While there are many means by which you can obtain an image to work with in Photoshop, from creating it yourself in another program to importing it from a photo CD or digital camera, to saving images from the web, scanning images is the method that poses the most problems for correcting the picture. Because the scanner, the photograph or both might have dust or dirt (often called "noise") on them, you'll often find that scanned images need more editing "help" to look the way you want them to. This class (and document) covers many of the steps you might need to take in adjusting a scanned image. Of course, you can use these techniques with images that you've acquired in other ways, too.

You can correct crooked scans by selecting Rotate Canvas from the Image menu . You will have the option of rotating the image 180 degrees, 90 degrees clockwise, or 90 degrees counter-clockwise. You can also flip the image horizontally (create a mirror image) or vertically (turn it upside down).

If you select Arbitrary , you can rotate your image anywhere between -359.99 and 359.99 degrees. The drawback of using the Arbitrary option is that no preview feature exists, so you must guess to what degree you need to rotate the image to straighten it. If you've guessed wrong, you have to return to the Arbitrary command and try again. Because Photoshop must interpolate pixel data every time you rotate an image using the Arbitrary command, beware: using it multiple times can degrade your image quality.

To avoid both the frustration and possible damage to image quality that result from using the Arbitrary command several times, you may want to use the Measure tool (I). This tool allows you to determine the length, width, and (most importantly for our purposes) angles of objects in your document window, or even the document size itself. By clicking and dragging the Measure tool parallel to the top or bottom edge of your image, you can determine the angle at which it's rotated. The Options bar for the Measure tool will display several pieces of information, including the angle of rotation, labeled A . If you now return to the Arbitrary command on the Image menu, you'll notice that Photoshop has automatically entered the angle data from the Info palette in the dialog box and selected either CW or CCW (whichever will correct your image's crookedness). When you click OK , you'll correct the rotation of the image perfectly, without any further guesswork.

You will often want to give a photograph a new focal point or remove unneeded background areas that distract from the photo. Cropping lets you select a rectangular area of an image and remove everything outside that selection. First, select an area with the Rectangular Marquee tool; then select Crop from the Image menu. Note: The Crop command will not appear if you had any feathering set in the Options bar for the Rectangular Marquee tool when you created your selection. Be sure to set the Feather to 0 before making your selection.

For more control over your selection area before you perform a crop, you may want to use the Crop tool. If you need the final product of your cropping to have specific width and height dimensions or a particular resolution (or both), enter the appropriate numerical values into the text boxes next to Width, Height and Resolution before you begin. (Some of these actions may involve resampling the image; see the Resampling Images section below for more information.) Should you want your final product to have the same width, height and resolution values as the current image, click on the Front Image button to have Photoshop enter those values automatically. (Alternatively, if you want to crop your image to the same dimensions as another image, bring that image forward first and press the Front Image button. Then make the desired image your active document window and perform the crop.) Then click and drag the Crop tool across the image to select a rectangular area.

Once you have a crop area active on the screen, the Options bar will display new features. To get an idea of what your image will look like once it's cropped, you can check on the Shield cropped area feature. Photoshop will then shield the area that will be discarded after you perform the crop; you can set the Color and Opacity of the shield in the Options bar. You can resize your crop area by clicking and dragging on the square handles that appear; to constrain the original proportions of the crop area, hold down <Shift> as you click and drag. To move the entire crop area, click and drag inside the area. Finally, if you want to change the 3-D perspective of the image (in, for example, a picture of a skyscraper), click on the Perspective option. The centerpoint of the crop area will move to the center of the image, and you can click and drag on the crop area's handles to adjust the crop to a polygonal shape, rather than a rectangle. When you have completed all your adjustments, click on either the checkmark (to perform the crop) or the cancel icon to cancel your crop.

If your image has an unnecessary border of uniformly colored pixels, you may find it easier to use the Trim command from the Image menu. To use this command, the pixel color that you want to eliminate must be located in eith the upper left-hand corner of your image, or the bottom right-hand corner, so you may need to use the Rotate Canvas commands before applying this command. Once your image is rotated appropriately, choose the command from the menu and set the pixel on which Photoshop should base its trimming. Then select whether you'd like Photoshop to trim the image at the top, bottom, left, right, or all of the above. Press OK for Photoshop to perform the trim.

