Video signal motion detecting method and noise reducer utilizing the motion

A first motion detection is performed for determining a motion or a still condition by comparing a difference signal .DELTA. between an input video signal of a current frame and a video signal of a previous frame with a preset threshold value. A second motion detection is performed for determining a motion or a still condition by using a difference signal .DELTA. between an input video signal of a current frame and a video signal of a previous frame at surrounding pixels and a center pixel as a target for the second motion detection. A wrong decision correction is performed for correcting an error in a result of motion detection in the first and/or second motion detections by changing a determination of a motion condition to a still condition, or a determination of a still condition to a motion condition, wherein an error is either a wrong determination of a motion condition for a still condition or a wrong determination of a still condition for a motion condition. A transition period detection is performed for detecting a motion-to-still transition period for a pixel whose data is determined as still during the correcting of a wrong decision.

BACKGROUND OF THE INVENTION 
This invention relates to a method for detecting motion from a video signal 
having noise or flicker superposed and a noise reducer for decreasing a 
noise component utilizing a frame correlation of the video signal and 
improving a signal-to-noise ratio of the video signal. 
Generally, the video signal is a signal which has video information 
repeating at the periods of the frames, and there is high auto-correlation 
between the frames On the other hand, since the noise component included 
in the video signal normally has little auto-correlation, if the video 
signal is averaged temporally for each frame period, the energy of the 
signal component hardly changes, and therefore, only the energy of the 
noise component decreases, so that the noise can be reduced. In order to 
obtain the above-mentioned average, a plurality of frame memories are 
required. Because the frame memories are expensive, the generally 
practiced method is not to use a non-recursive filter which requires a 
plurality of frame data but to use a one-frame first-order recursive 
filter. 
With regard to the noise reducer of a frame-cyclic arrangement which 
reduces noise by utilizing the frame correlation of the video signal, many 
methods have been proposed. One of those methods which describes the basic 
concept is carried in the Journal of the Institute of Television Engineers 
of Japan Vol. 33, No. 4 (1979). 
To help understand the present invention, description will first be made of 
a conventional noise reducer referring to FIG. 1. In FIG. 1, an input 
video signal 1, which comprises a component signal such as a luminance 
signal or one of color difference signals, primary color signal R, G and 
B, is supplied to the input terminal The input video signal 1 supplied 
through the input terminal is attenuated to (1-K) times the original 
energy by a variable attenuator 2 and becomes an input attenuated video 
signal 3, which is applied to an adder 4. On the other hand, a previous 
video signal 7, which has had noise reduced and then delayed by one frame 
period, is attenuated to K times the energy level held theretofore by a 
variable attenuator 8 and becomes a previous-frame-attenuated video signal 
9. This video signal 9 is added with the input attenuated video signal 3 
by the adder 4, and output as an output video signal 5 from an output 
terminal, and then stored in a frame memory 6. 
When the input video signal 1 is a completely still image, the frequency 
spectrum of this video signal is a line spectrum with a 30-Hz period, 
there is no energy loss of video signal by the circuit as shown in FIG. 1, 
and the degree of improvement in the signal-to-noise ratio can be 
expressed as follows: 
EQU Improvement of signal-to-noise ratio=10 log (1+K)/(1-K) (dB)(Eq. 1) 
FIG. 2 shows changes in the improvement in the signal-to-noise ratio with 
respect to coefficient K. It is obvious that the larger the K, the greater 
the degree of improvement in the signal-to-noise ratio becomes. 
On the other hand, generally, there is motion in the video signal, and if 
an image including motion is passed through the circuit in FIG. 1, an 
after image persists. The time constant T of the after image is 
EQU T=-1/(1n K) x 1/30 (sec) (Eq. 2) 
FIG. 3 shows after-image time constant characteristics with respect to the 
coefficient K. The after-image time constant T, namely, the after image is 
larger with a larger K. 
That is to say, the improvement of the signal-to-noise ratio and the 
occurrence of the after image are shadows to each other. For this reason, 
generally, the coefficient K is varied in the range of 0&lt;K&lt;1 according to 
the motion of the input video signal. To be more specific, when the motion 
of the video signal is large, the K is reduced to suppress the after 
image, and when the motion is small, the K is increased, thereby 
increasing the degree of improvement of the signal-to-noise ratio. This 
control of the K is done by a coefficient control circuit 10. 
