3 - Specifying a View

3.1 Camera Position
3.2 Field of View
3.3 Depth of Field
3.4 Stereo Rendering

When designing a rayshade input file, there are two main issues that must be considered. The first and more complex is the selection of the objects to be rendered and the appearances they should be assigned. The second and usually easier issue is the choice of viewing parameters. This chapter deals with the latter problem; the majority of the following chapters discuss aspects of objects and their appearances.

Rayshade uses a camera model to describe the geometric relationship between the objects to be rendered and the image that is produced. This relationship describes a perspective projection from world space onto the image plane.

The geometry of the perspective projection may be thought of as an infinite pyramid, known as the viewing frustum. The apex of the frustum is defined by the camera's position, and the main axis of the frustum by a "look" vector. The four sides of the pyramid are differentiated by their relationship to a reference "up" vector from the camera's position.

The image ultimately produced by rayshade may then be thought of as the projection of the objects closest to the eye onto a rectangular screen formed by the intersection of the pyramid with a plane orthogonal to the pyramid's axis. The overall shape of the frustum (the lengths of the top and bottom sides compared to left and right) is described by the horizontal and vertical fields of view.

3.1 Camera Position

The three basic camera properties are its position, the direction in which it is pointing, and its orientation. The keywords for specifying these values are described below. The default values are designed to provide a reasonable view of a sphere of radius 2 located at origin. If these default values are used, the origin is projected onto the center of the image plane, with the world x axis running left-to-right, the z axis bottom-to-top, and the y axis going "into" the screen.

eyep pos»: Place the virtual camera at the given position. The default camera position is (0, -8, 0).
lookp pos»: Point the virtual camera toward the given position. The default look point is the origin (0, 0, 0). The look point and camera position must not be coincident.
up direction»: The "up" vector from the camera point is set to the given direction. This up vector need not be orthogonal to the view vector, nor need it be normalized. The default up direction is (0, 0, 1).

3.2 Field of View

Another important choice to be made is that of the field of view of the camera. The size of this field describes the angles between the left and right sides and top and bottom sides of the frustum.

fov hfov [vfov]

3.3 Depth of Field

Under many circumstances, it is desirable to render objects in the image such that they are in sharp focus on the image plane. This is achieved by using the default "pinhole" camera. In this mode, the camera's aperture is a single point, and all light rays are focused on the image plane.

Alternatively, one may widen the aperture in order to simulate depth of field. In this case, rays are cast from various places on the aperture disk towards a point whose distance from the camera is equal to the focus distance. Objects that lay in the focal plane will be in sharp focus. The farther an object is from the image plane, the more out-of-focus it will appear to be. A wider aperture will lead to a greater blurring of objects that do not lay in the focal plane. When using a non-zero aperture radius, it is best to use jittered sampling in order to reduce aliasing.

aperture radius: Use an aperture with the given radius. The default radius is zero, resulting in a pinhole camera model.
focaldist distance: Set the focal plane to be distance units from the camera. By default, the focal distance is equal to the distance from the camera to the look point.

3.4 Stereo Rendering

Producing a stereo pair is a relatively simple process; rather than simply rendering a single image, one creates two related images which may then be viewed on a stereo monitor, in a stereo slide viewer, or by using colored glasses and an appropriate display filter.

Rayshade facilitates the rendering of stereo pairs by allowing you to specify the distance between the camera positions used in creating the two images. The camera position given in the rayshade input file defines the midpoint between the two camera positions used to generate the images. Generally, the remainder of the viewing parameters are kept constant.

eyesep separation

There are several things to keep in mind when generating stereo pairs. Firstly, those objects that lie in from of the focal plane will appear to protrude from the screen when viewed in stereo, while objects farther than the focal plane will recede into the screen. As it is usually easier to look at stereo images that recede into the screen, you will usually want to place the look point closer to the camera than the object of primary interest.

The degree of stereo effect is a function of the camera separation and the distance from the camera to the look point. Too large a separation will result in a hyperstereo effect that will be hard to resolve, while too little a value will result in no stereo effect at all. A separation equal to one tenth the distance from the camera to the look point is often a good choice.