How to input and output levels work in gimp? for example when we try to fix the color, brightness etc, using curves the vertical axis represents the output level and the horizontal axis represents the input levels. What do these levels mean? How do we predict what curves bring out what result? How do these affect the whole image?
For some images Im using Photopaint, not gimp becouse it has more editing modes for this explanation.
Middle values, a curve
I have a sample gradient (A)
And I afect the curves (B) with the "standard" one. This is subtile with the dark side and rapidly affects the white part.
There is a special kind of curve, gamma, wich "attacks" more agresively the dark part and slow down to the white part of the spectrum (C)
On the gradients you will notice that the blacks are afected more rapidly, so we have less dark gray.
This kind of graph is the one used in the levels on the middle selector, not only in gimp but as a standard value across aplications. (D)
Cropping values, black and white point
The input values crops the pixels beyond that point. If you have a light gray that was beyond that defined point, it is converted to white. The same with the black side. It crops the info in a linear way (E). You can see the effect clicking the Edit theese settings as Curves.
It is easy to see this also in the Output levels. I added a red line simulating moving theese sliders.
Some practical tips
When you know you need a white or black value on your image use levels, not curves.
When your photo is too dark, try first adjusting the gamma (using middle level) this will bring some detail more easy on the dark portion on an image.
Adjusting levels (or rurves on that matter) you could also affect saturation, so keep an eye on that.
It is better to controll brightness and contrast using curves and drawing S shape curves, not the linear crop that levels produce.