Constructive Filters


Four groups of filters provide tools to help improve image quality by changing the focus, or by smoothing transitions within an image. These filters belong to the Filter Blur, Noise, Sharpen, and Other submenus. These groups are the bread and butter of Photoshop filtering, the workhorses that are put to frequent, day-to-day use.

The constructive filters complement one another. For instance, Blur s effect is the opposite of Sharpen; Median acts on an image in exactly the opposite way from Add Noise. These complementary effects will be delineated in greater detail in this section.

Blur Filters

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The Blur group contains eight individual filters: Average, Blur, Blur More, Gaussian Blur, Lens Blur, Motion Blur, Radial Blur, and Smart Blur. Like their Sharpen counterparts, Blur, Blur More and the new Average filter are fully automated. You click, the filter does its job, and that s that ”no dialog box, no user input, no control. Blur and Blur More diminish contrast, resulting in softer, smoother edges and transitions. Average assesses the pixels in an image or selection and replaces them with a solid fill of the average color (see Figure 20.5).

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Figure 20.5: The Average filter replaces pixels with a solid fill of their average color

Gaussian Blur

Gaussian Blur is named for 18th-century German mathematician Carl Friedrich Gauss. Gauss s work with number theory and distribution patterns made him a pioneer of modern mathematics. He is ultimately responsible for the distribution curves employed in the filter that bears his name (he ll crop up again in Photoshop when we get to the Add Noise filter), and he was one heck of a dancer.

But seriously, if you learn to manipulate the niceties of the Gaussian Blur, you ll never need the Blur and Blur More filters. Even with only one variable in its dialog box (see Figure 20.6), Gaussian Blur offers much more control. By entering a number from 0.1 to 250 or by sliding along the Radius slider, you tell Gaussian Blur to apply its softening curve to a range of adjacent pixels. Lower values produce a slight , subtle blur effect, which can be used to smooth out rough transitions or blotchy areas; higher values can blur the image beyond recognition. You can create shadow effects by applying this filter to underlying layers . Figure 20.7 shows some of the Gaussian Blur filter s range.

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Figure 20.6: The Gaussian Blur dialog box
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Figure 20.7: The original image (left), and the image with the Gaussian Blur filter applied (right)
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With the Blur filters (as well as other filter types), the degree to which a given setting affects a particular image depends on the resolution of the image. The higher the resolution, the greater the settings have to be in order to achieve the effect.

Radically different effects can be achieved with Gaussian Blur, depending on whether you re applying it to a selection or freely to an image or layer as a whole. If Gaussian Blur is applied within a selection outline, its effect remains more or less constrained within the selected area. If you apply it to a layer or an entire image, the curve is allowed to carry its effect across the full range of the image. Compare the left image in Figure 20.8, where Gaussian Blur is applied within the confines of a selection, to the right image, where the same parameters were used on the entire layer.

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Figure 20.8: Gaussian Blur applied (left) within a selection and (right) to the unselected image

Smart Blur

The Smart Blur filter (see Figure 20.9) is a kind of counterpart to Unsharp Mask in that it applies its blur to the open or continuous areas of an image while retaining edge definition. Like Unsharp Mask, Smart Blur offers the option of defining what it sees as an edge and how far the effect should extend. It s often used for retouching grainy or textured photographs and for creative effects on line drawings. The Radius and Threshold settings work the same as in Unsharp Mask.

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Figure 20.9: The Mode option in the Smart Blur dialog box protects edges while blurring.

The options in the Quality pop-up list control the smoothness of the effect, with the High setting, logically enough, providing the smoothest transitions. The Mode control offers various methods of applying the blur. Normal mode applies the effect, well, normally, as seen in Figure 20.10. The Edge Only and Overlay Edge options trace the edges with white lines. Edge Only shows the edges as white lines against a black background, whereas Overlay Edge superimposes white edge lines over the visible image. The Normal mode is most likely the one you ll end up using 99% of the time with Smart Blur. Other edge-finding filters such as Filter Stylize Find Edges and Filter Stylize Trace Contour allow more control over your image and more interesting visual effects.

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Figure 20.10: The original image (left), and the effect of the Smart Blur filter (right)

The Blur submenu contains two more filters that apply a blur along motion lines. Motion Blur (see Figure 20.11) enables you to specify a direction of movement, expressed as an angle in degrees and a distance in pixels. Your image spreads out along a path in a linear distribution in the direction and distance indicated.

