Issue: Light value wrong scaled by scaling the light

[Micha]

I rendered a train interior with a long rectangular light over the whole ceiling of the car. The lighting setup was perfect. Then I had to attach a monitor to the ceiling and I had to cut the light in half lengthwise and place it in front of/behind the ceiling monitor. The light setup was now far too bright.

Please disable the automatic scaling of the light value or allow the user to switch off the automatic change. It does not work.

Interestingly, the light value is only “scaled” when you scale the light in the viewport, but if you adjust the u size value in the light parameters, the light value is retained and everything looks fine. Attached my test scene.

Best-
Micha

[iva_mancheva]

Hello Micha,

Thank you for the feedback.

The way it is currently done is in order to have consistency between V-Ray and Rhino rectangular lights, because the Rhino Rectangle light's intensity is a function of its area. When you scale the V-Ray Rectangle light in the viewport -> its intensity in gets adjusted in V-Ray. When you change its U and V Size in Asset Editor -> its intensity gets adjusted in Rhino (in Rhino's lights panel).
Keeping a consistent intensity between Rhino and V-Ray helps for achieving similar results between Rhino Rendered view and V-Ray and also Enscape and V-Ray.

Kind regards,
Iva

[Micha]

iva_mancheva

I did a few more tests and we seem to have several problems here.

(a) If I enlarge the light with _Scale1D, the size in the V-Ray light parameters remains constant. However, if I grab the light by a handle in the viewport and enlarge it, the size of the V-Ray parameters is adjusted. It's a bug.

(b) If I have a light surface with 1000W and illuminate a table and halve its light surface and then hang it twice over the table so that the same surface is illuminated again, then I do not need two surfaces with 2000W (together 4000W) but 2x500W. The power per light source does not have to be doubled but halved so that the total power remains constant.

(c) In practice, however, it is not the power that is used but the luminosity. If I place a luminous surface over a table, halve it and hang it up twice next to each other, then the luminosity must remain constant.

Please deactivate the scaling of the light value or at least allow it to be deactivated at VfR Configuration , because it only causes trouble.

[javierrentas]

I agree with Micha.
The new light behavior does not make sense. I don't mind if you have the scale behavior on the default unit, but when I'm using Lumens or Watts I would like to keep the same power no matter the size of the light.
If I have a 10" flashlight that produces 1000 lumens and another 2" flashlight that produces 1000 lumens of light, I should have the same illumination no matter the size of the flashlight.
Please bring the old behavior to the lights when using watts or lumens. used.
It is very annoying and unpredictable that when you scale the light the intensity changes.

[javierrentas]

Konstantin, Please, please, fix this issue. I do a lot of interior cabin renderings, and I have several lights for the ceiling ( I use Watts). When I need to make one of those instances of light smaller, I can see that the intensity of that light is different.
I also use a lot of mesh lights for the seat controller. They are illuminated icons, and because of the size issue, all the icons have different intensity, even though I'm using the same mesh light for all of them. Therefore, I get a different Lens effect on the buttons. Some of them do not even have the lens effect, when others have it.
Please, this is very critical to me. Remove that behavior when we are using Watts or Lummens.



Radiant Power (W) – Total emitted visible light power measured in watts. The intensity of the light does not depend on its size. This is not the same as the electric power consumed by a light bulb. A typical 100W light bulb only emits between 2 and 3 watts as visible light.

Luminous Power (Lumens) – Total emitted visible light power measured in lumens. The intensity of the light will not depend on its size. A typical 100W incandescent light bulb emits about 1500 lumens of light.



Please, do not update the documentation, give us the correct behavior back

[konstantin_chaos]

Hi javierrentas

We certainly have not changed the behavior of the area light sources intentionally.
Can you describe more precisely what has changed?

Your render of the spheres example looks correct to me.
W - Ensures that the total light emitted from a surface is consistent independent of the surface size. This means that when you make the light source surface bigger, the light intensity of each point on the surface will automatically decrease.

Konstantin

[konstantin_chaos]

Hi Micha,

I rendered a train interior with a long rectangular light over the whole ceiling of the car. The lighting setup was perfect. Then I had to attach a monitor to the ceiling and I had to cut the light in half lengthwise and place it in front of/behind the ceiling monitor. The light setup was now far too bright.

Please disable the automatic scaling of the light value or allow the user to switch off the automatic change. It does not work.

Interestingly, the light value is only “scaled” when you scale the light in the viewport, but if you adjust the u size value in the light parameters, the light value is retained and everything looks fine. Attached my test scene.

Are you referring to the fact that when a viewport light resize is performed, the intensity in the UI also changes?
If that is what you have reported - we'll investigate and fix it.

Otherwise I've tested your setup and everything seems to work as expected:



In my example I've used two separate light sources with identical intensities and different size.
The same will work if you have a single light scaled differently.

If you switch the units to total Lumens or Watts, you'll have to decrease the intensity of the 2 lights to half the intensity of the long one.

[javierrentas]

Koytse
I do not understand why you will need to change the intensity of the lights if you have Lumens or Watts. The intensity should not depend on the size.
Can you check my example above? That is a wrong behavior of the light. It doesn't matter the size, the illumination should be the same when we are using lumens and watts.

[konstantin_chaos]

Koytse
I do not understand why you will need to change the intensity of the lights if you have Lumens or Watts. The intensity should not depend on the size.
Can you check my example above? That is a wrong behavior of the light. It doesn't matter the size, the illumination should be the same when we are using lumens and watts.

The Lumens or Watts modes ensure that the total light emission from the entire light surface equals the value you have specified. To keep that true when the size of the light surface becomes bigger, the intensity of each point on the surface decreases.
The other modes do the opposite - they ensure that the light emitted from a light surface area with fixed size equals your value. In this case making the light bigger will keep the intensity of each point on the surface while making the total light contribution from the light stronger.

So you basically have the two options depending on what you're after.
Am I missing something here?

Konstantin

[javierrentas]

I think you have this concept inverted. When we use real-live units, the lights should keep the same intensity no matter the size of the light. You could even read this in the manual. The intensity of the light does not depend on its size. It doesn't matter the size of the light, the intensity will still be the same.
When you use the default unit, changing the size of the light will affect its intensity. Right now we have the opposite. If this is not going to be fixed. I guess having real-life units does not make a lot of sense. I guess I need to use Default units since now on, because it is acting as watts, and lumens should act.