I think you may have misunderstood the gamma correction bit. Of perhaps you did understand it, I'm not sure anymore.
In XSI, there are settings for Exposure and Display Gamma for HDR and OpenEXR images. This is for OpenGL and other preview.
You're not suposed to use the Image Clips Effects when you work with HDR or OpenEXR image.
If you Enable them, you will get the gamma corrected image sent to mental ray and that's the wrong result, because the gamma correction is implemented in the image source loader. (It's also terrribly memory intensive as images buffers are copied over the Mental Ray.) So don't enable the image clip effects with HDR...
With regards to running HSV correction on a 'normalized' version of HDR images, that doesn't work. It would only work reasonably if all the color channels in the image had the same scale and the image is pretty uniform, but then it wouldn't be a useful HDR image. If there are large differences between red, green and blue channels, you can't really say what's really "red", "green" or "blue" because it depends of where you're looking and what's the exposure.
If Maya does have a shader that does HSV correction on HDR images, and uses a min and max value, it probably doesn't normalize the entier image, but rather apply the LUT on a region of values and leaves the other values outside of it untouched, which is the proper thing to do.
If it was just up to me we wouldn't have image clip fx.., at least for color correction I mean why would have color tools there, then other kinds of tools in the RenderTree. A shader to do this in the RenderTree is totally reasonable, whereas I'm less keen on adding more feature creep to the Image Clip PPG
------------------
Luc-Eric Rousseau
Team Leader, User Interface
Softimage|XSI
> -----Original Message-----
> From: owner-xsi(at)Softimage.COM
> [mailto:owner-xsi(at)Softimage.COM]On Behalf
> Of Tim Leydecker
> Posted At: Wednesday, April 05, 2006 10:28 AM
> Posted To: xsi
> Conversation: (*.hdr) environment shader questions
> Subject: Re: (*.hdr) environment shader questions
>
>
> Hi Luc-Eric,
>
> I checked your article in the wiki (and the spam added/removed:-),
> of course helps a lot in getting an idea about linear colorspace,
> gammacorrection and image interpretation rules neccessary.
>
> DonÂt want to criticise any of that. ItÂs a great article.
>
> WhatÂs puzzled me is the Clipping to 8bit the moment you enable
> effects on an image (that has >8 bitdepth), if I understand you
> correct, the image is then gammacorrected (according to prefs) and
> passed to mR, resulting in a completely different rendering/backround.
> As the gammacorrection doesnÂt happen for the native HDR file but
> only for the clipped 8bit converted file.
>
> I *would* have instead expected an *.hr with the Exposure=0
> and the 8bit converted image to *look* identical, e.g. the clipping
> being based on the exposure set in the slider for example.
>
> Regarding the OGL display allways requiring a gammacorrection
> for floating point images, no critique here either. ItÂs logic.
>
> Yet, in terms fo working with fp images, IÂd think still think all
> colorconcepts are still adaptable if you normalize all images
> (not to be confused with a gammacorrection) to the 0-1 range.
> Which is what you can do with Maya, the min input is declared
> black, the max is pure white, for color operations. YouÂd need the
> displaygamma to correctly set the exposurelevel if you want a
> lowkey or highkey image (e.g. something without absolute black)
> out of that. ItÂs true, you canÂt load such an image without having
> an idea what it contains, the normalization always makes it a
> *perfect, full range* image by default. IÂd need to check the specs
> of the *.exr format, afaik they attach a certain exposure to
> the monitor
> displaygamma via the header, but I never working/fiddled with
> *.exr yet.
>
> I *think* itÂs like for exposure=0, thatÂs why I was puzzled
> initially.
> Maybe IÂm wrong and there is not such thing in HDR/*.exr files.
>
> Cheers
>
> tim
>
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