Brightness/Contrast...
This opens a pop-up window :
The top panel shows a histogram of pixel values :
The horizontal axis gives the range of values (from 0 to 255 for a 8 byte image). The number written on the left and right hand side are the minimum and maximum values displayed.
The vertical axis gives the number of pixel with a given value (the exact number is useless).
Minimum and maximum values may be modified using the horizontal bars. If you change those values, all pixel values are re-interpolated between the min and max, into a 0-255 scale (for a 8 byte image).
In this exemple, there are almost no pixels with very low or very high value. This makes a bad contrast in the image since the range covered by most pixels is small.
Then, you may increase the minimum value considered in the display, through the upper, red, horizontal bar. It is illustrated by the red vertical line in the top panel with the histogram.
You may also decrese the maximum value considered in the display, through the blue horizontal bar. It is illustrated by the blue vertical line in the top panel with the histogram.
Doing so, you increase the contrast (lower horizontal bar). You may also play directly with the contrast. If you increase it, both minimum and maximum value will converge towards each other. At the end, minimum equals maximum : you get a black and white image (no grey ; white if is above the selected value and black if it is below)
You may "Set" the minimum and maximum values by typing their values.
At any moment, you can "reset" to the initial values (or the one just before you choose 'Apply")
If you "Apply" your setting, the values of the image are cut below the minimum and above the maximum, and all values are re-interpolated into a 0-255 scale (for a 8 byte image). This cannot be undone (even with "reset" !!)
To confirm your modifications, just close the window.
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