Technical question on Gamma in VueScan for Hamrick

[Jonathan]

Hi Ed or any expert on this subject,

I am a big believer in working with 1.0 gamma (linear) images and then
transforming them to whatever gamma (2.5 in PC monitor) for output
purpose. As such, I have been searching for a scanner software that
can give me an 1.0 gamma output.

VueScan obviously can do this through the raw scan file output with
more than 8 bit color depth. The other potential way to obtain a
linear 1.0 gamma output in VueScan is through the Brightness control
(a gamma multiplier according to the user guide) in the Color tab.

My question is how does VueScan use this gamma multiplier internally?
According to the "How VueScan Works" section of the user guide,
VueScan seems to perform gamma correction (to the gamma of the color
space selected in the Color tab) in the color correction step. Does
VueScan first perform this color space gamma correction, produce an
intermediate image, then apply the brightness (gamma multiplier) to
the intermediate image to produce the final image?

Or does VueScan first calculate the final gamma by multiplying the
color space gamma by the gamma multiplier and use the final gamma to
perform just one time of color correction? (obviously, if I choose a
gamma multiplier that results a final 1.0 gamma, no gamma correction
will be done and the original linear data should be preserved)

I obviously would prefer the latter method as the forth and back
conversion in the former method will result in loss of information.

Ed: Do you also have the gamma value for each color spaces available
in the Color Tab?

Thanks & Regards,
Jonathan

 

[Erik Krause]

Hello, Jonathan
you wrote...

My question is how does VueScan use this gamma multiplier internally?
According to the "How VueScan Works" section of the user guide,
VueScan seems to perform gamma correction (to the gamma of the color
space selected in the Color tab) in the color correction step. Does
VueScan first perform this color space gamma correction, produce an
intermediate image, then apply the brightness (gamma multiplier) to
the intermediate image to produce the final image?

This would make no sense at all, I don't believe it is done this way. I
hope Ed comments on this. Usually he refuses to answer postings that
are addressed directly to him...

Or does VueScan first calculate the final gamma by multiplying the
color space gamma by the gamma multiplier and use the final gamma to
perform just one time of color correction? (obviously, if I choose a
gamma multiplier that results a final 1.0 gamma, no gamma correction
will be done and the original linear data should be preserved)

I'm pretty shure that this is the case. There is an additional gamma
for each color channel that are separate multipliers, too.

I obviously would prefer the latter method as the forth and back
conversion in the former method will result in loss of information.

Ed: Do you also have the gamma value for each color spaces available
in the Color Tab?

You find a complete list at
http://www.brucelindbloom.com/index.html?WorkingSpaceInfo.html

 

[Jonathan]

Hi Erik:

Just got an email from Ed. It is confirmed that the color space gamma
is first multiplied by the gamma multiplier and the raw image
undergoes only 1 gamma correction to get to the final image.