Filter for adaptive noise reduction and sharpness enhancement for electronically displayed pictures
Electronic images that are degraded by noise and data reduction, such as MPEG encoding, display artifacts in the reproduced image, such as ringing (“ripples”) and blocks (“huge pixels”), and noise in the image may be apparent as graininess. By performing image analysis, both on a frame-by-frame and pixel-by-pixel basis it is possible to identify and separate edges in the image, ringing artifacts and the boundaries between block transitions. By applying noise reduction according to the analysis, followed by sharpness enhancement, it is possible to clean up the image for further utilization.
1. A procedure for providing a cleaned-up picture from a bandwidth-limited, noise-reduced, and data reduced input stream of electronic data representing moving or stationary image information with noise content, said image information containing artefacts due to data reduction and bandwidth limitation, comprising the following steps
1) decoding the data reduced stream of data
2) feeding the data to a first analyzer (7) for global image analysis and to a second analyzer (4) for local image analysis,
3) the output of the first analyzer (7) being a frame-by-frame sharpness function and a frame-by-frame noise level function
4) the output of the second analyzer (4) being a pixel-by-pixel classification of the pixel as part of an edge, part of a ringing artefact, or part of a block edge artefact
5) the noise level output of the first analyzer (7) being combined in a limit calculation block (5) with the output from the second analyzer (4)
6) using the output from the limit calculation block (5) to determine the bandwidth of a noise reducing low-pass filter (6), the bandwidth in (6) being increased on the current pixel, in case of a detected edge, in order to obtain data relating to a noise-reduced image with good contrast
7) the sharpness output from the first analyzer (7) being combined with the output of the second analyzer (4) in a two-dimensional sharpness gain calculation block (8b)
8) applying the gain factor of gain calculation block (8b) in a sharpness enhancement limiter (8c), the pass band of the sharpness enhancement filter having its centre frequency locked to the sharpness value
9) the output being formed as the sum of sharpness-enhanced image detail from the limiter (8c) and the noise reduced signal from the noise reducing filter (6).
2. A procedure according to claim 1 , wherein the step of estimating the global sharpness value in the block (7) is performed by histogram analysis of the image details identified in the high frequency content of the decoded image data.
3. A procedure according to claim 2 , wherein for low measured sharpness values in block (7), the sharpness is enhanced with respect to horizontal (serial) signals only in block (9).
4. A procedure according to claim 1 , wherein the input stream of electronic data represents a colour image.
5. A procedure according to claim 4 , wherein the signal processing is performed in YCbCr (luminance and colour).
6. An apparatus for performing the procedure of claim 1 , wherein the apparatus comprises the following functional blocks:
a decoder for data reduced image information, an estimator (7) for global sharpness based on subtraction of a low-pass filtered signal from the total decoded signal, a noise estimator (7), an edge detector (4), a detector for ringing artefacts (4), a detector (4) for block boundary artefacts, a noise reduction block (6) performing pixel-by-pixel mixing of an input value and a low-pass filtered input value in dependence of detected edges and artefacts, a sharpness enhancement block (8b), a block (9) working on a horizontal line representation of the image, summing blocks (10,10b), a dynamics limiting block (8c), and a video signal output block.
7. Use of a procedure according to claim 1 on a data reduced signal in which the data reduction is performed according to the standards of the Motion Picture Expert Group (MPEG).