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Figure 20.11: The Motion Blur dialog box

Radial Blur

Radial Blur offers a different kind of directional movement in its dialog box. Two Blur Method options are available in this filter: Spin and Zoom. Spin distributes pixels by rotating them around a chosen center point, whereas Zoom s blur radiates pixels out from the center. The Radial Blur dialog box (see Figure 20.12) shows a Blur Center field containing a grid pattern. Click or drag the center point to reposition the starting point of the Spin or Zoom anywhere in your image. Enter an Amount to define how far the blur extends. (Why this slider is named Amount here but named Distance in the Motion Blur filter is a mystery. They both accomplish the same thing.)

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Figure 20.12: Click in the Blur Center box to position the Spin or Zoom starting point.

Draft, Good, and Best are offered as options under Quality. If Draft is chosen, the effect takes less time to accomplish, but the blur is applied more roughly ; Best provides a smoother blur but requires more time and memory capacity. Figure 20.13 compares the effects of the Motion Blur and Radial Blur filters.

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Figure 20.13: The hanging bulb: the original image (left); with Motion Blur (Angle: “9, Distance: 50) applied to a 300 ppi image (center); with Radial Blur (Amount: 18, Method: Spin centered at top-left corner, Quality: Good) applied (right)

Lens Blur

In a photograph, depth of field is characterized by a variation of focus. When the photograph is taken, the subject s distance from the lens is the determining factor of its sharpness. Photoshop s new Lens Blur filter simulates this effect.

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When you choose Filter Lens Blur, the formidable Lens Blur filter dialog box is displayed (see Figure 20.14). From this control center, you can blur specific areas of an image while maintaining the sharpness of others. The dialog box presents the following options on the next page.

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Figure 20.14: The Lens Blur filter dialog box

Preview Click the Faster radio button to generate faster previews. Choose it to preview the image while you make adjustments. Choose More Accurate after you ve previewed the effect to see the final results.

Depth Map Choose a depth map to determine what parts of the image will be affected. You can choose a Layer Mask, Alpha Channel, Transparency, or None as a source. When you choose None the entire image or the contents of a selection will be affected. Black areas in an alpha channel or layer mask will remain in focus while, white will be blurred. Gradual blurring is achieved with a gradient mask. Drag the Blur Focal Distance slider to set the focal length. If your focal distance is set at 100 for example, pixels with brightness values of 1 and 255 are completely blurred, and pixels closer to 100 are blurred less. If you click the preview image, the Blur Focal Distance slider matches the location you clicked, setting that depth as in-focus.

Iris Choose a shape for the iris of the lens. This simulates the effect of the iris of a camera lens. The more blades on the iris the more precise the blur will be. You can change blades of an iris by curving them or rotating them. The effect of changing iris curvature, shape, or rotation is very subtle, so examine the preview closely.

Specular Highlights A specular highlight is an area of absolute white. The Threshold slider assesses the brightness value of the lightest areas of an image. Pixels that have higher values are treated as specular highlights. The Brightness slider lightens selected areas.

Noise Film grain and noise are removed from the image when you apply the Lens Blur filter. Adding noise can help restore the grain and add consistancy to the image s tonal texture. Click either the Uniform or Gaussian radio button to choose a type of noise. Choose Monochromatic to avoid color shifts. The Amount slider controls the strength of the noise.

Figure 20.15 shows the result of applying the Lens Blur Filter. This image is on the CD. Open rebecca.psd in the Chapter 20 folder to experiment with the Lens Blur filter. It has a layer mask and an alpha channel that you can use as depth maps.

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Figure 20.15: The results of the application of the lens blur filter.

Noise Filters

You ll find the next group of filters in the constructive category in Filter Noise. Add Noise, Dust & Scratches, and Median all allow user control; the Despeckle filter is an automated effect that does its work without benefit of a dialog box. Like its hands-off counterparts in the Blur and Sharpen submenus, Despeckle s effects are easily surpassed by its more powerful siblings, Dust & Scratches and Median. These three filters work in various ways to eliminate noise; Filter Noise Add Noise, believe it or not, adds noise to an image.

