1. Field of the Invention
The present invention relates to a video signal processing technique, and relates to a video signal processing technique with which sharpening processing and smoothing processing of a video image are performed by use of a spacial filter.
2. Description of the Related Art
An Input video image includes various noises and distortions. Eliminating such causes of degrading a video image can improve viewability of the video image, and also, can extract and enhance useful information. Such processing is one of the important operations in image processing. Typical approaches of image processing include 1) contrast enhancement by density conversion, 2) smoothing for noise elimination, and 3) sharpening for edge enhancement. The smoothing processing/sharpening processing correspond to the enhancement processing of a low frequency component/a high frequency component of an image. If a density value of an image at a part such as a contour part, which density value should have a sharp change has a mild change, the image by nature has a figure with its contour blurred. Enhancing the change in the density of such an image to obtain a sharp image is called sharpening. One approach of the sharpening is a method of subtracting, from an original image, a result obtained by filtering the original image (sharpening operator).
Meanwhile, the smoothing processing includes an approach in which an operator of 3×3, for example, is moved along an original image and a computation is performed on each component value of the operator and a corresponding one of pixel values of the original image.
There has been known a circuit by which sharpening and smoothing an image in a display unit of a device such as a television receiver, a personal computer, and a mobile phone can be performed in accordance with the above-mentioned principle. FIG. 30 is a diagram illustrating a relationship between an edge amount input (X axis) and an edge amount output (Y axis) in sharpening processing which is described in a first embodiment of Patent Document 1 below.
                              β          ⁢                      :                    ⁢                                          ⁢          a          ⁢                                          ⁢          parameter          ⁢                                          ⁢          indicating          ⁢                                          ⁢          the          ⁢                                          ⁢          strength          ⁢                                          ⁢          of          ⁢                                          ⁢          sharpening                ⁢                                  ⁢                  Th          ⁢                      :                    ⁢                                          ⁢          a          ⁢                                          ⁢          parameter          ⁢                                          ⁢          determining          ⁢                                          ⁢          a          ⁢                                          ⁢          range          ⁢                                          ⁢          in          ⁢                                          ⁢          which          ⁢                                          ⁢          an          ⁢                                          ⁢          input          ⁢                                          ⁢          is          ⁢                                          ⁢          regarded          ⁢                                          ⁢          as          ⁢                                          ⁢          a          ⁢                                          ⁢          noise                ⁢                                  ⁢                  Y          =                      {                                                            0                                                                      (                                                                                          X                                                                    ≤                      Th                                        )                                                                                                                    β                    ·                    X                                                                                        (                                          Th                      <                                                                      X                                                                                      )                                                                                                          [                  Formula          ⁢                                          ⁢          1                ]            
Here, the unsharp mask (USM) of a sharpening processing algorithm will be described with reference to FIG. 1A and FIG. 30. Firstly, a difference (edge amount input) between an input video signal (RGB or YUV) and an input image signal around the input video signal (spacial filter output) is obtained. In this respect, the spacial filter output is data obtained by smoothing the input video signal. Hereinbelow, the smoothing is regarded as a synonym for noise elimination.
In the sharpening technique, the sharpening for enhancing the sharpness is performed by multiplying an edge amount by use of parameters and adding the result to target pixel data, the parameters being defined as a threshold (Th) set on the X axis and a gain (β) indicating the strength of the sharpening. As shown in FIG. 30, when an absolute value of the edge amount input is smaller than the threshold Th, the edge amount input is regarded as a noise and no sharpening is performed. This is because it is favorable in teens of image quality not to perform sharpening. A range in which the input is regarded as a noise can be specified by the threshold. The gain is a parameter used to convert the edge amount input. The strength of sharpening is adjustable by increasing or decreasing the gain. Note that the processing image by the USM will be described by using FIG. 1C later.
FIG. 31 is a diagram illustrating an overview of a technique for smoothing and sharpening which is described in a fourth embodiment of Patent Document 1.
                              α          ⁢                      :                    ⁢                                          ⁢          a          ⁢                                          ⁢          parameter          ⁢                                          ⁢          indicating          ⁢                                          ⁢          the          ⁢                                          ⁢          strength          ⁢                                          ⁢          of          ⁢                                          ⁢          smoothing                ⁢                                  ⁢                  β          ⁢                      :                    ⁢                                          ⁢          a          ⁢                                          ⁢          parameter          ⁢                                          ⁢          indicating          ⁢                                          ⁢          the          ⁢                                          ⁢          strength          ⁢                                          ⁢          of          ⁢                                          ⁢          sharpening                ⁢                                  ⁢                  γ          ⁢                      :                    ⁢                                          ⁢          a          ⁢                                          ⁢          parameter          ⁢                                          ⁢          of          ⁢                                          ⁢          an          ⁢                                          ⁢          intersection          ⁢                                          ⁢          with          ⁢                                          ⁢          an          ⁢                                          ⁢          edge          ⁢                                          ⁢          amount          ⁢                                          ⁢          output          ⁢                                          ⁢                      (            Y            )                    ⁢                                          ⁢          axis                ⁢                                  ⁢                  Th          ⁢                      :                    ⁢                                          ⁢          a          ⁢                                          ⁢          parameter          ⁢                                          ⁢          of          ⁢                                          ⁢          a          ⁢                                          ⁢          threshold          ⁢                                          ⁢          at          ⁢                                          ⁢          which          ⁢                                          ⁢          switching          ⁢                                          ⁢          is          ⁢                                          ⁢          made          ⁢                                          ⁢          between          ⁢                                          ⁢          a          ⁢                                          ⁢          smoothing          ⁢                                          ⁢          conversion          ⁢                                          ⁢          and          ⁢                                          ⁢          a          ⁢                                          ⁢          sharpening          ⁢                                          ⁢          conversion                ⁢                                  ⁢                  Y          =                      {                                                                                α                    ·                    X                                                                                        (                                                                                          X                                                                    ≤                      Th                                        )                                                                                                                                          β                      ·                      X                                        +                    γ                                                                                        (                                          Th                      <                                                                      X                                                                                      )                                                                                                          [                  Formula          ⁢                                          ⁢          2                ]            
Here, α is a parameter indicating the strength of smoothing; β is a parameter indicating the strength of sharpening; γ is a parameter of an intersection with an edge amount output (Y) axis; and a threshold parameter Th is a value of the edge amount input at which switching is made between a smoothing conversion and a sharpening conversion. The use of this technique (the fourth embodiment) enables smoothing in a case where an absolute value of the edge amount input is not more than the threshold, as well as sharpening in a case where the absolute value is not less than the threshold.    [Patent Document]Japanese Patent Application Publication No. Hei 3-245675