Patent Publication Number: US-2006007154-A1

Title: Optical detection of relative motion

Description:
FIELD OF THE INVENTION  
      The present invention relates to the analysis of images to detect motion, and in particular, to an optical computer pointing device, e.g., a computer mouse. The present invention may be equally applicable to other situations requiring motion detection from sequential images, and also relates to integrated circuits adapted for this purpose.  
     BACKGROUND OF THE INVENTION  
      An optical mouse has advantages in comparison to mouses that rely on electromechanical devices incorporating balls or rollers. U.S. Pat. No. 5,729,008 discloses an optical mouse in which a light beam is projected onto a work surface, such as a mouse pad. The resulting illuminated area of work surface is imaged by a 2D image sensor to produce a sequence of images, and XY movement signals are derived by signal processing which is based on correlation of sequential images.  
      This known technique requires the work surface to have sufficient granularity to provide image detail to enable the correlation to be made. It is also prone to errors caused by optical deficiencies, such as a non-uniform illumination of the work surface, which may be caused by misalignment of the projection system. Errors may also be caused by a poor focus of the imaging lens, and non-perpendicularity of the sensor axis with respect to the work surface.  
      U.S. Pat. No. 6,603,111 describes the use of spatial filtering in a computer mouse, but only in the context of enhancing motion along specific axes (45 degrees to the X and Y axes).  
     SUMMARY OF THE INVENTION  
      An object of the present invention is to overcome or reduce the above described problems. More generally, the present invention seeks to improve the processing of sequential image signals involving correlation between successive images.  
      This and other objects, advantages and features in accordance with the present invention are provided by a method for detecting relative motion between an optical assembly and an adjacent surface. The method may comprise illuminating the surface, imaging the illuminated surface onto an image sensor, generating picture signals representing sequential frames of the image, and deriving motion information by correlating successive frames. The picture signals are preferably high-pass filtered before performing the correlation.  
      The picture signals are in, or may be converted into, a digital form before being subjected to the high-pass filtering. The filtering may be performed by a matrix operation. The method may further comprise pre-processing the digital signals before the filtering. The pre-processing may comprise a correlated double sampling (CDS) method.  
      Another aspect of the present invention is directed to an apparatus for detecting relative motion between itself and an adjacent surface. The apparatus comprises a light source for projecting a light beam towards the surface for illumination thereof, a lens mounted to focus an image of the illuminated surface onto an image sensor, and an image sensor for generating picture signals representing sequential frames of the image.  
      The apparatus may further comprise image processing means or an image processor for correlating successive frames of the picture signal to derive an output representing the relative motion. A high-pass filter is preferably between the image sensor and the image processor.  
      The image sensor may comprise a pixel array in combination with read-out and analog-to-digital conversion circuits, and the high-pass filter may be between the analog-to-digital conversion circuit and the image processor. The high-pass filter may apply a matrix operation to the digitized signal, and may include gain control means or controller for separately adjusting the gain of high pass and low pass channels.  
      The image sensor may further comprise image pre-processing means or a pre-processor between the analog-to-digital conversion circuit and the high-pass filter. The apparatus of the invention preferably comprises or forms part of an optical mouse.  
      Yet another aspect of the present invention is directed to an integrated circuit comprising a light-sensitive image plane forming an array of pixels, a read-out circuit for reading out pixel values to form a picture signal, and an analog-to-digital conversion circuit for converting the picture signal to a digital format. An image processor may correlate successive frames of the picture signal to derive information therefrom. The integrated circuit preferably further comprises a high-pass filter connected between the analog-to-digital conversion circuit and the image processor.  
      The high-pass filter may apply a matrix operation to the digitized signal, and may include gain control means or a gain controller for separately adjusting the gain of high pass and low pass channels. The integrated circuit may also further comprise image pre-processing means or a pre-processor between the analog-to-digital conversion circuit and the high-pass filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      An embodiment of the invention will now be described, by way of example, with reference to the drawings, in which:  
       FIG. 1  is a schematic diagram of an optical mouse according to the prior art;  
       FIG. 2  is a schematic diagram of an optical mouse according to a first embodiment of the present invention;  
       FIG. 3  is a schematic diagram of an optical mouse according to a second embodiment of the present invention;  
       FIG. 4  is a block diagram illustrating in greater detail the filter shown in  FIGS. 2 and 3 ; and  
       FIG. 5  shows one example of a filter matrix that may be used in the filter illustrated in  FIGS. 2-4 .  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Referring to  FIG. 1 , in a known optical mouse a light source  10 , such as a LED, illuminates a work surface  12 . The illuminated surface  12  is imaged by a lens  14  onto an image sensor  16 . The lens  14  may be implemented by a single lens or by more sophisticated imaging optics.  
      The image sensor  16  will typically be a sensor having a two-dimensional array of N×M pixels, such as a CMOS single chip sensor. The output of the image sensor  16  is read out and converted to digital form in a read-out and ADC circuit  18  in a manner well known by those skilled in the art to provide digital image data of sequential frames to an image processing circuit  20 .  
      The image processing circuit  20  derives X and Y motion of the mouse relative to the work surface  12  by applying correlation techniques to successive image frames. Such techniques are well established in the art and need not be described here.  
      The image processing requires the digitized image to feature a minimum amount of texture, contrast, granularity, and uniformity of illumination. The image processing can be adversely affected by factors such as a non-uniform illumination of the work surface and poor alignment of the optics, such as the lens  14  not having its optical axis truly perpendicular to the work surface, or the image plane of the sensor is not being truly parallel to the work surface.  
      Referring now to  FIG. 2 , a first embodiment of the present invention contains the same elements as in  FIG. 1 , denoted by the same reference numerals. In this embodiment, however, a high-pass filter  22  is between the read-out and ADC circuit  18  and the image processing circuit  20 .  
       FIG. 3  shows a second embodiment, which comprises the same elements as in  FIG. 2 , denoted by the same reference numerals. In this embodiment, however, an image pre-processor  21  is interposed between the ADC  18  and the image processor  20 .  
      The image pre-processor can be used to manipulate the detected signals to increase the performance of the downstream correlation by detecting and removing noise from the detected image, or removing offset from the detected image, for example. Any suitable noise reduction technique can be used, such as a correlated double sampling (CDS), for example, either in standard or modified form.  
      Many methods to high-pass filter an image are known by those skilled in the art. One of the simplest methods uses a 3×3 matrix operation as shown in  FIG. 5 . This provides a spatial high pass filter, which removes image DC components and amplifies pixel-to-pixel transitions.  
       FIG. 4  shows a generalized representation of the filter step, with the possibility of tuning high-pass and low-pass gain. The gain tuning can be manual or automatic, for example, in a loop that analyses image statistics.  
      The filtering may assist motion detection on low granularity surfaces or in non-optimized lens focus conditions by enhancing high spatial frequencies and reducing low frequency shading effects. For instance, where the axis of the sensor is not perfectly perpendicular is one example.  
      The circuitry shown in  FIGS. 2 and 3  can be incorporated into a single integrated circuit chip. The image sensor  16  may suitably be a CMOS image sensor, and the other circuits can then be readily formed in the same CMOS chip. In a CMOS image sensor, the pixel readout will be analog, which is then converted to digital, but the invention can be applied to other forms of sensor, including those giving a direct digital output.  
      Reference is directed to a copending patent application (Attorney Docket No. 04EDI01352803) filed concurrently herewith and is assigned to the current assignee of the present invention, the entire contents of which is incorporated by reference herein. The copending patent application discloses the filtering technique described herein but is also directed to other features of an optical mouse.