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  1. 1.  Describe the process of measuring the image tonal balance from its histogram.
  2. 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. 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. 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. 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. 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. 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. 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.