2: Linear and nonlinear
In order to effectively process a image requires that it is linear and calibrated.
What is linear can easily be expressed as:
A star that is 10 times bright as another star also shall provide 10 times the signal.
Not so obvious that it is so!
More mathematically, a linear function satisfy:
- f (x + y) = f (x) + f (y) and f (a * x) = a * f (x)
When a camera is set to save images in RAW format is it usually in
its linear format.
However digital cameras even in raw mode, they have a certain compression in the higher range and then not exactly linear.
Normal film is strong nonlinear and also a digital image in jpg
format. A nonlinear reproduced image is very difficult to by math "count" back to linear format.