Advanced Camera Lens Modeling

January 2006

1 Introduction

Most computer graphics is done with an extremely simple camera model, as though we were using a pinhole camera with an instantaneous exposure (and infinitely fast film!) Renderman's depth of field and motion blur capabilities allow for a relatively sophisticated camera model, one that models the finite aperture and exposure time of real cameras. Still, the Renderman camera model is, by comparison with real optical systems, fairly simple. The lens aperture must be circular, illumination of the film plane is constant across the surface of the lens and the shutter opens and closes instantaneously.

With release 13, Renderman's camera model is enhanced to provide more control of the lens aperture shape and radial falloff, and to allow the shutter to take a finite amount of time to open and close.

2 The Lens Model

The lens shape is controlled by a new hider option:

    Hider "hidden" "aperture" [nsides angle roundness density]
The option is backwards compatible -- if you leave it out, you get the old behavior. The option's parameters are given in this table:

Parameter Name Meaning
nsides The number of sides of the aperture. For backward compatibility, if this value is less than 3, prman reverts to its previous behavior, using a circular aperture with exactly the legacy stochastic sampling pattern.
angle The polygon's orientation, in degrees from some arbitrary reference direction.
roundness A shape parameter. When 0, the aperture is a regular polygon with straight sides. When 1, it's a perfect circle. Intermediate values give polygons with curved edges bowed out. It can be negative, in which case the edges bow in. Legal values are from -1 to 1.
density The density (brightness) of the aperture need not be constant. This parameter sets the slope of the (linearly varying) density. If it's zero, the density is constant. If it's negative (resp. positive) the aperture is brighter near the center (resp. near the rim.) Again, the range of legal values is -1 to 1.

**Examples to come**

3 The Shutter Model

Shutter opening and closing times are controlled by another new hider option

    Hider "hidden" "shutteropening" [opentime closetime]
The option's parameters are measured in fractions of the shutter time (as set by the Shutter RIB command.) The shutter begins opening at time 0. At opentime it will be open to full aperture. At time closetime, it will begin closing. And at time 1 it will be fully closed. Specifying
    Hider "hidden" "shutteropening" [.25 .75]
has the shutter gradually open for the first quarter of the exposure, and gradually close for the last quarter.

Backwards compatibility with the pre-release 13 behavior can be had by omitting the option altogether, or by specifying

    Hider "hidden" "shutteropening" [0 1]

**Examples to come**


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