OpenGL Programming Guide

Chapter 3Viewing

Projection Transformations

Perspective Projection

The most unmistakable characteristic of perspective projection is foreshortening: the farther an object is from the camera, the smaller it appears in the final image. This occurs because the viewing volume for a perspective projection is a frustum of a pyramid (a truncated pyramid whose top has been cut off by a plane parallel to its base). Objects that fall within the viewing volume are projected toward the apex of the pyramid, where the camera or viewpoint is. Objects that are closer to the viewpoint appear larger because they occupy a proportionally larger amount of the viewing volume than those that are farther away, in the larger part of the frustum. This method of projection is commonly used for animation, visual simulation, and any other applications that strive for some degree of realism because it's similar to how our eye (or a camera) works.

The command to define a frustum, glFrustum(), calculates a matrix that accomplishes perspective projection and multiplies the current projection matrix (typically the identity matrix) by it. Recall that the viewing volume is used to clip objects that lie outside of it; the four sides of the frustum, its top, and its base correspond to the six clipping planes of the viewing volume, as shown in Figure 3-11 . Objects or parts of objects outside these planes are clipped from the final image. Note that glFrustum() doesn't require you to define a symmetric viewing volume.

Figure 3-11 : The Perspective Viewing Volume Specified by glFrustum()

void glFrustum(GLdouble left, GLdouble right, GLdouble bottom, GLdouble top, GLdouble near, GLdouble far);

Creates a matrix for a perspective-view frustum and multiplies the current matrix by it. The frustum's viewing volume is defined by the parameters: (left, bottom, -near) and (right, top, -near) specify the (x, y, z) coordinates of the lower left and upper right corners of the near clipping plane; near and far give the distances from the viewpoint to the near and far clipping planes. They should always be positive.

The frustum has a default orientation in three-dimensional space. You can perform rotations or translations on the projection matrix to alter this orientation, but this is tricky and nearly always avoidable.

Advanced

Also, the frustum doesn't have to be symmetrical, and its axis isn't necessarily aligned with the z-axis. For example, you can use glFrustum() to draw a picture as if you were looking through a rectangular window of a house, where the window was above and to the right of you. Photographers use such a viewing volume to create false perspectives. You might use it to have the hardware calculate images at much higher than normal resolutions, perhaps for use on a printer. For example, if you want an image that has twice the resolution of your screen, draw the same picture four times, each time using the frustum to cover the entire screen with one-quarter of the image. After each quarter of the image is rendered, you can read the pixels back to collect the data for the higher-resolution image. (See Chapter 8 for more information about reading pixel data.)

Although it's easy to understand conceptually, glFrustum() isn't intuitive to use. Instead, you might try the Utility Library routine gluPerspective(). This routine creates a viewing volume of the same shape as glFrustum() does, but you specify it in a different way. Rather than specifying corners of the near clipping plane, you specify the angle of the field of view in the x-z plane and the aspect ratio of the width to height (x/y). (For a square portion of the screen, the aspect ratio is 1.0.) These two parameters are enough to determine an untruncated pyramid along the line of sight, as shown in Figure 3-12 . You also specify the distance between the viewpoint and the near and far clipping planes, thereby truncating the pyramid. Note that gluPerspective() is limited to creating frustums that are symmetric in both the x- and y-axes along the line of sight, but this is usually what you want.

Figure 3-12 : The Perspective Viewing Volume Specified by gluPerspective()

void gluPerspective(GLdouble fovy, GLdouble aspect, GLdouble zNear, GLdouble zFar);

Creates a matrix for a symmetric perspective-view frustum and multiplies the current matrix by it. The fovy argument is the angle of the field of view in the x-z plane; its value must be in the range [0.0,180.0]. The aspect ratio is the width of the frustum divided by its height. The zNear and zFar values are the distances between the viewpoint and the clipping planes, along the negative z-axis. They should always be positive.

Just as with glFrustum(), you can apply rotations or translations to change the default orientation of the viewing volume created by gluPerspective(). With no such transformations, the viewpoint remains at the origin, and the line of sight points down the negative z-axis.

With gluPerspective(), you need to pick appropriate values for the field of view, or the image may look distorted. For example, suppose you're drawing to the entire screen, which happens to be 11 inches high. If you choose a field of view of 90 degrees, your eye has to be about 7.8 inches from the screen for the image to appear undistorted. (This is the distance that makes the screen subtend 90 degrees.) If your eye is farther from the screen, as it usually is, the perspective doesn't look right. If your drawing area occupies less than the full screen, your eye has to be even closer. To get a perfect field of view, figure out how far your eye normally is from the screen and how big the window is, and calculate the angle the window subtends at that size and distance. It's probably smaller than you would guess. Another way to think about it is that a 94-degree field of view with a 35-millimeter camera requires a 20-millimeter lens, which is a very wide-angle lens. "Troubleshooting Transformations," gives more details on how to calculate the desired field of view.

The preceding paragraph mentions inches and millimeters - do these really have anything to do with OpenGL? The answer is, in a word, no. The projection and other transformations are inherently unitless. If you want to think of the near and far clipping planes as located at 1.0 and 20.0 meters, inches, kilometers, or leagues, it's up to you. The only rule is that you have to use a consistent unit of measurement. Then the resulting image is drawn to scale.

OpenGL Programming Guide