System and method for lens shading correction of an image sensor using splines
An image sensing system provides for accurate lens shading correction even when there is significant lens shading asymmetry and non-uniformity. A two-dimensional B-spline technique is used to determine lens shading correction surfaces. The number of zones is selected to achieve accurate correction of center, edge, and corner regions of an image. Separate lens shading correction surfaces are calculated for a set of standard illuminants to permit lens shading correction to be adapted based on the illuminant used to capture the image.
1. An image sensing system, comprising:
a lens shading correction module to correct a gain of each pixel of a captured image for optical shading effects, the lens shading correction module storing a two-dimensional correction surface in which an image sensor is partitioned into at least three sections in an x-dimension and at least three sections in a y-dimension to form at least nine different rectangular-shaped zones with each zone having a first piecewise continuous quadratic polynomial correction curve in an x-dimension and a second piecewise continuous quadratic polynomial correction curve in a y-dimension, the number of zones being selected to achieve accurate shading correction of both a center portion of the image and edge portions of the image;
wherein there is a correlation in shading response between different color channels, the two-dimensional correction surface corresponding to a single color channel (C 1 ) having a first number of associated control points and wherein each additional color channel (C n ) has a correction surface represented as a correction surface normalized by the response of the single color channel (C n /C 1 ) and having a second number of associated control points less than said first number.
2. The image sensing system of claim 1 , wherein the two-dimensional correction surface corresponds to a two-dimensional quadratic B-spline having at least five control points in an x-dimension and five control points in a y-dimension to form at least three zones in each of the x and y dimensions.
3. The image sensing system of claim 1 , wherein the lens shading correction module stores shading correction surfaces for a set of at least two standard illuminants with the lens shading correction module adjusting the lens shading correction for the captured image based on a detected illuminant of the captured image.
4. The image sensing system of claim 3 , wherein the detected illuminant is identified by an auto white balance module.
5. The image sensing system of claim 3 , wherein the shading correction surfaces includes a main correction surface to perform shading correction for one standard illuminant and differential values to interpolate at least one other correction surface for other illuminants.
6. The image sensing system of claim 1 , wherein the two-dimensional correction surface corresponds to a two-dimensional quadratic B-spline having at least five control points in the x-dimension and five control points in the y-dimension.
7. The image sensing system of claim 1 , wherein said lens shading correction module is disposed in an image processor receiving pixel data from an image sensor.