Filters are automated ways to apply specific effects or image correction to large portions of an image (or the entire image). For example, instead of using the blur tool to slowly and perhaps unevenly blur a large portion of an image, you can apply a blur filter to a selected portion of the image. You can use filters to change an image's focus; to change its colors; to move pixels, creating bumps and extrusions; to obliterate image data and replace it with a new image; or to manipulate image colors, creating stylized versions of the original image.

Filters do not work in all color modes, so you may first need to convert an image into RGB mode (as described below) before you can apply filters to it. To apply a filter, simply select it from the Filter menu. (If the filter offers a dialog box, you must choose your settings and click OK before the filter will be applied.)

You can reapply the last filter you used by selecting it again from the menu, or by pressing <Ctrl><F> . You can lessen the effect of the last filter you applied by selecting Fade from the Edit menu. In the Fade dialog box, move the Opacity slider to reduce the filter's intensity. Sliding the Opacity setting all the way to the left makes your image look as if you never applied the filter at all. You can also adjust the blending mode for the filter here.

Photoshop comes pre-loaded with more filters than we can possibly cover here, so for the purposes of this document, we'll cover the ones that are most likely to be useful in correcting images. Most of Photoshop's filters are quite easy to use, so you can easily try them out on your own: take an image, save a copy of it, and try the filters on it, taking note of what each one does. Alternatively, Photoshop's Help program can provide descriptions of all the filters.

The sharpen filters make an image's detail more distinct, improve the focus, and prepare the image for printing. Almost any scanned image needs some sharpening to correct the softness caused by the scanning process itself. Large images will require more sharpening than small ones.

Like the blur tool, blur filters work by lessening the differences between adjacent pixels, creating a softening or blurring of the image.

All scanned images contain some sort of noise, whether from dust on the scanner, film grain, or the texture of the paper. Noise filters add or remove random pixels from images to correct scans or add grain to an image that's been edited so that it doesn't look artificial.

An easy way to make simple adjustments to the tonal range of the image, this command is almost always necessary when you scan in photographs. Choose Brightness/Contrast from the Adjustments submenu under the Image menu. In the dialog box that appears, use the slider bars (or type in a number) to adjust the brightness and contrast in the image; values can range from -100 to +100. Dragging to the left decreases the level; dragging to the right increases it. Click OK to apply the changes. If you'd like Photoshop to make these adjustments for you based on its perception of the image, choose the Auto Contrast command from the Adjustments submenu.

To make subtle adjustments to the shadows, midtones, or highlights of your image, use the Levels command. In the dialog box that appears, you can change the levels for the composite RGB channel. A good way to begin is to press the Auto button, so that you can have Photoshop do the work of calculating appropriate tonal variety for the image. (The Auto Levels command from the Adjustments submenu does the same thing as pressing this button.) Then you can make subtle changes to the tonal balance using the slider arrows beneath the histogram (the graphical representation of the distribution of shadows and highlights). To change the shadows in the image (or selection), click and drag on the black arrow to the far left just below the histogram. Use the gray arrow in the middle to change midtones and the white one to the right to alter highlights. (For now, you probably want the Output Levels settings to remain at 0 to 255.)

You can also choose to define a point in your image as the color level for shadows, for example. To do so, select the eyedropper tool filled with black paint and click on a dark point in your image. This color will be used to define the darkest point in your image; Photoshop will then distribute the remaining pixels between this shadow value and your highlight value. Choosing a very light color as your darkest point, then, will flatten out the histogram, indicating that there is little variety in shading in the image.

You can use the same techniques to define color levels for your image. For example, choose Red from the Channel drop-down menu (only available if you're working in RGB color). Using the slider bar below the histogram will alter the areas of red (and its complementary color, cyan) in the image.

