IES lights don't work as expected with Vray

[ern]

When I use the IES file in question, the light pattern on the walls looks odd.
My reference material/pictures are from normal scanline and MR. I also use Dialux and Relux.

@Vlado : I have sent you the scene and IES file, if you have the time please have a look at it

ern

[vlado]

I got the file, thanks. In general, V-Ray uses the same IES lights as the scanline renderer and I haven't noticed any differences in their appearance so far, but I'll take a look.

Best regards,
Vlado

[vlado]

Hello Erik,

In your scene you appear to be using a V-Ray camera with its exposure option ON,
which modifies the scene brightness similar to color mapping. If I turn this
option OFF, the scene renders identically with the scanline renderer and V-Ray.

Best regards,
Vlado

[ern]

Hi Vlado

I don't think it's a matter of exposure, I have sent you a mail
with two jpg's showing how the direct light looks with Vray vs. MR.

Vray gives a strange and hard edge like transition from light to shadow.

Regards

Erik

[vlado]

Hello Erik,

I see; this is because of the shadows; mental ray will produce area shadows automatically since this is a photometric area light; whereas V-Ray will not produce area shadows unless you specifically turn this option on in the VRayShadows properties. This happens with the scanline renderer as well unless you use Adv. raytraced shadows.

To produce the area shadows with V-Ray, check the "Area shadow" option, choose the "Box" type and set the dimensions to be the same as in the "Area light parameters" rollout (you can set the W size to 0.0 for a planar light).

It's probably possible for this to be done automatically, like for the Adv. raytraced shadows and I will look into it.

Best regards,
Vlado

[ern]

"Thanks. Vlado.. you are right, by the way I can see that Vray supports the "Area light sampling" parameter what is the difference between using that vs. turning on area shadows..."


Regards

Erik

[ern]

@Vlado... I'm sorry to say that changing to Vray Area shadows won't fix the problem, if I want to get "correct" direct lighting from IES linear/area lights I need to set the standard max setting "enable area sampling" well above 512 samples, and that will not correct the problem in all cases, but you will get huge render times when rendering GI...

I can mail you some renderings, to show what I'm talking about


Kind Regards
ern

[vlado]

It worked for me... I got identical renders in the end, but like I said, I had to adjust the VRayShadows (in particular, the U/V size).

Best regards,
Vlado

[ern]

@ Vlado... could you email me your file, if you still got it?
I have tried different files, and it seems that Vray dont like ies files measured with 5deg angels...


Kind Regards

ern

[vlado]

Yep, I will send you the file. However, like I said before, V-Ray has nothing much to do with the IES profiles themselves, only with the shadows. The actual IES lights are rendered by the same code that renders them in the scanline renderer.

Best regards,
Vlado