Color adjustments : brightness, contrast and gamma

Hi folks,

Today we are going to explain some terms related to color adjustments in digital images because all our products provide such features and since some have “esoteric” names (such as “gamma”) our general public might skip using them at image quality enhancement time.

Brightness and contrast are very well known image adjustments but let’s mention them nevertheless :

- Brightness reffers to the absolute value of colors (tones) lightness/darkness.
Increasing brightness of an image will light out all colours so the original light ones will become up to white.
Reversely, decreasing brightness will darken all colours so the original shaded ones will become up to black.

- Contrast  is about the distinction between lighter and darker areas of an image and it reffers to making more obvious the objects or details within an image.
Increasing contrast on an image will increase the difference between light and dark areas so light areas will become lighter and dark areas will become darker.
Reversely, decreasing the contrast will make lighter and darker areas stay approximately the same but the overall image becomes more “flat” and starts looking as if it were “washed out”.

- Gamma correction is not just as easy to understand and here is why: there is an important difference between how human eye perceives light compared to how image-capturing devices do (cameras, scanners, etc).
Digital image-capturing devices work based on a simple rule : if twice the photons hit a sensor then twice the signal will be generated.
Eyes don’t work the same way, as biology is almost never governed by linear simplicity : we are more sensible in perceiving changes occuring in dark tones than similar changes occuring in light tones.
When our eyes receive twice the photons, the visual sensation is not that of twice the brightness, it depends on the context so linearity here is an exception instead of a rule.
Gamma is about translating between digital sensitivity and human eye sensitivity, providing many advantages on one hand but adding complexity on the other hand.
Therefore, we will not go further presenting technical details or other various aspects such as gamma encoding in file creation, gamma corrections on image display, differences between CRT and LCD monitors, and so forth.
Instead, we’ll put it in a simplistic way : gamma adjusts the midtones from tonal scale but keeps the white and black.
In other words, gamma optimizes the contrast and brightness in the midtones.
This is particularly important for scanned documents because it can significantly improve pages readability.
For example, changing gamma settings on a very light document image will can make it readable without having to make it overall darker.

Cheers,

Bogdan

P.S.: In case you need explanations on technical terms used in or related to document imaging technology domain, please feel free to ask for in your comments.
We will be happy to provide them.

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