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1. Histogram - A histogram is a graph of all the different levels of pixels that can be picked up by your scanner. A typical grayscale image consists of 256 separate levels of gray. For each level, your scanner will pick up pixels (provided that there is gray in the original image). The same holds true for color images - with one exception. Color images consist of 3 channels: Red, Green, and Blue, with each channel holding 256 levels of color. For example, a typical image in RGB color scanning will have 256 levels of blue x 256 or green x 256 levels of red for a total of up to 16.7 million colors possible - all in one image!
Understanding Histograms
A histogram is a bar chart of the number of pixels in any given level. Suppose using the menu shown to the left that you wanted to make this image lighter. Use the point adjustment triangles (the little triangles that are just beneath the bar chart) to move the emphasis in the image. For example, moving the midpoint triangle (the one in the middle) to the left (towards the white end) will darken the image. Similarly, we can decide to throw out part of the image that we don't like. For example, in the image above, if the black point adjustor was set all the way to the right more dark area would be seen in the image. The key to using a histogram is remembering that anything that is inside your triangles will be used in the image, but anything outside the triangles will be ignored.
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| Gamma Curve |
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2. Gamma Curve - Many people use the scroll bar to adjust curves, but some people prefer to adjust a gamma curve manually. Gamma curves work in a similar fashion to histograms in that they represent different levels of pixel color. However, the two axes in a gamma curve represent the relationship between input and output levels, all in one graph.
To determine how to use a gamma curve correctly, we must first decide how to interpret what it represents. The horizontal axis represents what the scanner sees. The vertical axis represents how your final image will appear. If the input and the output curves matched exactly, you would have a 45-degree angle line, indicating no change in all levels. However, to change curves, we must change the emphasis, and create a tonal shift. A tonal shift is basically a change in the overall tone of an image (much like brightening or darkening an image).
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| Corrected Curve Menu |
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The Correction Curve menu will help you to decide what modifications you need to make to enhance or color correct an image before you scan it. As we have seen in the last few sections, Art-Scan provides a full range of options in adjusting the tonal values of an image. In the Correction Curve menu, you will find the following options:
- Auto White and Black Points - This setting allows Art-Scan to automatically decide the optimum balance of the overall tonal composition of your image. It is the default setting and may be modified. For example, sometimes you may prefer to make some adjustments in addition to what Art-Scan has set. In this case, simply select Image Range: Auto, and then make any additional modifications in your histogram, gamma curve, or other tonal adjustment tools.
- Full Range (Gray) - This setting will set Art-Scan to allow all levels of image information to be seen in the input histogram. This setting allows you to see the actual raw scan as the scanner sees it, without any enhancements or modifications. This is a good setting to use when you want to make all enhancements and tonal adjustments yourself. This generally results in grayer looking scans.
- Default Values (All) - When you select this menu item all values of the dialog will be set back to their default values. This is a great way to get back to normal if unintended changes were made.
- Solid Blacks and Whites - This setting works in much the same way as the Medium setting but it takes a higher percentage of both the highlight and shadow areas of the image and throws them out. We recommend using this setting only for images that lack a clearly defined highlight and shadow area. This setting will enhance an image that is very gray by giving you more contrast but may remove some data in the highlights and shadows.
- Set By Sliders or Droppers - This setting allows you to use your eye-dropper densitometers to set the highlight, midpoint, and shadow areas for your image. When you click on this option, you will find that your mouse pointer changes into an eye-dropper. Set your points as described in the previous section, covering the eye-dropper tool.
- Reset Curve Points - When adjusting by curves, selecting this item from the correction curves drop down will reset all five points of the curve to their neutral values. This is very useful to correct curves adjustment that caused undesirable effects in your image.
- Reset Except Curve Points - This selection allows you to use only the curve for image adjustments. All other settings are reset to neutral values.
- Newsprint/Laser ( 10% - 65% )
- Uncoated Offset Paper ( 15% - 85% )
- Coated Offset Paper ( 5% - 95% )
- No Dot Gain Adjustments ( 0 - 100% )
This menu allows you to set the dot gain percentage (output levels) of your imaging device. Whenever a dot is printed on paper, the paper will soak up and spread the ink over a larger area than was originally printing. This causes images that look fine on an un-calibrated monitor to look very dark when printed on paper. This menu gives you preset settings for the common types of paper to counteract this effect. Setting the dot gain correctly will cause your images on the final scan to look light (washed out) on your monitor in order to print correctly.
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