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HDR Gamma setting

Posted: Thu Jul 01, 2010 3:39 pm
by drm
Hi -

I'm a bit puzzled about the "for HDR Output" option next to Gamma-Gradation in General Options, and how it relates to Gamma expected in 48bit/HDR in HDR Studio.

According to Ian Lyons' tutorial, one should not check the option if one is using IT-8 calibration. But he also says the Gamma value from AI Studio should be inserted into "Gamma expected in 48bit/HDR". I don't quite understand this: surely the "for HDR Output" option is just automating the process, inserting the gamma into the TIFF file ? I'm concerned about the implication that "for HDR Output" is somehow in conflict with IT-8 calibration.

If I open a 48 Bit HDR with "for HDR Output" option checked in Photoshop, the image looks normal - just like the prescan, and identical to the same HDR opened in HDR Studio ... so I'm not quite sure what damage I could be doing.

If I understand correctly, a 48 Bit HDR with "for HDR Output" option checked and color calibration enabled is exactly the same as a "48 Bit Color", color calibrated, to the same gamma, EXCEPT it has no image processing. Is this correct ?

Thanks
David Mantripp

ps - and good luck against Argentina :-)

Re: HDR Gamma setting

Posted: Fri Jul 02, 2010 10:37 am
by LSI_Ketelhohn
drm wrote:Hi -

I'm a bit puzzled about the "for HDR Output" option next to Gamma-Gradation in General Options, and how it relates to Gamma expected in 48bit/HDR in HDR Studio.

Normally HDR files use a gamma of 1 (I would suggest to leave it that way).
If your Workflow requires you to use HDR files in another program which can not handle gamma adjustments you can set the expected value here.
This e.g would not be required for Photoshop as Photoshop can easily rise the gamma to 2.2.

drm wrote:According to Ian Lyons' tutorial, one should not check the option if one is using IT-8 calibration. But he also says the Gamma value from AI Studio should be inserted into "Gamma expected in 48bit/HDR". I don't quite understand this: surely the "for HDR Output" option is just automating the process, inserting the gamma into the TIFF file ? I'm concerned about the implication that "for HDR Output" is somehow in conflict with IT-8 calibration.

No this is wrong.
The setting has nothing to do with IT8 calibration.
The calibration is done at the normal set gamma (normally 2.2) and will be used when SilverFast HDR converts the image from gamma 1 to 2.2.

drm wrote:If I open a 48 Bit HDR with "for HDR Output" option checked in Photoshop, the image looks normal - just like the prescan, and identical to the same HDR opened in HDR Studio ... so I'm not quite sure what damage I could be doing.

No damage done. ;-)
you can also raise the normal gamma of 1 to 2.2 in Photoshop and the image would look the same.

drm wrote:If I understand correctly, a 48 Bit HDR with "for HDR Output" option checked and color calibration enabled is exactly the same as a "48 Bit Color", color calibrated, to the same gamma, EXCEPT it has no image processing. Is this correct ?

No.
The HDR image is not converted to a target colorspace and embeds the calibration profile for the scanner.
An 48bit image is converted to e.g AdobeRGB/ProPhoto/etc. and embeds that profile.

Thanks
David Mantripp

Your welcome
Arne Ketelhohn

drm wrote:ps - and good luck against Argentina :-)

p.s.: we will need no luck. :-D
...hopefully ;-)

Re: HDR Gamma setting

Posted: Sun Jul 04, 2010 9:48 pm
by drm
Thanks Arne -

0:4 ... and it looks like you still have all your luck unused :-)

I have a final question about manipulating HDR files - out of curiosity this weekend, I've been trying to replicate the Silverfast Histogram / Curves behaviour in Photoshop, applying a 2.2 Gamma correction using Levels to a 1.0 Gamma 48 bit HDR. The results are patchy: on "easy" images I can get very close, in fact almost exactly the same, but where there is high contrast and less than perfect exposure, it is much harder - in fact so far impossible. I've tried setting the blending mode to "Normal" or "Luminosity" - I assume "Luminosity" should be better, but no luck. The colour cast slider in Silverfast seems like magic ... and I've no idea why Adobe never copied the simultaneous colour channel display, it really makes life so much simpler. I'm clearly not expecting you to give away any trade secrets here, but are there any known methodologies for applying a gamma correction without colour shift ?

Thanks

Davod

Re: HDR Gamma setting

Posted: Tue Jul 06, 2010 2:23 pm
by LSI_Ketelhohn
Dear David,

I must admit I can't really help with that.
This is probably something a Photoshop crack must answer.

kind regards,
Arne Ketelhohn.

Re: HDR Gamma setting

Posted: Tue Jul 06, 2010 7:27 pm
by drm
Ok thanks -

I think I'll actually just use HDR Studio.... Life is too short for Photoshop :-)

Best wishes
David

ps - I must learn to spell my name correctly :-)

Re: HDR Gamma setting

Posted: Wed Jul 07, 2010 12:58 pm
by degrub
Try working in Lab color space and see if you get better results. Then convert into RGB.