Adobe PHOTOSHOP 5.0 User Manual page 121

To determine whether a scan captured
enough detail for high-quality output,
check the RGB pixel values of your highlight
and shadow areas in the Info palette—
generally, 240 and 10, respectively. The scan
must also be free of large gaps within these
ranges.
To check the histogram for an image:
Open the image you want to examine, and
1
choose Image > Histogram.
Note: To include pixel data from spot channels and
alpha channels, hold Alt (Windows) or Option
(Mac OS) when you choose Image > Histogram.
Histogram of gray levels in entire image
The x axis of the histogram represents the color
values from darkest (0) at the far left to brightest
(255) at the far right; the y axis represents the total
number of pixels with a given value.
Note: If part of the image is selected, the histogram
represents only the selected pixels. In addition, the
histogram for an adjustment layer depends on the
visibility of the layers beneath it.
The numerical values at the lower left of the
Histogram dialog box display statistical infor-
mation about the color values of the pixels:
Mean is the average brightness value.
•
Standard deviation (Std Dev) represents how
•
widely the values vary.
Median shows the middle value in the range of
•
color values.
Pixels represents the total number of pixels used
•
to calculate the histogram.
Cache Level shows the setting for the image
•
cache. If the Use Cache for Histograms option is
selected in the Memory & Image Cache Prefer-
ences dialog box, the histogram is based on a
representative sampling of pixels in the image
(based on the magnification), rather than on all of
the pixels.
For RGB, CMYK, and indexed-color images,
2
choose an option from the Channel menu. You can
plot either the color values in the individual
channels or the luminance of the composite
channel (with the Luminosity option).
To view information about a specific point on
3
the histogram, place the pointer there. To view
information about a range of values, drag in the
histogram to highlight the range.
The values at the lower right of the dialog box
change to display the gray level (Level) of the point
(from 0 to 255), the total number of pixels at that
level (Count), and the percentage of pixels at or
below that level (Percentile).
ADOBE PHOTOSHOP 5.0 113
User Guide