Light falloff question

[sharktacos]

So here's a basic lighting theory question (the following assumes I am using linear lighting setup and GI is on):

example #1: I place a rectangular light in a window frame in order to simulate the sky spilling into a room. When the light is bright enough on the floor and wall, because of the proximity of the light, the window frame itself will often get blown.

example #2: I have a hanging lamp from the ceiling. I place a sphere light in a lamp shade where the light bulb would be. Again, when the light is bright enough to illuminate the room the way a lamp would, the lamp shade itself is completely blown out.

Is the render behaving in a physically inaccurate way, or am I missing some vital step here?

[vlado]

This is exactly accurate, yes. As an example, try taking a photo in these conditions.

With that said, you can apply tone mapping to the resulting image to even out the apparent brightness of the different areas.

Best regards,
Vlado

[sharktacos]

Ah, right. I was looking with my eyes. When I take a picture of a lamp it does look the same as the render. Thanks for pointing that out! I'll try the tone mapping for that. Would you suggest something like Reinhard or HSV Exponential for this?

Now of course that does not happen with the sky on a window frame because the sky is not really a rectangular light placed there of course. So is there a better way to place a rectangular light so as to avoid this effect when trying to emulate sky coming into a room? I suppose I could create an HDR sky sphere object outside and make the rectangular light a portal. Would that be the correct approach? is there a more straight forward way to get the desired behavior (not having a blown out window frame)?

[svetlozar.draganov]

Hello sharktacos

We are very sorry about our very late reply.
If you still have this issue , please attach a simple scene and we will try to help you with it.

Thank you very much in advance.

[joconnell]

is there a more straight forward way to get the desired behavior (not having a blown out window frame)?

There is no other way to do it aside from putting in a different sky in post. The biggest thing is how large the windows on your scene are. If you've got a small room with a tiny window, then the walls are blocking most of the light that's outside from getting in. If you've got huge windows in your scene, there's less light being blocked and the difference in "volume" of light is less between inside and out. If the light is quite dark then you get an issue with balancing your exposure - if you set your camera open enough so that the room appears in "normal" brightness then you're exposing for the volume of light inside. If the volume of light outside is 5 or 10 times more, then the outside will appear totally overblown. It's a natural thing and you'd expect to see it in a photo. Vray will make the light behave as it would in real life, so if you've small windows that means turning up your light outside hugely to fill the volume of your room. You'll have to have a second image as the background outside the windows or do a post thing to fake it, but it might look unnatural if you get a balanced exposure of inside and outside at the same time!

[Stehn]

You need to get the tone mapping right. Either within Vray with color mapping controls - HSV expotential is great, either do it in post. You can render 32 bit images and tone map them with Photoshop or any other HDRI tone mapping capable software (Photomatix). You can also cheat like they do on movie sets, use lots of additional lights to even the lighting troughout the scene.