Abstract:
An image sensor may selectively produce an effect, such as simulating a night vision scope, by controlling existing hardware to vary anti-vignetting and gamma.

Description:
FIELD OF THE INVENTION  
       [0001]    The invention relates to the production of special effects in image sensors. 
       BACKGROUND OF THE INVENTION  
       [0002]    Image sensors are in common use in consumer products, such as mobile phones with cameras, webcams and digital still cameras. There is a desire to enable such image sensors to provide special visual effects. However, such special visual effects commonly require additional circuitry, which adds to cost, complexity and power consumption. 
       SUMMARY OF THE INVENTION  
       [0003]    An object of the present invention is to provide an apparatus and method for selective provision of special effects in image sensors, especially but not exclusively, in consumer products, such as mobile phones. 
         [0004]    This and other objects, advantages and features in accordance with the present invention are provided by a solid state image sensor comprising an array of color pixels and means or circuit for reading out pixel values to provide an image signal. The image sensor may have anti-vignetting means or circuit and gamma control means or circuit. The anti-vignetting means may vary the gain across the image to counteract vignetting caused by an imaging lens, or by interaction between an imaging lens and microlenses on the sensor. The gamma control means may vary gamma in each of the color channels. 
         [0005]    The image sensor may also comprise a selectively operable special effect generator which, when operated, provides one or both of the following: 
         [0006]    (a) controlling the anti-vignetting means to have a low or negative value at an edge of the image to produce a dark border around the image, and 
         [0007]    (b) controlling the gamma control means to have a selected gamma or gain factors in the color channels to produce a monochrome image. 
         [0008]    The special effects generator is effective to produce both (a) and (b) simultaneously. The pixel array is typically an array of RGB pixels, and the special effects generator may set either the gamma or gain for red and blue channels to zero, thereby producing a green monochrome image. 
         [0009]    The image sensor may include means or circuit for changing the output of selected rows and columns of the pixel array. The special effects generator is may be operable to change the output of a row, or a plurality of adjacent rows and a column, or a plurality of adjacent columns, thus forming a cross on the image. 
         [0010]    The means for changing the output of selected rows and columns is operable to set the output of the selected row or column to black or to white, or to apply an offset to the data values, or to invert the data values, or to exclusive-or the data with a fixed value. The selected row(s) and column(s) may be in the center of the image. 
         [0011]    Another aspect of the present invention is to provide a method of generating special effects in an image sensor. The image sensor may have an array of color pixels and means or circuit for reading out pixel values to provide an image signal, anti-vignetting means operable to vary the gain across the image to counteract vignetting caused by an imaging lens or by interaction between an imaging lens and microlenses on the sensor, and gamma control means for varying the gamma in each of the color channels. 
         [0012]    The method may comprise one or both of he following: (a) controlling the anti-vignetting means to have a low or negative value at an edge of the image to produce a dark border around the image; and (b) controlling the gamma control means to have selected gamma or gain factors in the color channels to produce a monochrome image. Both (a) and (b) may be produced simultaneously. 
         [0013]    The pixel array is typically an array of KGB pixels, and either the gamma or gain for red and blue channels are set to zero, thereby producing a green monochrome image. 
         [0014]    The image sensor may include means or circuit for changing the output of selected rows and columns of the pixel array. The method may include selectively changing the output of a row, or a plurality of adjacent row and a column, or a plurality of adjacent columns, thus forming a cross on the image. 
         [0015]    The output of the selected row or column may be set to black or to white, or an offset may be applied to the data values, or the data values may be inverted, or the data may be exclusive-or&#39;d with a fixed value. The selected row(s) and column(s) are preferably in the center of the image. 
         [0016]    Other aspects of the present invention are directed to a camera comprising the foregoing image sensor, which may be a webcam or a digital still camera, a mobile telephone including such a camera, and an electronic game incorporating the foregoing method. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]    An embodiment of the present invention will now be described, by way of example only, with reference to the drawings, in which: 
           [0018]      FIG. 1  is a block diagram of an apparatus embodying the present invention; and 
           [0019]      FIG. 2  illustrates the effect achieved by an apparatus embodying the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Referring to  FIG. 1 , an image sensor comprises a pixel array  10  and read-out circuitry  12 . The pixel array  10  may, for example, be a CMOS array in which each pixel comprises a photodiode and associated transistors, as is well known in the art. An image is formed on the array  10  by an imaging lens (not shown) and color filters associated with the pixels. The color filters will most suitably be RGB filters arranged in a Bayer pattern. 
         [0021]    The read-out circuitry  12  is operable to read out pixel values sequentially to form an image data stream, and will typically include noise reduction features such as correlated double sampling, and analog to digital conversion. 
         [0022]    The image processor further comprises a co-processor  14  that performs signal processing functions on the data stream from the read-out circuitry  12 . As is conventional, the signal processing functions in the present example include anti-vignetting (i.e., the adjustment of gain across the area of the array to compensate for reduction in brightness from center to edge caused by variation of optical performance of the imaging lens), and gamma correction on each of the RGB channels (i.e., correction for different performance of kGB phosphors and for their non-linearity). 
         [0023]    The co-processor  14  in the present example also has a feature that is known in some existing image sensors, which is the ability to selectively change the output from selected rows and columns. This feature is provided in order to perform tests and adjustments during manufacture, but remains embedded in the image sensor. The selectable change is typically setting all pixels in the chosen row or column to black, but other changes may be used for the same purpose, such as setting all pixels to white, or varying the pixel values by an operation such as applying an offset, inverting or performing exclusive-or with data of a fixed value. 
         [0024]    All of the foregoing is conventional and well known in the art. However, the image sensor advantageously produces special effects. As shown in  FIG. 1 , the image sensor has a special effect enable input  16 . This may be a specially provided key, or a selection from a software menu. When operated, the special effect enable input  16  causes the co-processor  14  to perform three operations, namely: 
         [0025]    (1) The anti-vignetting control is set to a negative value that forces a circular image with a black surrounding. 
         [0026]    (2) The gamma controls on the red and blue channels are set to zero or low values and gamma for the green channel is increased, thus producing an image that is essentially green monochrome. 
         [0027]    (3) The central row and central column of pixels are disabled, thus producing a black cross centered across the image. 
         [0028]    With regard to step (2), as an alternative to using gamma the same effect can be produced by controlling gain in the color channels. In a typical coprocessor or processor the RGB data undergo matrixing before gamma is applied, and it would be possible to set either the input or the output of the matrix to zero for the red and blue channels. 
         [0029]    An example of the resulting image is seen in  FIG. 2 . The image simulates the popular conception of a night scope, image intensifier view. Such an image provides an optionally useable playful effect to a camera system that adds a differentiating feature and adds value. The effect can be achieved without adding to the camera circuitry, and can thus be provided at minimal cost. 
         [0030]    It is desirable that selection of the special effect also causes the zoned exposure control that is conventionally provided in the image sensor to be set to use the center of the image so that correct exposure is achieved. 
         [0031]    Detailed methods of performing these functions will be readily apparent to those in the art and will not be described in detail herein. It will typically be most convenient to use embedded software to control the special effect function, for example by the co-processor host writing appropriate values to registers controlling the anti-vignetting, gamma, row/column disable and exposure control zone functions. 
         [0032]    Modifications of the above embodiment may be used. For example, the special effect may comprise only one or two of the above changes (1)-(3). Also, a different color effect may be used by suitable adjustment of channel gamma values, for example, to obtain a sepia effect. 
         [0033]    The image sensor may typically be used in a camera within a mobile phone. However, it may also be used in other types of cameras, or incorporated in an electronic game.