From the input video signal 1 from the input terminal, the subtracter 
subtracts the previous frame video signal 7 which has had noise reduced 
and has been delayed by one frame period, and a resulting inter-frame 
difference signal .DELTA. is input into the coefficient control circuit 
10. The probability of the inter-frame difference signal .DELTA. being a 
noise component is generally high for smaller inter-frame difference 
signal .DELTA., while the probability of the inter-frame difference signal 
.DELTA. being a motion component of the signal is high for larger 
inter-frame difference signal .DELTA.. Therefore, when the inter-frame 
difference signal .DELTA. is small, the K is increased to increase the 
degree of improvement of the signal-to-noise ratio. When the inter-frame 
difference signal .DELTA. is large, the K is decreased to suppress an 
occurrence of the after image insofar as possible. 
The value of K is controlled as shown in FIG. 4 according to the 
inter-frame difference signal .DELTA. input to the coefficient control 
circuit 10. 
In FIG. 4, the K is a function of the inter-frame difference signal 
.DELTA., and can be expressed by Eq. 3 below. 
##EQU1## 
Thus, by the conventional method, it is possible to reduce noise while 
minimizing the occurrence of the after image. 
On the other hand, a method for detecting motion from a noise-superposed 
video signal has been reported in ITEJ Technical Report TEBS112-1 (1986, 
7, 27). This method will be described with reference to FIGS. 5 and 6, and 
Table 1. 
FIG. 5 indicates the relation between noise and a motion signal The 
frequency of zero cross of the inter-frame difference signal .DELTA. 
including noise is considered to differ in the still-image region and in 
the moving-image region. By utilizing this phenomenon, the motion is 
detected as follows 
With the inter-frame difference signal .DELTA. at a target pixel for motion 
detection and the preset surrounding pixels around the center target pixel 
in the detection range, the number of plus pixels CP and the number of 
minus pixels CN are calculated, and .xi. is calculated by the following 
equation. 
EQU .xi.=min (CP, CN)/max (CP, CN) (Eq. 4) 
where 
min (A, B) : a value of A or B whichever is smaller 
max (A, B) : a value of A or B whichever is larger 
In comparing .xi. with a preset threshold value th (0&lt;.xi.th&lt;1), when 
0.ltoreq..xi..ltoreq..xi.th, a decision as "motion" is made, and when 
.xi.th&lt;.xi.&lt;1, a decision as "still" is made. 
Table 1 shows a summary of correspondence between the values of .xi. and 
the decision results in the motion detection when the range of the 
surrounding pixels, by which the decision was made, was set as 5.times.5 
pixels around the center target pixel as shown in FIG. 6 and the threshold 
value was .xi.th=0.35. In the case of FIG. 6, since CP=16 and CN=9, 
.xi.=0.56, so that decision was "still". 
TABLE 1 
______________________________________ 
plus minus 
(CP) (CN) .xi. decision 
______________________________________ 
0 25 0 motion 
1 24 0.04 motion 
2 23 0.09 motion 
3 22 0.14 motion 
4 21 0.19 motion 
5 20 0.25 motion 
6 19 0.32 motion 
7 18 0.39 still 
8 17 0.47 still 
9 16 0.56 still 
10 15 0.59 still 
11 14 0.67 still 
12 13 0.92 still 
13 12 0.92 still 
14 11 0.67 still 
15 10 0.59 still 
16 9 0.56 still 
17 8 0.47 still 
18 7 0.39 still 
19 6 0.32 still 
20 5 0.25 motion 
21 4 0.19 motion 
22 3 0.14 motion 
23 2 0.09 motion 
24 1 0.24 motion 
25 0 0 motion 
______________________________________ 
As described above, by the conventional motion detection method, a decision 
can be made as to the motion based on the inter-frame difference signal at 
the center pixel as the target of motion detection and the surrounding 
pixels in the preset detection range. 
However, in the above-mentioned conventional noise reducer, noise reduction 
is performed by using only a statistical feature that the inter-frame 
difference signal .DELTA. is highly likely to be a noise component when 
the inter-frame difference signal .DELTA. is smaller, while the 
inter-frame difference signal .DELTA. is highly likely to be a motion 
component when the inter-frame difference signal .DELTA. is larger. So, 
this is not motion detection in the strict sensor of the word. By this 
method, a small motion in the inter-frame difference signal .DELTA. is 
removed in the same way as noise. This removal of small motion causes an 
after image to occur in the moving image, which has been a problem. 
The conventional motion detection method has another problem in which an 
omission of detecting the motion such as misjudging the motion as a still 
or an error such as misjudging the still as a motion occur very often 
owing to the effects of noise and flicker. 
