[Mihai]

uuh.....

Thomas, ofcourse the 0 value and the 255 don't change, meaning a totally black pixel on your monitor has 0 voltage input, so you can multiply it with whatever you want, it will still remain 0, it will be turned off. For the brightest pixel, the electron gun can't make any brighter pixels than it's designed to do. So ofcourse these two values remain the same.

But, the whole idea of changing the gamma is to display an image on the screen as our eyes perceive it.

The intensity to voltage response in the pixels on a screen are not linear, meaning if you want a pixel on the screen to be displayed at 0.2 brightness, it's actual brightness will be far less, so you need to boost it. Very dark pixels will need a large boost in brightness, while lighter pixels will need smaller boost in brigthness, thus forming the gamma curve.

That's why when you adjust the gamma, that curve makes sure you are adjusting the image so that the perceived brightness of the image changes equally. No contrast increase or decrease is produced.

Ofcourse full off, and full on pixels, will not be changed.

[Thomas An.]

Mihai,

Gamma, includes the concept of contrast in it. (It is not pure contrast, but it does effect contrast)

....Gamma is a measure of contrast in an image, typically in the midrange grays (mid-tones). Many image editing programs express gamma as a curve which can be manipulated by moving points to change the contrast of the image. Adjusting the gamma allows you to correct mid-tones without noticeable changes in the highlight and shadow areas. Gamma is also the way the brightness of an image is interpreted by computer hardware. Many monitors and graphics cards let you adjust the gamma level to alter the monitor appearance or to compensate for brightness or color in a room. This is one reason why the same image may look very different on two different monitors.

[Mihai]

Well I think that's misleading....

Gamma is not a contrast adjustment. Ofcourse if you make radical gamma adjustments (depending first of all on what info the initial image had), you will indeed change the distribution of light vs dark pixels, because you are clipping the values.

This is especially misleading:

Adjusting the gamma allows you to correct mid-tones without noticeable changes in the highlight and shadow areas.

Try doing that with an image that has good contrast (good distribution lf all greylevels), and see what happens to the darker shades for example.

Perhaps they are confusing again the curves command in PS, where you can set points to control exactly which areas you want influenced. But that's not a gamma adjustment.

well, anyway.... Maybe with a hdr image you can actually change the gamma and get a more "contrasted" light and because it has so much brightness information, the light parts will continue to remain light, even if you make radical gamma adjustments. So I'm sorry I brought this up...

[tom]

Well, it seems like dielectrics over and let's play with gamma...
IMO, both (Mihai and Thomas) you say is correct.
My personal opinion is:
Contrast is a linear change which you can also alter start and end points.
Gamma is a non-linear change which has fixed end points due to hardware.
Surely, they affect the final image contrast.

[Micha]

Hmm ... what is with my wish: I want to change the gamma of the HDRI? I have different HDRIs with different gamma value. I would prefer to change the gamma in the light parameter, so I could modify the mood of the light.