Photoshop also includes some fun-filled color mapping commands, so-called because they change the colors of your image in specific ways. Two of them, Invert and Equalize, don’t even have any options. They’re akin to single-step filters that you apply and forget.
Invert
Invert simply reverses all the colors and tones in your image, creating a negative image. Photoshop changes black tones to white; white tones to black; dark grays to light grays; and colors to their complements. For example, a light yellow color becomes a dark blue, and so forth. Some folks mistakenly think they can use this command to create a positive (or color correct) version of a scanned color negative. It ain’t so simple because color negatives have an orange mask overlaying the color information. To really do that correctly requires a lot of color correcting and tweaking. Don’t try this at home!
Equalize
This command locates the lightest and darkest pixels in an image, defines them as white and black, respectively, and then changes all the other pixels in between to divide the grayscale values evenly. Depending on your image, this process may increase contrast or otherwise alter the color and tones as the values are evenly distributed.
Threshold
Threshold converts your image to black and white, with all pixels that are brighter than a value you specify represented as white, and all pixels that are darker than that value as black. You can change the threshold level to achieve different high-contrast effects.
Posterize
This color mapper creates an interesting graphic effect by reducing the number of colors in your image to a value you specify, from 4 to 255. Low values provide distinct poster-like effects. As you increase the number of color levels, the image begins to look either more normal, or a bit like a bad conversion to Indexed Color.
Cleaning Up a Line Art Scan
Line art consists of (you guessed it!) lines, rather than the continuous tones of a photograph or painting. Line art can consist of outlines, shapes (like you’d find in a bar chart), patterns (like the fills in the bar chart), or freehand drawings like those produced in pen or pencil. Often, line art consists of just one color, plus white, such as black lines on a white background, or
colored lines on a plain background. Line art can also include several distinct colors: Your bar chart may have bars in a variety of different hues. What you don’t want to see when you scan line art is an extra color: the background color of the paper, which should be nominally white, but often doesn’t scan that way. Often the paper appears as a dull gray, and you may see other artifacts you don’t want, such as wrinkles or spots in the paper.
Luckily, Photoshop has a handy Threshold command that you can use to determine which tones appear as black and which are dropped altogether. When you set a threshold for your image, anything darker than the brightness you choose is rendered as black, and anything lighter is converted to white. You end up with a nice black-and-white line art image with all the intermediate tones removed. Follow these instructions to clean up a piece of line art:
1. Open a line drawing in Photoshop.
2. Choose Image➪Adjustments-->Threshold from the menu bar.
The Threshold dialog box includes a chart called a histogram. The histogram includes a series of vertical lines showing how many of an image’s tones are represented by a certain brightness level. You can see that a relatively small number of tones are represented by a brightness value of 93, marked by the gray triangle at the bottom of the histogram. Many more tones are used at the other levels, forming a sloping mountain in the chart.
3. Move the slider to the right until the tones you want to appear in the image are shown.
The more you move the slider to the right, the darker the image gets. A threshold of about 189 seems about right for this image.
4. Click OK to apply the modification.
Some small artifacts may remain in your image. These are spots and parts of wrinkles that are darker than the page background, approaching the darkness of the line art itself.
5. To clean up these slight defects, use the Eraser tool.
Invert
Invert simply reverses all the colors and tones in your image, creating a negative image. Photoshop changes black tones to white; white tones to black; dark grays to light grays; and colors to their complements. For example, a light yellow color becomes a dark blue, and so forth. Some folks mistakenly think they can use this command to create a positive (or color correct) version of a scanned color negative. It ain’t so simple because color negatives have an orange mask overlaying the color information. To really do that correctly requires a lot of color correcting and tweaking. Don’t try this at home!
Equalize
This command locates the lightest and darkest pixels in an image, defines them as white and black, respectively, and then changes all the other pixels in between to divide the grayscale values evenly. Depending on your image, this process may increase contrast or otherwise alter the color and tones as the values are evenly distributed.
Threshold
Threshold converts your image to black and white, with all pixels that are brighter than a value you specify represented as white, and all pixels that are darker than that value as black. You can change the threshold level to achieve different high-contrast effects.
Posterize
This color mapper creates an interesting graphic effect by reducing the number of colors in your image to a value you specify, from 4 to 255. Low values provide distinct poster-like effects. As you increase the number of color levels, the image begins to look either more normal, or a bit like a bad conversion to Indexed Color.
Cleaning Up a Line Art Scan
Line art consists of (you guessed it!) lines, rather than the continuous tones of a photograph or painting. Line art can consist of outlines, shapes (like you’d find in a bar chart), patterns (like the fills in the bar chart), or freehand drawings like those produced in pen or pencil. Often, line art consists of just one color, plus white, such as black lines on a white background, or
colored lines on a plain background. Line art can also include several distinct colors: Your bar chart may have bars in a variety of different hues. What you don’t want to see when you scan line art is an extra color: the background color of the paper, which should be nominally white, but often doesn’t scan that way. Often the paper appears as a dull gray, and you may see other artifacts you don’t want, such as wrinkles or spots in the paper.
Luckily, Photoshop has a handy Threshold command that you can use to determine which tones appear as black and which are dropped altogether. When you set a threshold for your image, anything darker than the brightness you choose is rendered as black, and anything lighter is converted to white. You end up with a nice black-and-white line art image with all the intermediate tones removed. Follow these instructions to clean up a piece of line art:
1. Open a line drawing in Photoshop.
2. Choose Image➪Adjustments-->Threshold from the menu bar.
The Threshold dialog box includes a chart called a histogram. The histogram includes a series of vertical lines showing how many of an image’s tones are represented by a certain brightness level. You can see that a relatively small number of tones are represented by a brightness value of 93, marked by the gray triangle at the bottom of the histogram. Many more tones are used at the other levels, forming a sloping mountain in the chart.
3. Move the slider to the right until the tones you want to appear in the image are shown.
The more you move the slider to the right, the darker the image gets. A threshold of about 189 seems about right for this image.
4. Click OK to apply the modification.
Some small artifacts may remain in your image. These are spots and parts of wrinkles that are darker than the page background, approaching the darkness of the line art itself.
5. To clean up these slight defects, use the Eraser tool.
Playing with the Color Mappers in Photoshop
![Playing with the Color Mappers in Photoshop]()
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