Add Noise

When you Add Noise to an image, the result is a grittier, grainier look ”a bit of texture. Noise can work wonders in helping to smooth tonal transitions; it can help prevent banding in gradations and stair-stepping in your tonal range. In a grayscale image, noise is added in black, white, and gray grains; in color images, noise is added individually to each color channel, producing natural-looking hues of noise that blend together at random. Figure 20.16 shows the Add Noise dialog box with its three option areas.

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Figure 20.16: The Add Noise dialog box

Amount Enter a number from 0.1% to 400% to set a color range for your noise. This controls how far the noise can differ from the existing pixel colors ”the higher the number, the greater variation in color of the added noise.

Distribution Two options are available to define the way noise is distributed across an image. If you click the Uniform button, colors are applied at random throughout the image. Otherwise, Photoshop applies the noise in a Gaussian curve (you were warned that Carl Friedrich would turn up again). Gaussian generally results in a more pronounced effect than Uniform delivers.

Monochromatic Check this box if you wish to distribute noise uniformly across all color channels. Otherwise, Add Noise applies its effect randomly to each channel separately, resulting in more color variation. Monochromatic ends up producing grayscale noise; if you re working on a grayscale image, the effect is the same whether or not you check this option. Take a look at Figure 20.17 for some samples of noise in action.

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Figure 20.17: The original image (left); and the image with the Add Noise, Gaussian filter, Amount: 64, applied (right)
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Adding noise when retouching an image is a common task with little concern for RGB users, but it does have one small pitfall when working in CMYK. Noise is inadvertently added to the black plate, which can result in an overall graininess being added to the image. To avoid this problem, Shift click the CMY channels in the Channels palette and apply the Noise filter only these channels

Dust & Scratches

The Dust & Scratches filter was created for the purpose of removing those spots and splotches that many old photos have acquired from years of neglect or abuse. Because it s used for photo restoration, the Dust & Scratches filter is covered at length in Chapter 19, Photo Retouching, and further demonstrated in Hands On 7, Restoring a Color Photograph.

Median

The Median filter is similar to Dust & Scratches, but its dialog box (see Figure 20.18) lacks the Threshold slider. Median is used to average the numerical values of adjacent pixels that are based on the Radius setting (with a range of 1 through 100). Median ignores pixels that are radically different so that the effect reflects a center-weighted average. The result is a softened, molded quality almost like a mild posterization. Higher values can destroy image detail, though, so take it easy on this one. Figure 20.19 provides one example of the Median filter at work.

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Figure 20.18: The Median filter s dialog box
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Figure 20.19: The original image (left), and the image with the Median filter applied with a Radius of 22 (right)

Sharpen Filters

The Sharpen group contains four filters: Sharpen, Sharpen Edges, Sharpen More, and Unsharp Mask. The first three of these are fully automated ”that is, the user has no control over their effect. When you click one of these, no dialog box appears. Photoshop simply applies the predefined effect of increasing contrast. Sharpen increases contrast overall; Sharpen More has a stronger effect; and Sharpen Edges focuses on the areas of highest contrast in an image.

Much more powerful than any of these is Unsharp Mask, which we visited in Chapter 16. Unsharp Mask gets its unusual name from an old photographic technique of shooting through a blurred negative as a mask to increase the edge contrast in a film positive.

Photoshop s engineers have transformed that arcane bit of low technology into an incredibly useful enhancement tool. Its three-variable dialog box allows for minute adjustments and a very fine level of control. If the other three Sharpen filters were discarded, Photoshop users would still have all the sharpening capability they d ever need with this one tool.

The Unsharp Mask dialog box (see Figure 20.20) appears when you choose Filter Sharpen Unsharp Mask. These options appear for your input:

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Figure 20.20: The Unsharp Mask dialog box includes options that are common to many filters.

Amount Values from 1% to 500% can be entered to define the degree of sharpening ”the higher the value, the greater the effect.

Radius This setting defines the thickness (from 0.1 to 250 pixels) of an edge. As illustrated in Figure 20.21, lower values produce crisp, sharp edges, whereas higher values define edges as thicker and generate greater overall contrast throughout an image.

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Figure 20.21: Unsharp Mask applied with various Radius settings. The original image (upper left); Radius of 2 (upper right); Radius of 100 (lower left); Radius of 250 (lower right).

Threshold Entering a value from 0 to 255 enables Unsharp Mask to determine what s considered an edge. The number indicates the difference in brightness values necessary to recognize an edge. Lower numbers include lots of pixels in the effect; the higher the number, the more exclusive the value.

