VRay Sun - Intensity/size relationship

V-Ray V-Ray Common Render Theory
1

Hi all,

Have been playing with the VRay Sun - Intensity/size parameters & am lost to find the correct physical relationship between the intensity & size when trying to achieve cloudy skys.

Obviously the default parameters of size [1.0] & intensity [1.0] are for a clear sky. Is there a way to know how much the size parameter should change if the intensity is say halved to 0.5 ?? Should the size be double = 2.0

Is there a physically correct relationship between these two parameters or is it a matter of just doing what looks good/right.??
Thanks
Jamie

2

easy way around that Jamie. Use a cloudy HDR in a VRay dome light.

3

Thanks

Thanks jacksc02

I’ll give that a try!

Would HDR be faster or slower than the VRay Sun/sky setup??

Cheers
Jamie

4

it really depends on the HDR you use and the settings on the VRaylight

5

The VRaySky and VRaySun system is not like the VRayLight object, where the intensity/light output is dependent on the lights dimensions. Increasing the size of the VRaySun will not change it’s light/energy output. This is only done with the Multiplier value.

So theoretically, if the VRaySun’s correct multiplier is 1.0 and the VRaySky multiplier is also 1.0, then combined they produce a certain amount of light (let’s say x) on a clear day.
On an overcast day, if you reduce the Sun’s multiplier to 0.5 then you would need to also increase the VRaySky multiplier to 2.0, in order to conserve the same amount of light in your scene.
Whether this is actually physically correct is doubtful. You would expect that on a cloudy/overcast day, less light would reach the earth’s surface because more light would be dispersed and reflected by the clouds.

An overcast HDR would be the way to go, but you will still need to determine the multiplier for the hdr to make sure it is emiting the correct amount of light.

6

Yes, just look at the difference in exposure of photographs with the same shutter and f-stop value with the only difference being clear or cloudy day.

So, I would say the “correct” way would be that your rendered image will also reflect this.

7

Originally Posted by dejaVu View Post
You would expect that on a cloudy/overcast day, less light would reach the earth’s surface because more light would be dispersed and reflected by the clouds.
Yes, just look at the difference in exposure of photographs with the same shutter and f-stop value with the only difference being clear or cloudy day.

WWX
So, I would say the “correct” way would be that your rendered image will also reflect this.

Thanks for the help guys!
This all seems to make sense to me although I was more interested in the relationship between the sun intensity & the sun size …ie: the area shadows created on a partly cloudy day.
As an analogy it would be like a clear light bulb [a clear day] & a diffuser light bulb [a cloudy day]. A diffuser light bulb will create a larger area for the light source than a clear bulb which is more like a point light. This appears to be the same as you get when the sun is shinning through thin clouds. The sun appears larger and produces softer shadows.
Any ideas on this relationship???
Thanks again

8

Aaaa now i understand your question better…but don’t have an exact answer. Just that the bigger the sun size the softer your shadows will be. For now i would say play around with the size unilt you get what you like or until one of the really knowledgable users reply;)