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Count value indicates the number of pixels at the Input value. If
•
Input value is 2 and Count value is 1300, then there are 1300
pixels in the image at the input value of 2.
Percent value is the percentage of all pixels in the image where
•
color value is less than or equal to the input value. If Percent is
15% and Input value is 2, it means that 15% of all pixels in the
image have a value of 2 or less.
The Channel button lets you control the Shadow and Highlight
g
settings for a particular color channel (red, green or blue) or for
the Master channel (red, green, and blue simultaneously).
The Original/Enhanced box lets you choose which histogram
h
to see: the original, before the image was modified; or
enhanced, after the image had been modified (with shadows
and highlights or any other tool).
How to read and correct a histogram
A histogram shows how the brightness and darkness levels are
distributed in an image. The darkest pixels are at the left, and
the lightest pixels are at the right.
An image with good contrast will have a histogram with verti-
cal lines spread across the scale from left to right. Here, the his-
togram is heavily skewed to the left, where the darkest pixels
are, indicating a dark image.
To change the histogram (and thus the image), use the three tri-
angles below the histogram.
In the original histogram, the pixels are mostly to the left where
the black triangle is, indicating a dark image. The range of
spread is also broad and flat, with almost no pixels for the mid-
tones and highlights where the gray and white triangles are.
In the corrected image, the triangles have been moved to new
locations. The net effect is to narrow the distribution range of
the pixels and lighten the image.
Generally, the best thing to do is to move the black and white
triangles to the start and end of the curve.
Reference
3-65
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