The minimum grey level is 0. The maximum grey level depends on the digitisation depth of the image. For an 8-bit-deep image it is 255. In a binary image a pixel can only take on either the value 0 or the value 255. In contrast, in a greyscale or colour image a pixel can take on any value between 0 and 255.
In a colour image the grey level of each pixel can be calculated using the
Grey level = 0.299 * red component + 0.587 * green component + 0.114 * blue component
This formula takes into account the colour sensitivity of the human eye making the presentation of the grey levels independent of colour and limited solely to the brightness of the individual pixels.
A grey level histogram indicates how many pixels of an image share the same grey level. The x-axis shows the grey levels (e.g. from 0 to 255), the y-axis shows their frequency in the image. This information can be used to calculate a threshold.
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