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INTRODUCTION TO PHOTOSHOP 4.0 (Continued)
BASIC IMAGE CORRECTIONCOLOR MANAGEMENT - Because of the way scanners scan color, monitors display color, and inks print color, precise color-matching can be elusive, even to the best artist. In addition, individual and environmental factors, such as lighting, can present other challenges to matching the color of your on-screen images to the printed image.
Color management consists of two parts. First, you adjust the overall contrast, or tonal range, of an image, and then you make adjustments to the color values, if necessary.
DETERMINING TONAL RANGE - is performed by the way an image's pixels are distributed throughout the image's highlights, midtones, and shadows (from darkest to lightest pixel). This can be seen using the histogram, which charts an image's range of pixel's with regard to their brightness (see figure 4). The horizontal axis plots the pixel values from 0 (black) to 255 (white). The vertical axis shows the number of pixels with that level of brightness. Fig. 4 - Low Key Histogram DETERMINING IMAGE TYPE - Images can be classified into one of three key types, determined by the visual distribution of tones within the image. Fig. 5 - Avg Key Histogram Low-key image - an image composed of mostly dark tones Average-key image - an image composed an equal mixture of light and dark tones High-key image - an image composed of mostly light tones ADJUSTING TONAL RANGE - The most commonly used adjustment tools are: Fig. 5 - High Key Histogram Brightness / Contrast - the simplest but most limited adjustment control. It offers the least amount of control over the tonal range of an image because the adjustment affects the image globally. It does not let you adjust individual channels within a document. Auto Levels - Automatically adjusts the overall contrast of an image by defining the lightest and darkest (highlight and shadow) pixels in an image as white and black, and then distributes the remaining tones in the image between them. The result is determined by the characteristics of the original image. Levels - Lets you control the tonal balance by setting the lightest and darkest points in the image yourself. You can use Levels to affect an entire image, a selection, or a specific channel within an image. It also lets you adjust the gamma in an image. Gamma measures the contrast that affects the midlevel grays (graytones), and can be adjusted without significantly affecting the shadows and highlights in an image. Curves - Provides the most sophisticated con controls. In addition to using Curves to set the black and whith points, you can control the midtones in an image with greater precision.
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