 4.0 pixels to arrive at the final image shown in Figure 8-42.

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    Figure 8-42: After merging the layers, the Saturation of the image was increased by 25 percent and the Unsharp Mask filter applied with an Amount value of 100 percent and a Radius of 4 pixels.

Depending on how your image looked before you started, you may or may not achieve something resembling absolute perfection . For example, the sample photo still suffers compression artifacts and some undesirable color aberrations. Nevertheless, the image looks far better than it looked in the beginning (see Figure 8-37). It also looks significantly better than it would have had Hue/Saturation and Unsharp Mask been applied directly to the original image without creating a duplicate layer and employing Median and Gaussian Blur.

Note that these steps work well for sharpening other kinds of compressed imagery as well, including old photographs that you over-compressed without creating backups and images that you've downloaded from the Internet. If applying the Unsharp Mask filter brings out the goobers, try these steps instead.

Using the Average filter

If the Median filter is a refined, dignified tea party in the countryside, then Photoshop's Average filter is the most reckless, wild Mardi Gras free-for-all you've ever seen. Whereas Median performs a pixel-by-pixel examination of your image and averages the color values smoothly based on a radius you select, the Average filter sizes up all of the color values present and then with a resounding "Wahoo!" smashes them into one gigantic, uncontrollable, uniform mass.

Choosing Filter Blur Average will, on most color images, immediately replace your image with a grayish wall of nothing. Why, thank you, Photoshop. That gray void is what you usually end up with when you average all of the varied color values in an image. Consequently, on its own, Average isn't much more than a minor curiosity . But believe it or not, there are some uses for this filter. The trick is to use it in tandem with selections and blend modes.

When just a portion of your image is selected, the Average filter makes calculations from and applies changes to only the selected area. Marquee over a patch of grass, and the averaged area becomes a flat, solid green. Use the Magic Wand to grab a handful of sky, and Average turns it into a smooth, perfect firmament. And though these averages may not be all that photographic or impressive on their own, when you begin to blend averaged sections of an image with the original underlying pixels, the filter really proves its worth.

For example, say you've got an image of a lovable face, marred only by slight imperfections in color. Create a selection that includes only the skin areas of the face and press Ctrl+J ( z +J on the Mac) to copy your selection to a new layer. Choose Filter Blur Average and watch as the various color differences in your floating face layer completely disappear. The new layer is clean, yes, though not entirely believable. Lower the layer's opacity to around 70 percent and set the blend mode to Hue. Now you're applying the smooth color of the averaged layer with the texture detail of the original image. As if by magic, your image is jaundiced no more.

You can also experiment with blend modes to achieve stylized results, as shown in Figure 8-43. Here, the background layer was duplicated to a layer above it. Next, the different color zones of the face were selected on the new layer using Select Color Range and the Average filter was applied to each one individually. Then a few blend modes were applied and Opacity values tested on the layer until a combination that worked was found. Finally, different elements of the face were manually selected and copied to new layers, then they were selected and the Average filter applied, for a somewhat above-average effect.

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Figure 8-43: Here the Average filter was used together with the Vivid Light blend mode and a little manual labor to smooth out color transitions and create some nice stylistic effects.


Photoshop CS2 Bible
Photoshop CS2 Bible
ISBN: 0764589725
EAN: 2147483647
Year: 2003
Pages: 95

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