camera exposure

[grenouille]

Hi!

 

I usually work with Max and Vray and both have a camera exposure options. 

I can change ISO, fstops,... and I can use them like a real camera parameters.

 

But when I try to use these knowledges inside Houdini they don't work in the same way, or not at all. (for example, F-stop determines blurriness of depth of field effects, but not the aperture of diaphragm and the level of the brightness).

 

http://www.sidefx.com/docs/houdini13.0/nodes/obj/cam

 

How do you make correctly exposed renders using Mantra? 

or saying in other words, what should I do to match them with a real footage brightness without changing every light in the scene? 

 

Thanks!

[Guest tar]

You can render to a float format like exr and change the exposure in post.

[grenouille]

Thanks Marty, 

but I want to avoid changing the exposure in post and use it only for refining.

 

Change the sensibility of camera is a very useful tool and I can't believe that Mantra doesn't have it.

Furthermore when we can find tools like these in Maxwell for Houdini:

 

http://support.nextlimit.com/pages/viewpage.action?pageId=5079881

 

 

[Guest tar]

It could be useful though with PBR how many lights do you have to adjust in your scene? You can link up the intensity controls pretty easily I imagine.

[symek]

 

Thanks Marty, 

but I want to avoid changing the exposure in post and use it only for refining.

 

Change the sensibility of camera is a very useful tool and I can't believe that Mantra doesn't have it.

Furthermore when we can find tools like these in Maxwell for Houdini:

 

http://support.nextlimit.com/pages/viewpage.action?pageId=5079881

 

 

 

This is simply redundant in a pipelines Mantra is usually used, but I see your point. You can install on camera "White Point" property (Gear icon->Edit Rendering Properties). It usually exists on Mantra ROP, and is set to 1.0. It's not in exposure metrics, and it won't give you a physical camera model (with vignetting and such), but at least allows you to control camera sensitivity.  Alternatively atmosphere object can be used as well.

 

hth,

skk.

 

ps You can link white point with f-stop parameter with the expression like 1/pow(sqrt(2), ch("fstop")) (or whatever the math should be)*.

 

* - I'm a last person to ask, but without shutter and ISO this is rather meaningless, I would assume that a diffuse with reflectivity of 0.18 at default 5.6 f-stop, should give us ~0.4 pixel value on grid facing camera, at a given lighting condition and ISO, right?

Edited March 7, 2014 by symek

[jrockstad]

 

Thanks Marty, 

but I want to avoid changing the exposure in post and use it only for refining.

 

Change the sensibility of camera is a very useful tool and I can't believe that Mantra doesn't have it.

Furthermore when we can find tools like these in Maxwell for Houdini:

 

http://support.nextlimit.com/pages/viewpage.action?pageId=5079881

 

 

 

Interesting that you bring up Maxwell, as I was going to suggest it as something you might look into. I haven't played around with the Houdini plug-in much yet, but I've been using it a lot in Maya lately and enjoying it quite a bit. Of course I understand that purchasing a license for a new rendering engine probably isn't your ideal solution here, but it is something to consider if you enjoy that more photographic approach to lighting and rendering.

[jon3de]

The "White Point" method is pretty nice. Thank you very much Szymon.

"This is simply redundant in a pipelines Mantra is usually used" Can you explain that further? Changing the exposure globally seems like a useful thing to me. In any pipeline. 

[Hazoc]

I think he means it's usually the director of photography/compositor/director who defines camera details and this is done in post to ensure consistent look between different shots. But I agree with you having them available would be nice.