PDA

View Full Version : HDRI capture technique


ossosso
07-07-2009, 11:24 AM
Hi!

I've bought a 3" Polished Chrome Steel sphere, (it have to arrive yet).

I'd like to use it to create my own HDRI image and use these last to illuminate 3d scenes.

Well..
In the meantime the sphere arrives, I'm looking for a tutorial which could explain me how to capture an HDRI image.

I know yet I've to put the sphere on a trivet and make different shoots with my camera to the sphere at different exposures.

But in depth:
- how many shoots?
- which distance the camera to the sphere?
- which Focal Length?
- cmaera and sphere at the same height?

If some of you could link me towards documentation/tutorial of it, or could give me some advice I'll appreciate a lot.

Thank you very much!

SaphireS
07-07-2009, 11:35 AM
Yes, camera and sphere should be at the same height.
There are several photoshop tutorials on how to create HDRIs, they usually use 6-7 different shoots.
Try to get far away from the sphere and zoom in as far as possible, to reduce your own reflection on the sphere.

ossosso
07-07-2009, 02:04 PM
thank you for your advices

cbamber85
07-07-2009, 04:40 PM
http://gl.ict.usc.edu/HDRShop/

How about a tutorial from the man himself Paul Debevec? This is all centred around using HDRShop, don't worry - it's free!

noouch
07-07-2009, 06:02 PM
- how many shoots?

This depends on your scene. Normally you should shoot enough images to capture some shadow detail, the non-clipped highlights of the scene, and enough images to reliably capture the range of light. I find starting at 2 stops over what the camera would normally expose to, and increase by 2 stops progressively until I have a shot in which the brightest lights are exposed to grey works quite well. You can increase the step if it's a very high range scene or time is short, but above a certain level (normally 5 stops difference or more) you're prone to introduce error and noise to your HDRI that way.

- which distance the camera to the sphere?
- which Focal Length?
- cmaera and sphere at the same height?

You should select the longest focal length available. It's best to have a tele (>100mm) lens, since you minimize the size of the camera, tripod, etc. on the sphere, and you have less perspective distortion of the sphere. The sphere should be framed so it takes up the entire image at maximum focal length (don't cut off the sides though).
Generally the camera and sphere should be at the same height, but it's not absolutely necessary, as you can rotate the HDRI in your 3D package.

ossosso
07-07-2009, 07:59 PM
Thank you very much for your precious advices.

Kinematics
10-20-2009, 04:18 PM
Hello there,

Did you guys manage to succesfully take two images of the mirrorball at the 90 degree angles and combine them nicely together without misalignment issues? I always seem to be having issues with that.

cbamber85
10-20-2009, 08:29 PM
I always take a metre stick and a protractor with me, and I do the whole procedure a few times so I've definitely got a good one.

Kinematics
10-21-2009, 04:16 AM
Alignment issue Issue (http://www.twitpic.com/kozzd)

Do take a look at this guys, why does this happen? I tried multiple times already. I am pretty close to the camera because I am using a 70mm but its on a crop factor camera so it probably gives me about 105mm. The ball is much bigger now but still, they don't align.

Am I missing a step in HDRshop? Do you guys save the cropped files and reopen them before selecting points? And do you wrap image A first then select points and then take down the coordinates? Or before you wrap it....take down the coordinates then hit wrap which apparently changes the coordinates?

noouch
10-21-2009, 08:18 AM
You probably have some sort of parallax shift going on somewhere in the process. For the most part, this isn't a bad problem and your HDRI will still do what you want it to.

Make sure that the camera is at the exact same distance and pointing precisely at the mirror ball. You probably won't get it perfectly aligned, but it helps to eliminate as many sources for parallax shift as you can.

ossosso
10-21-2009, 09:03 AM
once I've taken all shoots I need I use Photoshop or Photomatix to merge them together into an unique hdri file; both of the softwares have an automatic tool to align all 5 shoots together before merge them.

Once I have to hdri images (one at 0 degree and the other at 90 degrees) I pass to hdrshop to create a lightprobe.

Kinematics
10-21-2009, 10:55 AM
You probably have some sort of parallax shift going on somewhere in the process. For the most part, this isn't a bad problem and your HDRI will still do what you want it to.

Make sure that the camera is at the exact same distance and pointing precisely at the mirror ball. You probably won't get it perfectly aligned, but it helps to eliminate as many sources for parallax shift as you can.

Thats what I thought. The HDRI does work but I'd like a pretty close to perfect HDRI mirrorball image that I managed to clone myself out of. I hate the idea that it never lines up.

I will try again. maybe this time on a table to really check for this. Do you know the room for error here? I am working with tripod stands and stuff. Its a little bit of a clunky workflow but it should work.

once I've taken all shoots I need I use Photoshop or Photomatix to merge them together into an unique hdri file; both of the softwares have an automatic tool to align all 5 shoots together before merge them.

Once I have to hdri images (one at 0 degree and the other at 90 degrees) I pass to hdrshop to create a lightprobe.

I use Sofortbild (http://www.sofortbildapp.com/) which is great for the HDRI stuff though after making the .HDRI file I have to save them as a floating point tiff in photoshop before I can import them into HDRshop and actually see them for some reason. But I get a bunch of errors just before the image pops up. Do you guys get these issues?

CGTalk Moderation
10-21-2009, 10:55 AM
This thread has been automatically closed as it remained inactive for 12 months. If you wish to continue the discussion, please create a new thread in the appropriate forum.