1.  Describe the process of measuring the image tonal
balance from its histogram.

2. In This Chapter, you’ll learn on:
 Explain the term histogram
 Evaluate exposure with a histogram
 Differentiate between
o a basic histogram and
o a colour histogram (RGB)

3.  Histogram
 A histogram is a statistical graph that allows the intensity
distribution of the pixels of an image; also refer to as the
number of pixels for each luminous intensity to be
represented by convention. A histogram represents the
intensity level using X-coordinates going from the darkest
(on the left) to lightest (on the right).

 Thus, the histogram of an image with 256 levels of grey will
be represented by a graph having 256 values on the X-
axis and the number of image pixels on the Y-axis. Let us
consider, for example, the following image made up of
levels of grey:

4. HIstogram
 The histogram reveals that there are many more light grey
tones present in the image than dark grey tones.The tone of
grey that is most used is the 11th from the left.

5. HIstogram
 Understanding the Histogram
 As we had mentioned, a histogram is a graph that displays how
light is distributed in your picture. The left side of the graph
represents the shadows, while the highlights are on the right.
Here's what that means: If the histogram has a high peak on
the left, you can deduce that a lot of pixels in the picture are
dark, or in shadow. A peak on the right of the graph means
that a lot of pixels are bright, or in highlights. Peaks in the middle
of the graph represent pixels in the midtones of your exposure.

6. HIstogram
 Understanding the Histogram
 And here's the real key to unlocking the power of a histogram:
There should not be any peaks that get "cut off" at either end of
the graph, as if they want to continue past the edge of the
graph. When the histogram starts or ends with a peak that's
already in the air, then you know that colour information has
been lost because the camera's exposure settings weren't
correct for that picture.

7.  Colour Histogram
 In image processing and photography, a colour
histogram is a representation of the distribution of
colours in an image. For digital images, a colour
histogram represents the number of pixels that have
colours in each of a fixed list of colour ranges, which
span the image's colour space, the set of all possible
colours.

8.  Colour Histogram
Several histograms are necessary for colour images. For
example, for an image coded in RGB there are:
 A histogram representing the luminance distribution,
 three histograms representing the distribution of the values of
the red, blue and green components respectively.