Useful hints for editing those HDRs

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Christian Bloch
Guide,
60 Messages

wrotes on Thursday, 11:45:50 10 July 2003 :



Yes, most of these links have been mentioned in the Lightbox before, but for your convenience I post them again in one place:

HDRView: http://athens.ict.usc.edu/FiatLux/hdrview

HDRShop: http://www.debevec.org/HDRShop

HDRIE: http://www.acm.uiuc.edu/siggraph/eoh_projects/eoh2002.html

HCR-Edit: http://www-public.tu-bs.de:8080/~y0013390/hcredit/

Photosphere (OS X only): http://radsite.lbl.gov/pickup/photosphere.tar.gz


OK, now here comes the tricky part.
Why do these specific images look so flat in any of the above programs?
Because they have been adjusted to be seen at Gamma 1. Yes, the actual pixel values are scaled to a Gamma 2 already. So, if you want to edit them, you NEED to set your Display Curve to a Gamma of 1. Otherwise the embedded Gamma AND the Display Gamma sum up to a Double Gamma of somewhat like 4 - and the image looks flat, grey, washed out.

confused?

Here is a picture to illustrate what I did.



Why did I do this?

Because this is how rendering engines import and see the images by default, at Gamma 1. So, if a picture looks good in HDRShop (at HDRShops default 2.2), it is guaranteed to look VERY dark in the rendering system (bottom right image). This is most notably on the Background.HDR, but applies to the environment as well.

To compensate, I used to do a gamma correction in Lightwave when I load the images. As far as I know, 3DMax users used to do the same by using Input Override. But when making the first translation tests, we figured out that for example MaxonCinema does not allow a Gamma correction on the textures (Hi Pyro).
That's why I decided to edit the HDRs themselves to work at in any other 3d Software at the default Gamma 1 (top right image).

Sorry for the fuzz.
I guess if Chris Tchou or Paul Debevec were here, they would prove me wrong with the blink of an eye. No, there is no scientific reason for doing so, I was just trying to make it work in off-the-shelf rendering systems.

If you disagree, and think I messed it up, let's talk about it!

Blochi


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