What's a "gray ball pass" in making HDR panoramas?

monkeyscience

I'm working on processing HDR panoramas into lighting and I'm trying to find out what exactly a "gray ball pass" is good for and how it works. I know they use it in movies, you take an HDR photo of both a chrome ball and a 50% matte gray ball and somehow fix and normalize extreme lighting conditions, like direct sunlight. I need the scientz tho. Anyone know how it works or have any good links or papers?

Panupat

The grey ball serves as lighting reference. It shows the light color and direction.

Eric Chadwick

Nick says basically the same thing, but in more detail.
http://www.max-realms.com/modules/tutorials/guide_to_hdri_lighting.php


Some related info from HDR Shop...
[ QUOTE ]
...mirrored spheres are never 100% specular, and you should compute and adjust for the percent reflectivity of any sphere used for measuring incident illumination. This can be done by placing the sphere near white or gray diffuse surface in such a way that that the surface is visible directly and is also visible as a reflection in the sphere. Photograph this with a camera for which a response curve is available, and then divide the pixel value of the reflection by the pixel value of the surface to obtain the reflectivity in the red, gree, and blue channels. Then, divide the pixel values of any image taken of the sphere by these numbers to have a probe image that is calibrated with repsect to images (such as background plates) taken directly by the camera. Typical metal sphere reflectivities are in the range of fifty to sixty percent.

You should take care to clean the ball of dust and grease before using, and to handle it with a white cotton glove such as those sold at photography stores to handle photographic negatives. If a ball bearing gets wet, it will rust, so make sure not to leave it outside overnight.

[/ QUOTE ]