Optical modules are generally designed to be positioned orthogonal to the projection surface. When a projector is tilted relative to the projection surface, or vice versa, the image will become trapezoidal in shape due to a change in the distance between the projection lens and the edges of the projected image. To correct for this effect, some projection systems include a feature called keystone correction, which manipulates the image in software to fit it into a rectangular subset of the projected pixels (see Figure 5-1).

Keystone correction can be manually adjusted or automatic with an accelerometer which measures the tilt angle of the system. Vertical keystone correction is most common, but some projection systems also offer horizontal keystone correction. Note that keystone correction is a software feature, not a hardware feature. The DLPC343x controller has vertical keystone correction functionality for most DMDs, however for the DLP4710 chipset, keystone correction must be implemented external to the DLP controller. Consult the controller datasheet for more detailed information about how to implement keystone in your product.

Note that a shorter throw ratio optical module will result in a more distorted image when the product is tilted relative to the projection surface.

Figure 5-1 Keystone Correction