Image signal interpolation

Details Image signal interpolation Image signal interpolation

1998-07-30

2003-05-13

Garber, Wendy R. (Department: 2612)

Television

Camera, system and detail

Combined image signal generator and general image signal...

C348S222100, C358S532000

Reexamination Certificate

active

06563537

ABSTRACT:

This application is based on Japanese patent application No. 9-206856 filed on Jul. 31, 1997, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
a) Field of the Invention
The present invention relates to techniques of processing image signals, and more particularly to image signal processing techniques with interpolation.
b) Description of the Related Art
FIG. 11A
shows a fraction of image data picked up with a charge coupled device (CCD) camera. Image data is constituted of red (R), green (G), and blue (B) signals. A three-chip type sensor is made of three sensors for R, G, and B signals. In a single chip type sensor, R, G, and B signals are disposed on one sensor in a matrix such as shown in FIG.
11
A. Various types of matrices are known.
FIG. 11A
shows an example a Bayer matrix. In the Bayer matrix, R and G signals are alternately disposed in one row (horizontal line), and in the next adjacent rows, G and B signals are alternately disposed. Therefore, a row of R and G signals and a row of G and B signals are alternately disposed.
The example shown in
FIG. 11A
shows 3×3 image data. There are four R signal pixels at the four corners of the 3×3 unit. R signal at the other pixel positions cannot be obtained so that it is necessary to obtain R signal through interpolation. In order to obtain R signal at the pixel position of G
1
signal, it is interpolated through averaging of R signals at the adjacent right and left two pixels. R signal at the pixel of G
4
signal is interpolated in the similar manner. R signals at the pixels of G
2
and G
3
signals are interpolated through averaging of R signals at the adjacent upper and lower pixels. R signal at the pixel of B signal is interpolated through averaging of R signals of the obliquely adjacent four pixels (at four corners of the 3×3 unit). B signals are also interpolated in the similar manner to R signals.
Next, interpolation for G signal will be described. The center pixel in the unit is B signal which has no information on G signal. It is therefore necessary to interpolate G signal. Assuming that G signal at the center pixel is Gc signal, this Gc signal can be obtained through averaging by the equation (1).
Gc
=(
G
1
+
G
2
+
G
3
+
G
4
)/4  (1)
where G
1
, G
2
, G
3
, and G
4
signals are G signals at four pixels adjacent to Gc signal.
FIG. 11B
shows 3×3 image data containing one horizontal edge. Pixels with hatching in
FIG. 11B
indicate that they have larger signals than other pixels. If Gc signal at the center pixel is obtained through averaging of four pixels, the horizontal edge of the image data becomes unsharp. In order to avoid this, if image data contains a horizontal edge, Gc signal is obtained by the following equation (2).
Gc
=(
G
2
+
G
3
)/2  (2)
A method of judging whether or not 3×3 image data contains a horizontal edge will be described. If image data contains an edge and the following conditional formula (3) is satisfied, it is judged that the image data contains a horizontal edge.
|
G
1
−
G
4
|>|
G
2
−
G
3
|  (3)
FIG. 11C
shows 3×3 image data containing one vertical edge. Also in this image data, if Gc signal at the center pixel is obtained through averaging of four pixels, the vertical edge of the image data becomes unsharp. In order to avoid this, if image data contains a vertical edge, Gc signal is obtained by the following equation (4).
Gc
=(
G
1
+
G
4
)/2  (4)
A method of judging whether or not 3×3 image data contains a vertical edge will be described. If image data contains an edge and the following conditional formula (5) is satisfied, it is judged that the image data contains a vertical edge.
|G
1
−
G
4
|<|
G
2
−
G
3
|  (5)
In the examples of
FIGS. 11B and 11C
, the image data contains one edge. In this case, by obtaining GC signal by the equation (2) or (4), a resolution of the edge can be retained.
FIG. 11D
shows 3×3 image data containing two horizontal edges which are a horizontal edge between the first and second rows LN
1
and LN
2
and a horizontal edge between the second and third rows LN
2
and LN
3
. It is not possible to judge from the conditional formulas (3) and (5) whether the image data has either a horizontal edge or a vertical edge, because the conditional formulas (3) and (5) become |G
1
−G
4
|=|G
2
−G
3
|.
Even if none of the conditional formulas (3) and (5) are satisfied, it is not proper to obtain Gc signal from the equation (1). It is rather preferable in this case to judge that the image data contains a horizontal edge and to obtain Gc signal from the equation (2). If Gc signal is obtained from the equation (1), only Gc signal takes a value different from the other values on one line constituting the second row LN
2
.
If one edge is contained in image data, proper interpolation of G signal is possible. However, if image data contains two edges, proper interpolation of G signal is impossible.
If image data contains noises, discrimination between horizontal and vertical edges may lead to a false judgement. If interpolation is performed with a false judgement of an edge, a proper interpolation is impossible.
With improper interpolation, an edge (contour) becomes unsharp and the resolution of an image is lowered. A pseudo color (essentially absent) is developed lowering reproductivity of an image.
G signal contains a large amount of luminance components and greatly influences a resolution. If an interpolation precision of G signal is increased, the resolution of an image can be improved. From this reason, the abovedescribed interpolation depending upon an edge direction is performed in interpolating G signal.
In contrast to this, since R and B signals less influence the resolution, it has been considered that a simple interpolation method is sufficient. R and G signals have been interpolated therefore by simple averaging of adjacent pixels.
As the interpolation precision of G signal is improved, the relative interpolation precision of R and B signals lowers. Namely, G, R and B signals are unbalanced. There is a limit in improvement on the image quality if the interpolation precision of only G signal is improved. If the interpolation precision of not only G signal but also R and B signals is improved, the image quality can be expected to be improved further.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide image signal processing techniques capable of properly interpolating image signals having various patterns.
It is another object of the present invention to provide image signal processing techniques capable of properly interpolating image signals containing noises.
It is still another object of the present invention to provide image signal processing techniques capable of improving an interpolation precision of chrominance signals constituting an image.
According to one aspect of the present invention, there is provided an image signal processing apparatus comprising: first edge judgement means for judging from four pixels adjacent to an object pixel whether the object pixel constitutes a single edge; first edge direction judging means for judging whether the single edge is horizontal or vertical, if the first edge judging means judges that the object pixel constitutes the single edge; and interpolating means for interpolating the object pixel in accordance with at least right and left two pixels adjacent to the object pixel in a horizontal direction, if the first edge direction judging means judges that the single edge is horizontal, and for interpolating the object pixel in accordance with at least upper and lower two pixels adjacent to the object pixel in a vertical direction, if the first edge direction judging means judges that the single edge is vertical.
After it is judged whether the object pixel constitutes a single edge

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