lecture9

Image Histogram
The histogram of an image is a plot of the gray _levels values versus
the number of pixels at that value.
A histogram appears as a graph with "brightness" on the horizontal
axis from 0 to 255 (for an 8-bit) intensity scale) and "number of pixels
"on the vertical axis. The histogram gives us a convenient –easy -to -read
representation of the concentration of pixels versus brightness of an
image, using this graph we able to see immediately:
1- Whether an image is basically dark or light and high or low contrast.
2- Give us our first clues( ر ا )a bout what contrast enhancement would
be appropriately applied to make the image more subjectively pleasing
to an observer, or easier to interpret by succeeding image analysis
operations.
So the shape of histogram provide us with information about nature of the
image or sub image if we considering an object within the image. For
example:
· Very narrow histogram implies a low-contrast image
· Histogram skewed toword the high end implies a bright
image
· Histogram with two major peaks , called bimodal, implies an
object that is in contrast with the background
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Figure(1) histogram of image
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Histogram features
The histogram features that we are considered are statically based
features where the histogram is used as a model of the probability
distribution of the gray levels. These statistical features provide us with
information a bout the characteristic of the gray – level distribution for
the image or sub image. We define the first – order histogram probability
P(a) as :