Exposure discrepency in VRay Camera

[Dynedain]

I wanted to play around a bit with compositing using the new features in 1.5

I took this photo:

Camera settings:
focal length: 37mm
shutter speed: 1/10 sec
ISO speed: ISO-400
F-stop: 5
Taken outside my office at 7:20pm (daylight savings time)

So I setup a scene to match, using a VRay Sun and Sky (set to the right location by attaching to the Max daylight system and turning off the Max lights).

I also placed a VRay camera and set the values to match my real camera. But something is way off:



I have to take the shutter speed all the way down to 1/100 (instead of 1/10) in order to get a useable result:



Why is there such a discrepency? Why must I set the VRay camera to 1/100 instead of 1/10?

[themaxxer]

have you checked the 'exposure' parameter in vraycam?

best regards
themaxxer

[Da_elf]

was your shot done on a totally bright and sunny day?

[Dynedain]

Exposure is on in the VRay Camera, that's why 1/100 sec produces different results than 1/10 of a second. The Vray Sun and Sky are using their default settings.

The shot was taken on a clear day, around sunset.

The point of this is to figure out a workflow that will work well for compositing our architectural work. I'm trying to find out why such a discrepency would occur and how to predict it so we can adjust accordingly.

Color mapping is linear 1.0/1.0 because exposure should be handled properly by the camera (in theory).

[jacksc02]

anychance you could re-link the images? they seem to be down

[dapeter]

Here's a test I did for someone else:

http://www.chaosgroup.com/forum/phpB...295&highlight=

I'll take a look at your scene when I get a chance... we'd also need to know which direction is north. What camera are you using, also? And is there any tree canopy that would block some of the sky?

Here's my test, linear workflow, gamma correction mult=1, inverse gamma=.454, gamma=1. I agree, the exposure is way off set as you stated (unless the LA air is really smoggy that day), in the test below I set the ISO to 100...



Material is the architectural material concrete cast in place flat gray 1.

[Dynedain]

dapeter, check your pm for the file.

The camera is a Cannon EOS Digital Rebel XT

Site north is setup with a compass in the max file. The only tree stuff that would block things is the bamboo you can see on the right, and I have an object in the scene to account for it already.

Like I said, I can get useable results, but I'd like to nail down why there is so much difference in the exposure settings so I can predict things.

This is with a shutter speed of 1/100sec (instead of the 1/10 the camera captured). Useable, but not what I was expecting.

[dlparisi]

Off topic (sorry): How are you getting the shadows on the ground in your image? Yeah, yeah I know - set vray properties to matte, -1, affect shadows, alpha, etc. I can't seem to get it to work correctly with the physical camera though (normal camera is fine except for the exposure). Everytime I try it I get a black rectangle where the matte plane is (the alpha looks correct however), meaning I'd actually have to composite it to preview how it looks. Is this a bug with the physical camera or am I missing something?

David

[Dynedain]

Off topic (sorry): How are you getting the shadows on the ground in your image? Yeah, yeah I know - set vray properties to matte, -1, affect shadows, alpha, etc. I can't seem to get it to work correctly with the physical camera though (normal camera is fine except for the exposure). Everytime I try it I get a black rectangle where the matte plane is (the alpha looks correct however), meaning I'd actually have to composite it to preview how it looks. Is this a bug with the physical camera or am I missing something?

David

Make sure you are using a black background, and uncheck pre-multiply alpha when saving.

Other than that, I'm doing exactly as you described (vray properties).

[dlparisi]

So you are compositing it as a post process? Shouldn't the background show through the matte plane except receive shadows and affect the alpha in shadow areas?

[Dynedain]

So you are compositing it as a post process? Shouldn't the background show through the matte plane except receive shadows and affect the alpha in shadow areas?

Yes... but because I used a black background, and saved with premultiply turned off, I can composite onto any background plate I want.

The background plate is never in my Max file.

[dlparisi]

Yes... but because I used a black background, and saved with premultiply turned off, I can composite onto any background plate I want.

The background plate is never in my Max file.

I understand why you'd want to composite as a post process for the final output and not have it as part of the max file but my question is more concerned with test renders which are a lot easier if you render over the background image. Like I said, if you use the normal max camera (and change the sunlight level down to like .01) the matte plane renders fine over the background. If you use a phys. camera and uncheck "exposure" AND "Vignette" it will render correctly though.

[jacksc02]

how come your using an architectural material?

[Neilg]

how come your using an architectural material?

I think he means the texture included with the architectural materials pack.

[jacksc02]

ah right.