The Curves command makes the same sort of adjustments that the Levels command does, but it allows you to set up to 15 fixed points rather than only three. (Normally, you would not use both the Curves and Levels adjustments on the same image, since they set the same information, just in differing ways.) You can click and drag the curve to adjust your image; each time you click, you will create a set point value that Photoshop will use to adjust your image accordingly. Here are a few basic shapes that will help you correct RGB images:

Just as with the Levels command, you can manually define a point from your image as a defining point on the curve using the eyedropper tools. Finally, for more unusual effects, try using the Pencil tool rather than the Curves tool (you can select it from the lower right-hand corner of the dialog box). Instead of dragging out color curves, you can now draw them in any shape you like.

Sometimes you'll need to adjust the colors in your image. In much the same way that the Levels command corrects the levels of black and white in your image, you can use Color Balance to correct color levels. First, if you're working in RGB color mode (discussed later in this document), you should check on the Preserve Luminosity feature. Next, select the tonal range of the image you want to adjust: shadows, midtones, or highlights. Finally, slide the triangle toward colors that you want to increase and away from those you want to decrease. Alternatively, if you'd like Photoshop to make such adjustments for you based on its interpretation of the image, you can select the Auto Color command from the Adjustments submenu.

You may also consider using the Variations command (in the Adjust submenu) to get a quick look at the color balance in your image. This command will display your image as adjusted towards the six colors available in the Color Balance dialog box. You can, just as in the Color Balance command, choose to work with different tonal ranges of the image, but the Variations dialog box will show you what each adjustment will look like instantly. This is a good command to use when checking for a color cast in your image (that is, the image is shaded toward a particular color, making even the whites appear in that color. Blue color casts often appear in digital photographs, for example.).

The Hue/Saturation command offers the opportunity to alter the hue, saturation and lightness values for either a composite version of your image ( Master ) or a single color shade. Use the Edit drop-down menu to choose the shade you'd like to adjust, then use the sliders to apply changes to your image. The color bars at the bottom of the dialog box show you the normal color-wheel spectrum (on top) and the spectrum as you've adjusted it (on the bottom). You can use the Colorize command to create a sepia-toned image from one in Grayscale; checking that feature in a color photo, however, may have the effect of converting it to black-and-white.

If you choose to work in a single shade (rather than Master), you will also be able to choose a specific color range using slider bars that appear between the two spectrum bars. You can also use the eyedropper tools to select the specific color range you want to adjust. Note that if you select, say, Reds from the Edit drop-down menu but choose a color range (using either the spectrum bars or the eyedropper tools) that is in yellow, Photoshop will automatically switch the name in the Edit menu to Yellows .

Use the Desaturate command much like you use the Sponge tool's desaturate feature, to "gray out" the colors in your image (the Sponge tool is covered in [ http://help.unc.edu/?id=3548 ] Photoshop: Intermediate Editing ). This command does not offer a dialog box for a range of adjustments; to adjust the intensity of the desaturation, use the Edit, Fade command.

When you want to replace one color with another, replacing a blue car with red, for example, use the Replace Color command. In the dialog box, click the radio button for Selection if you want to see a masked version of your image (unmasked areas, or selected areas, will appear in white, while masked areas will appear in black) or Image if you're working with a magnified version of your image.

Use the Eyedropper button to select the color from your image that you want to replace. Adjust the selection area with the Eyedropper+ and Eyedropper- icons (adding to and subtracting from it, respectively). Next, adjust the Fuzziness slider bar, which sets the tolerance for the selection area (including related colors, depending on the setting). Then click and drag the Hue, Saturation and Lightness sliders until you're satisfied with the new color. Click OK to finish.

If you simply want to add more space to an image, an extra inch around the borders, for example, you'll use the Canvas Size command under the Image menu. You can change the dimensions of the image in the Width and Height drop-down menus. If you check on the Relative option, Photoshop then adjusts the canvas size by proportion, so that Width and Height settings of 2 and 2 would double the available canvas space. Finally, set the location of the original image using the Anchor feature. (For example, if you want the image to be surrounded evenly on all sides by the new border, choose the center position.)

To change image size, select Image Size from the Image menu. First, uncheck the Resample Image box at the bottom of the dialog box (we'll cover resampling below). Now the second section (which is the only section available) shows the image's Print Size. Here you can change the image's resolution and physical dimensions. Any changes to the resolution and the physical dimensions only change the image's output--that is, how it prints out or is read in other applications. To see how an image will print out on a page, click on the file size information on the status bar at the bottom of the screen or choose Print Size from the View menu.

