HDRI exposure

[Gijs]

I was wondering if one can change the exposure of an HDR image. If I turn down the multiplier it seems to do that linearly. Is this possible with VRay HDRI map loader, or do I have to modify my HDR image within HDRIShop ?

Thanks,

Gijs

[cocolas]

I usually modify mine into HDRshop, but I think you can do it w/ the vray map loader as well...

paul.

[Syclone1]

Yea I do the same as cocolas, I use HDRshop to edit the HDRI images.

-dave

[flipside]

I don't think the hdr loader multiplies it linearly. If you turn it down, light through windows for example stay bright untill the end. Not sure though...

[Gijs]

Flipside, that's exactly my point. If you turn down the HDRI within the map loader, then you will not see the same effect as pressing the - button in HDRIShop. In HDRIShop you can get to see what was outside the window. I think it would be nice if you did not have to create a new version of your HDRI and rather change the exposure within the map loader.

Gijs

[J_Bug]

Maybe it has something to do with what I've read that HDRIs can be created separately for diffuse as well as specular in HDR shop. It seems to me that the multiplier in the HDR loader works the same as pressing +or- in HDR shop.

--Jon

[Gijs]

Well, more or less. I did a few tests and it appears to me that dividing by 2 is a stop.

So if you need to have 5 stops underexposed it is 2^-5 (about 0.031)

Maybe useful for vray.info?

Gijs

[J_Bug]

Interesting that you figured it out the way you did . For the human eye to naturally perceive an "exposure change" the light level must either double or halve. This is exactly what the F-stop is based on. The Inverse Square Rule seems to also be related to F-stops, which are a manufactured not physical scale. Basically if you double the distance a light source you will get a quarter of the light falling on the surface of the subject and in turn if you half the distance the light level will be squared.

--Jon

[Gijs]

Now you say so I realize...I guess I was a bit dense yesterday.
It just did not seem very straightforward. But like many things, once you know it seems all so simple

Gijs

[samuel_bubat]

hi

There is a "Tone Mapping" plugin for HDRShop available.
http://www.gregdowning.com/HDRI/tonemap/Reinhard/

this gives you more detailed control over the image "exposure".
But it´s not very interactive.

If you have a copy of "digital fusion", use gamma/gain to adjust
the hdri exposure in a more interactive way.