These three variables work together to do the job of heightening focus and contrast. A little experimentation and practice might be in order to find the right combination of settings to achieve the desired effect. Subtle effects can be produced by keeping the Amount setting under 100%; raising it over 300% can create results that some might consider undesirable.

Sometimes repeated applications of a lower Amount setting can produce better results than a single application at a higher setting.

When identifying edges, Unsharp Mask uses the Radius setting as its criteria. The effectiveness of this setting depends entirely upon the resolution of the image. Screen images and Web graphics will require a much lower setting (say, in the 0.5 range) than high-resolution images intended for fine printing (usually nicely sharpened in the 2.0 range).

Radius values can be set all the way up to 250, but the higher values produce results more suitable for special effects than for day-to-day image correcting.

Raising the Threshold value has the effect of increasing the definition of an edge. In other words, higher values require more contrast between pixels to be recognized as an edge; lower values recognize edges between pixels with closer brightness values.

Other Constructive Filters

Some other useful constructive filters reside in the Filter Other submenu. Filter Other Offset (see Figure 20.22) is effectively a Move tool in a (dialog) box. You enter numbers in the Horizontal and Vertical fields to tell Photoshop how far to move a selection. Positive numbers move the image to the right or down; negative numbers move it to the left or up. You can make the same precise moves by using the arrow keys in conjunction with the Move tool. The difference is that Offset enables you to determine what happens to the unselected areas by clicking one of its three radio buttons . Set To Background fills the emptied area with the current background color. If the image to be moved is on a layer, then the Set To Background option changes to Set To Transparent and the emptied area will become transparent. Repeat Edge Pixels fills the area with duplicates of the pixels at the edge of the selection, and Wrap Around takes pixels from the opposite side of the selection and duplicates them. These settings reappear frequently in some of the destructive filters you ll see shortly, particularly those in the Filter Distort and Filter Stylize submenus.

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Figure 20.22: The Offset dialog box

Minimum and Maximum

The effect of the Minimum and Maximum filters will be familiar to anyone who has spent time in a lithography darkroom. To make colors in a printed image overlap, or trap, printers change the colors borders to enable one of the colors (generally the lighter of the two) to spread into the other. When applied to a masking channel, the Minimum filter shrinks the selection. The number you specify using the Radius slider is the amount your selection contracts. Maximum does the exact opposite: input a Radius amount, and the filter expands your selection precisely by that number of pixels. This way you can create precise traps mathematically. Masks created using Maximum and Minimum can be used to overlap different colored areas so that just the right amount of trap is created. This prevents dreaded white space from showing where two colors meet during printing.

High Pass

The High Pass filter offers a tool for finding and isolating areas of high contrast. On its own, it seems at first glance an awkward and unlikely filter. But when applied to individual color channels or to selection masks, it can help create interesting line effects or imbue unremarkable images with added color. High Pass s Radius slider allows a range from 0.01 to 250 pixels. Lower numbers leave your image a flat, midrange gray. Higher values let the higher-contrast areas show through in lighter gray. This isn t so great when applied to an image as a whole, but try applying this filter to individual color channels and you have a completely different animal. If you increase contrast by applying High Pass to individual color channels, you end up adding color at either end of the scale.

You can achieve a line-drawing effect by using High Pass in combination with the Threshold command and then reapplying the modified image to the original. Figure 20.23 shows how this can be achieved. First, make a duplicate layer of your image, and apply High Pass to the duplicate, using the Radius to find a desirable effect. Choose Image Adjust Threshold at the desirable value. Then use the Opacity slider in your Layers palette to blend the line image with your original. Voila`! a line-drawing effect.

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Figure 20.23: The High Pass filter is applied to a 300 ppi image. Step 1: High Pass, Radius 7 (top). Step 2: Threshold, Level 120 (center). Step 3: The Opacity slider in the Layers palette adjusted to 50% (bottom).
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Many users favor variations on the basic High Pass sharpening technique. You can duplicate a layer and apply the High Pass filter from 0.3 to 3 pixels (depending on the image content and pixel dimensions). Blend the duped layer in a blending mode such as Overlay, Soft Light, Hard Light, or Linear Light and reduce the opacity until the desired effect is achieved.




Photoshop CS Savvy
Photoshop CS Savvy
ISBN: 078214280X
EAN: 2147483647
Year: 2004
Pages: 355

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