Field of the Invention
The present invention relates, in general, to digital video noise reduction processing. In particular, aspects of the present invention provide a method for reducing artifacts in a digital video sequence of image frames.
Description of the Related Art
Temporal video noise reduction methods include motion adaptive and motion compensative methods. A motion adaptive temporal filter method works best in stationary video sequences because it assumes that noise causes the only difference between successive frames and that the noise is random with a mean of zero. The motion adaptive temporal filter removes noise by averaging the target pixel with the corresponding pixels in the previous frames for which it is relatively detected as a still pixel, therefore reducing the noise without impairing the spatial resolution of the image. A motion compensative temporal filter method functions similar to the motion adaptive temporal filter, but it performs filtering along moving object trajectories. The motion compensative temporal filter method requires more processing time than the motion adaptive methods, but produces better results. It is often desirable to apply a motion adaptive or motion compensated recursive temporal filter to reduce the input video noise and thereby improve the compression efficiency of the encoder.
Contour artifacts are seen on images when there are not enough bits to represent the change of pixel intensity in the image. This effect is particularly visible in smooth gradient regions such as dark video with low contrast where a small gradient of 8-bit digitized pixel levels span across a large area of the image. These artifacts can be especially objectionable in image areas having spatially smooth intensity variations such as sky, skin, and large homogeneous surfaces such as walls. In natural video, the visibility of the contour is often masked due to the dither effect caused by random noise in the input video. The random noise introduced by the dithering into the image effectively breaks up the contouring artifacts. In digital video, one method of reducing the contour artifacts is to use dithering to add Gaussian noise to the image to break up the contour lines, but the addition of Gaussian noise can also reduce the image contrast. Moreover, adding Gaussian noise to the image creates new artifacts and increases the number of bits in the compressed image.
The motion adaptive (or compensated) temporal filter applies a temporal recursive filter to stabilize the video frames when there is only a small difference between the current pixel and the previously filtered pixel at the same location (or along the motion trajectory). The temporal recursive filter is an effective technique to reduce random noise in the video, but it eliminates the natural fluctuations in the signal level of the video frames, thereby making the contour artifacts more visible. It is also desirable not to introduce extra noise to the image such as dithering.
A method and encoding system for reducing artifacts in a digital video sequence of image frames. The method acquires a current frame of the digital video sequence, and retrieves a previous frame of the digital video sequence from a frame delay. The method applies a recursive temporal filter to the current frame and the previous frame to generate a filtered frame. The method then applies a mixer to the current frame and the filtered frame to generate an output frame. The method stores the output frame in the frame delay.
The recursive temporal filter computes a pixel difference for each pixel in the current frame, and each corresponding pixel in the previous frame. To generate a filtered frame, the recursive temporal filter computes a pixel value for each pixel in the filtered frame based on the pixel difference. When the pixel difference is above an upper threshold, the pixel value is equal to a value of the pixel in the current frame, and when the pixel difference is below the upper threshold, the pixel value is a sum of the value of the pixel in the current frame and an adjustment based on the pixel difference. In another embodiment, when the pixel difference is below a lower threshold, the pixel value is equal to the value of the pixel in the current frame.
The mixer computes a pixel difference between each filtered pixel and each input pixel at a corresponding location. To generate the output frame, the mixer computes a pixel value for each pixel in the output frame based on the pixel difference. When the pixel difference is above an upper threshold, the pixel value is equal to a value of the pixel in the filtered frame, and when the pixel difference is below the upper threshold, the pixel value is a sum of the value of the pixel in the current frame and an adjustment based on the pixel difference.
For simplicity and illustrative purposes, the principles of the embodiments are described by referring mainly to examples thereof. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the embodiments. It will be apparent however, to one of ordinary skill in the art, that the embodiments may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the embodiments.
Before describing in detail embodiments that are in accordance with the present invention, it should be observed that the embodiments reside primarily in combinations of method steps and apparatus components related to reducing the contour artifacts that are present after processing digital video through a recursive temporal filter, while preserving the effectiveness of the recursive temporal filter. Accordingly, the apparatus components and method steps have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.
In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element preceded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element.
O[x,y,t]=O[x,y,t−1]+f(|I[x,y,t]−O[x,y,t−1]|)×(I[x,y,t]−O[x,y,t−1]),
where I[x,y,t] is the input pixel value at location (x,y) at time (frame number) t, O[x,y,t] is the output pixel value at location (x,y) at time (frame number) t, and f(δ) is a non-linear function that has the characteristics shown in
In one embodiment, the functions performed by the adaptive IIR filter kernel 310 and the adaptive function 315 are the same as the functions performed by the adaptive IIR filter kernel 110 and the adaptive function 115 shown in
O[x,y,t]=O[x,y,t−1]+f(|I[x,y,t]−O[x,y,t−1]|)×(I[x,y,t]−O[x,y,t−1]),
where I[x,y,t] is the input pixel value at location (x,y) at time (frame number) t, O[x,y,t] is the output pixel value at location (x,y) at time (frame number) t, and f(δ) is a non-linear function that has the characteristics shown in
The leaky motion adaptive recursive temporal filter 300 reduces the contour artifacts introduced by the adaptive IIR filter kernel 310 by allowing a small amount of noise to “leak” through the filter. The output of the adaptive IIR filter kernel 300 is blended with the input pixels by the adaptive mixer 320. The input pixels are passed through the adaptive mixer 320 when the difference between the filtered output pixels from the adaptive IIR filter kernel 310 and the input pixels is less than or equal to a just noticeable perceptual threshold. With 8-bit representation of the luminance signal of the image, the value of 1 is empirically chosen as the threshold. The adaptive mixer 320 has the following characteristic:
Y=X+g(|X′−X|)×(X′−X),
where Y is the output pixel value, X is the input pixel value, X′ is the filtered output pixels from the adaptive IIR filter kernel 310, and g(Δ) is a non-linear leak function 325 that has the characteristics shown in
The leaky motion adaptive recursive temporal filter 300 shown in
This disclosure describes aspects of the present invention in the context of a motion adaptive recursive temporal filter. In another embodiment, the present invention also reduces the contour artifacts that are present after processing digital video through a motion compensated recursive temporal filter, while preserving the effectiveness of the motion compensated recursive temporal filter.
Although the disclosed exemplary embodiments describe a fully functioning system and method for reducing artifacts in a digital video sequence of image frames, the reader should understand that other equivalent exemplary embodiments exist. Since numerous modifications and variations will occur to those reviewing this disclosure, the system and method for reducing artifacts in a digital video sequence of image frames is not limited to the exact construction and operation illustrated and disclosed. Accordingly, this disclosure intends all suitable modifications and equivalents to fall within the scope of the claims.
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Number | Date | Country | |
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20090273716 A1 | Nov 2009 | US |