A Linear v Non-Linear

[pscammp]

Hi all,
    This has probibly been asked a million times but a google search does'nt seem
to produce an answer....So.....

In Lamens terms.....

What is difference between a Linear image and a Non-Linear image ??

And Please dont say.......Ones stretched & one is not    LOLOL

Cheers
Paul

[pfile]

in a linear image the brightness of features is linearly dependent on how many photons were received from the feature. the gain, quantum efficiency of the sensor and what filter you were using will determine how many ADUs are registered for a given source of light.

a nonlinear image has had it's brightness changed in some nonlinear way, where the brightness of some pixels are increased more than the brightness of other pixels in the image. as you mention this is usually accomplished by some kind of stretch, where midtones and darks in the original image are boosted more than the brightest parts.

rob

[Josh Lake]

Eor Equis, a fellow PI user and mod on Reddit's /r/astrophotography, made a very clear video about linear vs. non-linear, stretching, and histograms. Definitely worth a look:

https://www.youtube.com/watch?v=lWXj6Pc_hog

[oldwexi]

Hi Paul!

And Please dont say.......Ones stretched & one is not    LOLOL

Paul

In addition to robs perfect explanation please note,
If you use a linear stretch like add, multiply the image is still linear only the angle
of the line of the linearity increases or decreases as long there is no clipping,  a linear stretched image is still linear!
A linear stretched image can be retourned to the original status after a linear stretch by a simple linear unstretch.
A nonlinear stretch cannot be reversed.

Gerald

[Geoff]

A nonlinear stretch cannot be reversed.

Gerald

As long as the function doing the stretch is 1-1 (which will usually be the case) it will have an inverse which can be used to reverse the stretch. In practice, it will not be easy to discover the appropriate function just by examination of the image.
Geoff

[cdesselles]

Eor Equis, a fellow PI user and mod on Reddit's /r/astrophotography, made a very clear video about linear vs. non-linear, stretching, and histograms. Definitely worth a look:

https://www.youtube.com/watch?v=lWXj6Pc_hog

Yet another useful Youtube video.  Thanks.