SUMMARY OF THE INVENTION 
The first object of the present invention is provide a motion detection 
method for detecting only a true motion with high accuracy and being 
hardly effected by noise and flicker. 
A second object of the present invention is to provide a noise reducer 
which suppresses the occurrence of an after image. 
To achieve the above objects, a motion detection method according to the 
present invention comprises the steps of: 
a first motion detection for determining motion by comparing a difference 
signal between an input video signal and a one-frame-delayed video signal 
with a preset threshold value and/or a second motion detection for 
detecting motion by using the above-mentioned difference signal at 
N.times.N surrounding pixels and the center pixel as the target of motion 
detection; 
a wrong decision correction for correcting an error in a result of motion 
detection by the first motion detection and/or second motion detection; 
and 
a transition period detection for detecting a transition period of the 
motion from a motion detection result of the previous frame. 
By this motion detection method according to the present invention, it is 
possible to detect with high accuracy a true motion signal from an input 
video signal on which noise and flickers are superposed. 
In addition, the noise reducer according to the present invention comprises 
a motion detection circuit for detecting motion from an input video signal 
and a previous-frame video signal, which has been delayed by one frame 
period. By this motion detection circuit, a decision is made as to which 
of a moving image part, a still image part and a motion-to-still 
transition part of the picture the respective pixels represent, and 
separate noise reduction processes are performed by a moving image part, a 
still image part and a motion-to-still image part. The after image can be 
reduced to a small degree even with a motion signal having a relatively 
small amplitude.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be described with reference to 
the accompanying drawings. FIG. 7 is a block diagram showing an embodiment 
of the noise reducer according to the present invention. In FIG. 7, 
reference numeral 101 denotes an input video signal comprising component 
signals. Reference numeral 102 denotes a subtracter (a subtraction 
circuit) which subtracts from the input video signal 101 an output signal 
111 of a non-linear circuit 109 serving as a signal processing circuit in 
order to obtain an output video signal 103. Reference numeral 104 denotes 
a frame memory (delay circuit) for delaying the noise-reduced output video 
signal 103 by one frame period. Reference numeral 106 denotes a motion 
detection circuit for deciding a mode of the target pixel by using an 
input video signal 101 and a previous-frame video signal 105, and the 
motion detection circuit 106 controls the characteristics of the 
non-linear circuit 109 by adaptive control according to a mode decided for 
each pixel. The modes produced by the motion detection circuit 106 are a 
moving image mode for the moving image part, a still image mode for the 
still image part and a motion-to-still transition period mode for the 
motion-to-still transition part. Reference numeral 107 is a subtracter 
(difference signal detection circuit) which subtracts a previous-frame 
video signal 105 from an input video signal 101 in order to obtain an 
inter-frame difference signal .DELTA. 108. Reference numeral 109 
designates a non-linear circuit (signal processing circuit) for performing 
a non-linear process on an inter-frame difference signal .DELTA. 108 
produced by the subtracter 107. Reference numeral 110 denotes a control 
signal output from the motion detection circuit 106 to perform the 
above-mentioned adaptive control, while numeral 111 denotes an output 
signal from the non-linear circuit 109. 
Description will now be made of the operation of the above-mentioned 
embodiment. In this noise reducer according to the present invention, in 
order to minimize an after image, which image has been a problem with 
conventional noise reducers, the motion detection circuit 106 is used to 
detect the motion in the moving image signal from an input video signal 
101 and a noise-reduced previous-frame video signal 105, and the 
characteristics of the non-linear circuit 109 are controlled in adaptive 
control according to results of the motion detection to suppress the after 
image in the moving image part. 
More specifically, the motion detection circuit 106 detects the motion in 
the input video signal 101 in pixel units from a difference between the 
input video signal 101 and the previous-frame video signal 105, and 
decides modes for the individual pixels. The available modes are a moving 
image mode, a still image mode and a motion-to-still transition period 
mode. To implement different kinds of signal processing according to modes 
decided, the motion detection circuit 106 controls the non-linear circuit 
109 by adaptive control according to a control signal 110. 
For the input/output characteristics of the non-linear circuit 109, an 
equation Eq. 5 for example, can be used, which is shown below. 
