Method and apparatus for automatic focusing in an image capture system using symmetric FIR filters

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an automatic focusing method and apparatus developed for digital image capture systems such as digital imaging and video cameras (to be generally referred to herein as "camera").
During the focusing phase, a sequence of images at different focal lengths is captured.
After being captured, each image is filtered by a symmetric finite impulse response (FIR) filter.
A focus value, which indicates the level of camera focus, is derived from the filtered image.
An FIR filter is adopted by the algorithm as it may be implemented by a fixed function digital signal processing (FFDSP) hardware with smaller computation costs and also avoid the error accumulation problem normally seen in infinite impulse response (IIR) filters.
Furthermore, using a symmetric filter can reduce the number of multiplications for filtering the image roughly by half, as compared to an asymmetric filter.
In one embodiment, the focal distance at which the captured image has the largest focus value is considered the optimal focal distance for the scene and is output by the algorithm.
In one embodiment, a processor and a memory are used to process the images to extract focus values for determining when the image capturing system is in focus.
As mentioned above, the processor may be a digital signal processor or an application specific integrated circuit (ASIC).
The processor may also be a general purpose processor.
The memory may be any storage device suitable for access by the processor.
FIG.
1 is a diagram illustrating an image 100 with a width X and a height Y.
Image 100 is composed of a set of pixels, each overlaid with a color filter from a color filter array (CFA) 102.
In one embodiment, CFA 102 is in a Bayer pattern, with a repeated red (R), green (G), green (G), and blue (B) filter pattern.
In addition, FIG.
1 also includes an area of interest 104, which has a size of N pixels and M rows.
FIG.
2 is a flow diagram illustrating the determination of a focal value (V) from an image such as image 100 of FIG