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Topic: HDRI: Locating and understanding this elusive beast
posted by CHESSJR
archived on 10.5.2002
Where can i get HDRI maps?
Follow-ups
A: try www.debevec.org . Or try a google search... google.com is still the best way to find answers! =)
Comment: Now you can also take a look through the links section of this site.
Q: I have encountered many links to this guys site but none seem work for me. And indeed, why are these HDRI maps so difficult to find ? Maybe their file sizes are too large for download?
Mysteries of the CG world. I think thats why I love this stuff, there is always information which is elusive and never handed to you on plate - slowly but surely it has to be pieced together.
A: Take a look at
The HDRI Library
Comment: The dificulty in finding these images is demonstrated below with an explanation of there internal workings.
Comment: It seems what HDR images from The HDRI Libraryare not "real" HDR images but set of one shot with levels correction.
Comment: They work fine for me.
Comment: Yes, of course. But HDR images technology are different.
Comment: I made my own in terragen using their exposure control
Q: Would you elaborate on how these differ from the true HDRI?
A: A raw-file from a digicam covers the dynamic range of ~6-7 f-stops if it's in 12bit/channel format(usual for modern cameras) ..a normal jpg have even a range of at least 4 f-stops!!!
A true HDRI can cover a much higher range.. and this depends on how it was created. ...single images from 8sec. exposure time up to 1/10000 sec. ~15 f-stops
So if i create a tiff with my digicam i get a kind of HDRI, but the range is to low to get sharp shadows... you can use it to color your rendering and get some basic shadows.
Virtual Vienna
Profipage
these HDRIs have 9-10 f-stops...even not enough for sharp shadows.
;)
eh .. to explain f-stop:
it's the classical decrease of exposuretime if the aperature size is not changed... here we go:
8 sec.
4
2
1
1/2 sec.
1/4
1/8
1/15
1/30
1/60
1/125
1/250
1/500
1/1000
1/2000
1/4000 sec.
every line is one f-stop.
Q: Does a hdr image with 15 fstops provide a noticably improved image?
Would it be possible to have a couple of samples uploaded?
A: Theoretical a Low dynamic range image have the RGB range from 0-255 steps for each color. ..and yes it have a dynamic range.
A high dynamic range image uses a floating point format like from 0.0000 to whatever.
If you use terragen then look at the tutrorial from Marko Dabrovic:
www.rna.hr/tutorial01/default.htm
And here a tut about skypanos with a range of 10 f-stops:
www.cgtechniques.com/lightbox/tutorials/fisheyepano/index.php
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