##EQU2## 
The .DELTA. is an input to the non-linear circuit 109, and is an 
inter-frame difference signal derived from the input video signal 101 and 
the noise-reduced previous-frame video signal 105. The .phi.(.DELTA.) is 
an output of the non-linear circuit 109 evolving from the .DELTA.. The TH 
is a threshold value which is preset. The K.sub.0 (0&lt;K.sub.0 .ltoreq.1) is 
a parameter which is controlled in adaptive control according to a result 
of motion detection. In practice, various values are set for the K.sub.0 
according to a mode decided by the motion detection circuit 106, that is 
to say, K.sub.0 =.gamma. in the still image mode, K.sub.0 =.alpha. in the 
moving image mode, and K.sub.0 =.beta. in the motion-to-still transition 
period mode, where 0&lt;.alpha.&lt;.beta.&lt;.gamma.&lt;1. With larger TH and K.sub.0, 
the degree of improvement in the signal-to-noise ratio is better, but the 
after image occurs more greatly. 
In the noise reducer in this embodiment, a motion of the input video signal 
101 is detected, and in the still image mode, the signal-to-noise ratio is 
improved sufficiently by controlling the K.sub.0 of the non-linear circuit 
109 so as to be a large value, while in the moving image mode, the K.sub.0 
is controlled so as to be a small value to minimize the after image. 
However, if there is a great difference in the degree of improvement of 
the signal-to-noise ratio between in the still image mode and in the 
moving image mode, a false contour appears at the border area, and if the 
false contour moves, this causes a flicker, which is very disturbing to 
the viewer. Therefore, a transition period from a moving image to a still 
image (motion-to-still transition period) is detected, and in the 
motion-to-still transition period, the signal-to-noise ratio is improved 
to a degree intermediate between the moving image mode and the still image 
mode. To be more specific, the K.sub.0 of the non-linear circuit 109 is 
controlled to be a value intermediate between in the still image mode and 
in the moving image mode. By so doing, the border part between the still 
image part and the moving image part is smoothed away, so that the motion 
in the picture becomes natural. 
As is clear from the foregoing description, by the noise reducer according 
to the above embodiment, the coefficient K.sub.0 of the non-linear circuit 
109 is made small in the moving image part, so that the after image can be 
limited to a minor degree. 
Referring to FIGS. 9, 10 and 11, description will then be made of the 
motion detection method in the motion detection circuit 106. The motion 
detection circuit 106 operates in, e.g., four steps (first motion 
detection, second motion detection, correcting a wrong decision, and 
motion-to-still transition period detection) which are described below. 
The data of each pixel in the input video signal 101 is subjected to a 
decision by the motion detection circuit 106 to specify a mode--the still 
image mode, the moving image mode or the motion-to-still transition period 
mode--by which the data is to be processed. 
(1) First motion detection 
A difference value .DELTA. (i, j) is calculated as an inter-frame 
difference by subtracting a pixel value PY (i, j) of the noise-reduced 
previous frame from a pixel value IY (i, j) of the current input signal, 
and the inter-frame difference is compared with TH in Eq. 5, 
1) when .vertline..DELTA.(i, j).vertline.&gt;TH, the "moving image mode" is 
decided, and 
2) when .vertline..DELTA.(i, j).vertline..ltoreq.TH, the second motion 
detection, described below, is performed. 
This first motion detection is done by comparison with a threshold value. 
However, when .vertline..DELTA.(i, j) &gt;TH, it is no doubt possible to 
determine with fairly high accuracy that the pixel concerned represents 
motion. For example, when the superposed noise is supposed to be Gaussian 
noise, even if the signal-to-noise ratio of the input image is as high as 
26 dB, the probability of noise level exceeding TH =40 is less then 0.3 
per cent. 
(2) Second motion detection 
In the first motion detection, when .vertline..DELTA.(i, j).vertline.&lt;TH, 
motion detection is performed by using the above-mentioned difference 
signal .DELTA.(i+m, j+n) of the N .times.N surrounding pixels and the 
center pixel as the target of motion determination, where m and n are 
##EQU3## 
when N is an even number, or 
##EQU4## 
when N is an odd number. 
Description will be made of details of the step of the second motion 
detection with reference to FIG. 9. 
For an inter-frame difference signal .DELTA.(i+m, j+n) (-2.ltoreq.m, 
n.ltoreq.2) at , e.g., 5.times.5 surrounding pixels and the center pixel 
as the target of motion determination, 
the number of pixels each having a difference signal .DELTA.(i+m, j+n) 
which is greater (&gt;) than a threshold value 2 is denoted by p.sub.-- num, 
the number of pixels each having a difference signal .DELTA.(i+m, j+n) 
which meets a condition that a threshold value 1.ltoreq..DELTA.(i+m, 
j+n).ltoreq.the threshold value 2 is denoted by z.sub.-- num, 
the number of pixels each having a difference signal .DELTA.(i+m, j+n) 
smaller (&lt;) than the threshold value 1 is denoted by n.sub.-- num, and 
decision is made as follows. 
