xgamma what is gamma? - X

This is a discussion on xgamma what is gamma? - X ; Hi AFAIK the definition of gamma is : luminance = (Pixelvalue / 255 ) ** gamma (** means exponent) So if I increase gamma, a midtone pixelvalue, lets say 128, should get darker. If I increase the setting of gamma ...

1. xgamma what is gamma?

Hi
AFAIK the definition of gamma is :
luminance = (Pixelvalue / 255 ) ** gamma (** means exponent)
So if I increase gamma, a midtone pixelvalue, lets say 128, should get
darker.
If I increase the setting of gamma with xgamma or kgamma, the midtones get
brighter. What is wrong?

Best regards

2. Re: xgamma what is gamma?

Victor H wrote:
>Hi
>AFAIK the definition of gamma is :
>luminance = (Pixelvalue / 255 ) ** gamma (** means exponent)
>So if I increase gamma, a midtone pixelvalue, lets say 128, should get
>darker.
>If I increase the setting of gamma with xgamma or kgamma, the midtones get
>brighter. What is wrong?
>
>Best regards

luminance = (Pixelvalue / 255 ) ** (1 / gamma)

3. Re: xgamma what is gamma?

Kip Rugger wrote:

> Victor H wrote:
>>Hi
>>AFAIK the definition of gamma is :
>>luminance = (Pixelvalue / 255 ) ** gamma (** means exponent)
>>So if I increase gamma, a midtone pixelvalue, lets say 128, should get
>>darker.
>>If I increase the setting of gamma with xgamma or kgamma, the midtones get
>>brighter. What is wrong?
>>
>>Best regards

>
> luminance = (Pixelvalue / 255 ) ** (1 / gamma)

This is interesting, but does not include the build in monitor gamma of
about 2.2 of the PC monitor. So, can we say:

luminance = (Pixelvalue / 255 ) ** (monitor-gamma / x-gamma) ?

With x-gamma means the setting value of gamma in the xgamma program.

4. Re: xgamma what is gamma?

Victor H wrote:
>Kip Rugger wrote:
>
>> Victor H wrote:
>>>Hi
>>>AFAIK the definition of gamma is :
>>>luminance = (Pixelvalue / 255 ) ** gamma (** means exponent)
>>>So if I increase gamma, a midtone pixelvalue, lets say 128, should get
>>>darker.
>>>If I increase the setting of gamma with xgamma or kgamma, the midtones get
>>>brighter. What is wrong?
>>>
>>>Best regards

>>
>> luminance = (Pixelvalue / 255 ) ** (1 / gamma)

>
>This is interesting, but does not include the build in monitor gamma of
>about 2.2 of the PC monitor. So, can we say:
>
>luminance = (Pixelvalue / 255 ) ** (monitor-gamma / x-gamma) ?
>
>With x-gamma means the setting value of gamma in the xgamma program.

I'm not sure what you are asking.

If the gamma value recorded in the monitor (retrieved by DDC) is 2.2,
this means you should correct pixel values to

(Pixelvalue / 255) ** (1 / 2.2) * 255

It is the 2.2 that you give to the xgamma program. What the server
does with that value depends on a lot of things, including the version
of X, the driver, and the video card.

If you are asking how to combine two gamma values photometrically,
they would combine as arc-cotangents

arccot(A) + arccot(B) = arccot( (A*B - 1)/(A + B) )

5. Re: xgamma what is gamma?

Kip Rugger wrote:

> I'm not sure what you are asking.
>

I will try to make it clear:
There are charts from the Website of Norman Koren to test Display gamma.
This shows a Display gamma of about 2.5 on MY screen. X-gamma reports a
gamma of 1.0. When I increase the xgamma to 1.1 then the charts show a

In the meantime I found a document from w3-consortium called
'A Standard Default Color Space for the Internet - sRGB'
where they propose a display gamma of 2.2 .
I was confused by formula 0.2 where the say

display gamma = CRT gamma * LUT gamma (0.2)
(with LUT Look up table , means graphic card)

but later in the text they use in the examples

display gamma = CRT gamma / LUT gamma = about 2.2.

I think things are clear now

luminance = (Pixelvalue / 255 ) ** (CRT-gamma / x-gamma)

Bye