Knowing how to accurately measure pixel values is an important first step in being able to evaluate and correct digital images. Many times, you have to determine the tonal value of an image area or compare the value of two different areas. It is important to measure these values numerically, because pixel values change from monitor to monitor based on contrast settings, ambient light, and monitor brands. Photoshop uses the Info palette to measure pixel values; the Eyedropper and Color Sampler tools also are helpful aids.
Open the Info Palette
With the image open, choose Window, Info. The Info palette opens on the desktop.
Select a Color Model
The top-left section of the Info palette represents the actual color value of the currently selected pixel. Click and hold the Eyedropper icon to select the color model in which you want the pixel's color specified. (Refer to Part 1, Task 6, "How to Select a Color," for information about color models.) Choose the Opacity option to measure the degree of transparency in a given layer.
Select a Variable Color Model
The top-right section of the Info palette tracks a second set of color values, which allows you to compare the same pixel value with two different models. Click and hold the Eyedropper icon and select the desired color model from the list. Move the mouse cursor over the image (don't click) and watch the corresponding readings appear in the palette.
Enter Coordinate Units
With so many pixels in an image, it can be hard to sample the same one more than once. The pixel coordinates section in the lower-left corner of the Info palette tracks the exact cursor position based on the x,y axis. Click and hold the cursor icon to choose the units of measure.
Create Sample Points
To accurately track the same value through the course of your imaging session, you should use eyedropper sample points. Click the Eyedropper tool in the toolbox; a submenu of two eyedroppers appears. Select the Color Sampler tool (the second tool). Drag the tool over the image and click to place a sample point. A new section is created in the Info palette for each sample point you create.
Track Sample Values
Drag the sample point to move it; drag the point off the image to delete it. Each image supports up to four sample points. If you want, you can set a color model for each point area in the palette, just as you did in Steps 2 and 3.
Use the width/height section in the lower-right corner of the Info palette to measure the exact size of an active selection. This capability is especially valuable with the Marquee tool, which gives real-time dimensions as you drag. For more information on selections, see Part 3, "Selection Techniques."