Light Falloff

This transformation can apply or correct for brightness variations in the input image that depend only on the distance from the center of the image. Given a sample image of a uniformly illuminated field, it can also compute the necessary curve to correct the image for any light falloff.

This transformation can be used to correct for lens vignetting or to create vignetting effects. It can also be used like the Hotspot transformation to darken the central hot spot that appears in some infra-red images.

Curve

The curve control lets you specify how you want the brightness of the image to vary with the distance from the center of the image. The x-axis represents distance from the center with 0.0 at the far left and the distance to one of the corners of the image at the far right. The y-axis represents the factor by which the image brightness is multiplied with 0.0 at the bottom and 1.0 at the top. The default curve illustrated above has no effect on the image since it multiplies the brightness by 1.0 everywhere. This transformation can only darken the image.

Settings Menu

In addition to the normal menu items, the settings menu has one extra:

Compute Falloff Curve

Selecting this item analyzes the input image and generates a 33-point falloff curve automatically. This command assumes the input image is a photo of a uniformly illuminated field such as a white screen on a monitor. A falloff curve is then generated that corrects the image for light falloff at the edges by darkening the center as necessary. For example, using the Cosine Falloff test pattern as an input:

 Input Image                                                         Falloff Curve                                                         Output Image