1) when z.sub.-- num.gtoreq.zero.sub.-- th, the decision is "still" 
2) in cases other than 1), .eta. is calculated by the following equation: 
EQU .eta.=min(p.sub.-- num, n.sub.-- num)/max(p.sub.-- num n.sub.-- num) (Eq. 
6) 
where 
min (A, B) : a value of A or B whichever is smaller 
max (A, B) : a value of A or B whichever is larger 
the calculated .eta. is compared with a preset threshold value .eta.th 
(0&lt;.eta.th&lt;1), and if 0.ltoreq..eta..ltoreq..eta.th, the decision is 
"motion" 
and if .eta.th &lt;.eta..ltoreq.1, the decision is "still". 
This decision method is to detect a bias of the inter-frame difference 
signal .DELTA.(i+m, j+n) to the plus or minus side, and, motion detection 
is made from this bias. .eta.th is provided as the threshold value. 
The above-mentioned threshold values 1 and 2 are provided to prevent an 
error of motion detection from occurring owing to noise or flicker 
superposed on the input signal. 
The step of the second motion detection is followed by the step of 
correcting a wrong decision. 
(3) Correcting a wrong decision 
A check is made of a result of a decision in the second motion detection, 
and a decision considered faulty is corrected (correction of a wrong 
decision). Correction is carried out as follows. 
(3-1) Correcting a failure to detect 
For the pixels which are determined as having data of a still state in the 
second motion detection, the result of a decision about motion at eight 
surrounding pixels is checked as shown in FIG. 10, if four or more of the 
eight surrounding pixels are determined as having motion data, the target 
pixel is forcibly treated as having motion data. 
(3-2) Correcting a detection error 
For the pixels determined as having motion data in the second motion 
detection, the result of decision about motion at the eight surrounding 
pixels is checked as shown in FIG. 10, if the pixels determined as having 
motion data are two or less, the center target pixel is forcibly treated 
as having data of a still state. 
After a wrong decision is corrected, 
1) when the result of correct decision is "motion" the "moving image mode" 
is decided 
2) when the result of correct decision is "still" 
The subsequent step of motion-to-still transition period detection is 
performed. 
(4) Detecting a motion-to-still transition period 
For the pixel whose data is determined to be still as a result of 
correction of a wrong decision, detection is made for a motion-to-still 
transition period in the step of detecting a motion-to-still transition 
period. With reference to FIG. 11, description will be made of detection 
of a motion-to-still transition period. With regard to a pixel of the N 
frame, 
1) if the result of motion detection at the corresponding pixel of the 
previous N-1 frame is "moving image mode", 
the "motion-to-still transition period mode" is decided 
2) if the result of the above-mentioned detection is "still image mode", 
the "still image mode" is decided. 
FIG. 12 is a flowchart showing the flow of the steps of detection mentioned 
above. To be more specific, at step 121, the first motion detection is 
performed, and when the pixel is found to have motion data, the "moving 
image mode" is decided. When the pixel is found to have data of a still 
state, the second motion detection is made at step 122. At the next step 
123, a wrong decision correction is performed, and if the correct decision 
is "motion", the "moving image mode" is decided. And, if the correct 
decision is "still", a motion-to-still transition period is detected at 
step 124. At step 124, if the decision of previous frame is "moving image 
mode", the "motion-to-still transition period mode" is decided, and if the 
decision of previous frame is "still image mode", the "still image mode" 
is decided. 
In the above embodiment, description has been made of a case in which both 
the first and second motion detections are carried out, but either one of 
the two motion detections may be omitted. More specifically, it is 
possible to proceed to the step of wrong decision correction when 
.vertline..DELTA.(i, j).vertline..ltoreq.TH in the first motion detection. 
Or otherwise, the second motion detection may be conducted from the 
beginning without doing the first motion detection. 
The motion detection method according to this embodiment has an advantage 
that this method can detect only true motion with much higher accuracy and 
is hardly effected by noise or flicker than the conventional methods. 
As has been described above, an effect of this motion detection method 
according to the present invention is that even if noise or flicker is 
superposed on the input video signal, only true motion can be detected 
with excellent accuracy. 
In addition, an effect of the noise reducer according to the present 
invention is that this noise reducer detects true motion from the input 
video signal with remarkable accuracy, controls a noise reduction process 
by adaptive control according to results of motion detection, can limit 
the after image to a minor degree even when the motion signal has a 
relatively small amplitude.