Note that when you decrease the resolution (lowering the image quality), the print size becomes bigger. That's because resolution (usually in pixels per inch) and print size (here, in inches) are both linked to pixels. Let's think about resolution like population density: if you originally had 300 people per inch in a one-inch-square, then dispersed them to only 72 people per inch, they'd spread out,making the area bigger (4.17 inches square, to be exact). You didn't get rid of any people, so you had to put them all somewhere, causing your area to get bigger. The same concept holds true for pixels in your image in relation to the print size. This principle also means that when you decrease the print size, the image's resolution will increase to maintain the same number of pixels spread across a smaller area.

Resampling an image changes the number of pixels in the image. Resampling is the only way to change the file size of your image (a particularly important issue if you're working with graphics for the web, a topic covered further in [ http://help.unc.edu/?id=3717 ] Photoshop: Web Graphics ). In the case of resampling, you'll change the number of pixels in your image directly. (Not incidentally, you will also change the print size of your image.)

To resample an image, select Image Size from the Image menu. Check the Resample Image box. You can now change the pixel count, and the dialog box will recalculate all of the dimensions just by changing one dimension. You can turn off this automatic calculation by unchecking the Constrain Proportions box. (Note: unchecking Constrain Proportions may allow you to distort your image.)

Photoshop offers an easy way to negotiate resizing images. From the Help menu, choose Resize Image . This wizard will walk you through resizing your image in a way appropriate to its final output (print or online).

If you need to see the numerical color values for your image (for example, to make sure the colors you see on your screen will match the output of a professional printer), use the Color Sampler tool. It can be accessed by clicking and holding down on the Eyedropper tool in the toolbox, and it allows you to select up to four different pixels in your image in order to identify the precise numerical settings for each color, which will be visible in the Info palette. To get rid of the "bulls-eye" markings on your image, hold down the <Alt> key and point your cursor to each bulls-eye. When you see a scissors icon appear, click to remove the point.

Beneath the list of color modes, you will notice a bit depth for your image of either 8 Bits/Channel or 16 Bits/Channel . Bit depth indicates the level of color information saved with a pixel; a pixel with a bit depth of 1, for example, has only two possible values: either black or white. Higher bit depth makes for better color matching between the image you see on your screen and the image you output. Occasionally, you may notice that some of Photoshop's features (such as filters) may not work in 16-bit depth; to turn "greyed-out" options back on, you can change the bit depth to 8. All of Photoshop's features are available at this bit depth.

There are a number of basic types of scanners: drum scanners, flatbed scanners, and film scanners. Drum scanners are very expensive, have exceptional quality, and are used mainly by service bureaus for pre-press work. Flatbed scanners make up the bulk of the market. Many of the higher-priced models also come with film scanners to acquire image negatives or to scan slides. The difference in price is a reflection of quality, software, dynamic range, bit-depth, and options. Following are three features you should research in any scanner you are planning to purchase.

Each scanner comes with its own driver, the software controlling the behavior of the scanner. It is usually a plug-in Import module for Photoshop (it appears on the Import sub-menu of the File menu). In order to use the scanner, you need to connect it according to the manufacturer's instructions, then install the driver as instructed in the scanner manual. Once you've completed that process, you may need to "introduce" Photoshop to your new scanner using the Import command for Twain_32 Source. (Depending on the brand of scanner you've purchased, you may see a different name before the word "Source.")

Once everything is set up, you start to scan by placing your image on the scanner bed, closing the scanner's top, then selecting the scanner driver from the Import sub-menu under the File menu. For example, if you are using an HP Scanner, you will probably select Twain_32 from the Import sub-menu. You can then begin the scanning process (with a "Scan" or "Preview" button).

Depending on the scanner driver you're using, you may have various control options. Because Photoshop generally allows for more efficient manipulation of your image, we suggest that you accept the manufacturer's default settings and simply make adjustments once the image is imported into Photoshop. Two exceptions to this rule: