JP2014053700A - Image pickup and image processing apparatus and image processing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce degradation of image quality in a screen center area in an image pickup apparatus that assumes distortion aberration by image processing.SOLUTION: Degradation of resolution performance of a screen center is prevented by specifying a residual distortion amount at a time of lens design, prioritizing image processing distortion correction on the periphery, limiting sharpness recovery processing in a meridional direction, and setting an amount of correction on the basis of a design value as a reference.

Description

  The present invention relates to an image processing apparatus and an image processing method, and more particularly to an image processing apparatus and an image processing method for processing a signal from an imaging apparatus having an axially symmetric imaging optical system such as a camera and an endoscope.

  In recent years, in an imaging apparatus such as a compact digital camera, in particular, on the wide-angle side of a zoom lens, the number of wide-angle lenses has increased, and the amount of distortion aberration that depends on the angle of view increases. Further, there is an increasing number of examples in which distortion aberration correction in the optical design of the photographing lens is reduced and distortion aberration correction is performed by image processing by digital image processing.

  In general, distortion correction by digital image processing is to correct barrel distortion generated at the wide-angle end by stretching an image. Hereinafter, the description will be limited to barrel distortion.

  As is well known, distortion is a phenomenon in which the lateral magnification changes nonlinearly with the image height. What is called barrel-shaped distortion occurring at the wide-angle end is such that the lateral magnification becomes non-linearly smaller than on the axis as the image height increases. By stretching the image so as to cancel out this distortion according to the amount of distortion, the distortion can be reduced by post-processing. As a result, since distortion remains in the lens, the necessity for correcting distortion in the lens design is reduced, and the degree of freedom in lens design is increased. The increased degree of freedom can be directed to downsizing and cost reduction, such as reducing the number of lenses and the number of aspheric surfaces used.

  On the other hand, since distortion correction by image processing is based on an image enlargement operation, there is a problem in that the resolution performance is degraded.

  Distortion correction by image processing is mainly performed by coordinate conversion by image stretching in the meridional direction and interpolation processing associated therewith. At this time, the amount of enlargement increases nonlinearly as the image height increases as described above. The reduction in resolution performance increases according to the amount of enlargement. Therefore, the lower the resolution performance becomes, the higher the image height, that is, the peripheral portion of the image. In order to solve this problem, there has been proposed a solving means for increasing the aperture correction amount on the high image height side (Patent Document 1).

  In addition, such a wide-angle lens generally tends to deteriorate image quality due to the influence of astigmatism at the periphery of the screen, that is, at a high image height. The indication of this subject and the proposal of the solution means to perform the process which differs the aperture correction amount by the meridional direction and the sagittal direction are made | formed (patent document 2).

  However, when the distortion is gradually increasing from the axis, there is a problem that the resolution is already deteriorated from the vicinity of the center of the screen by performing the distortion correction by the image processing.

  Further, as shown in Patent Document 1, when the correction amount of the aperture correction process is increased on the high image height side in order to alleviate the influence of the distortion correction by the image processing, the correction is performed in the sagittal direction. It has been found that it becomes excessive and has a negative effect on image quality.

  On the other hand, as shown in Patent Document 2, the imaging performance difference based on the difference between the F-number of the off-axis light flux of the optical system and the sagittal direction, and the image plane difference between the meridional direction and the sagittal direction due to astigmatism. Different edge emphasis processing is applied in the meridional direction and the sagittal direction to the resolution degradation caused by defocusing off-axis. In this case, since it is affected by the individual manufacturing variation of the optical system, it is necessary to apply conditions suitable for each individual.

JP 2004-242125 A JP 2006-21112 A

  The present invention has been made in view of the above-described facts, and an object of the present invention is to propose an imaging apparatus capable of reducing a decrease in resolution performance near the center of the screen by correcting distortion by image processing. To do.

  Distortion is proportional to the cube of the image height in the third-order aberration region. Therefore, when distortion aberration correction is performed by image processing, the image is gradually stretched from the center of the screen, and is affected by resolution degradation from the vicinity of the center of the image, in addition to the periphery of the screen with a large stretch amount.

  As a result, there is a problem that the image quality is somewhat sleepy even near the center of the screen, and only a clear image can be obtained on the entire screen.

  In the method according to the present invention, a condition is added to the barrel distortion remaining in the wide-angle imaging lens while suppressing an increase in image processing load, and a resolution performance recovery unit suitable for the condition is set.

The configuration of the image processing apparatus according to the present invention is as follows.
An imaging optical system in which barrel distortion remains optically, and an image processing apparatus that processes a signal from an imaging apparatus having the imaging optical system,
It has a function of correcting distortion by correcting distortion by image processing,
Distortion as a function of image height,

When expressing
The barrel distortion remaining in the optical system is
-3% ≦ dist, up to 50% of image height
-20% ≦ dist ≦ −10% at 90% of image height
Because
For images that have been subjected to distortion correction by image processing, image sharpening processing such as aperture correction processing or edge enhancement processing is performed only in the meridional direction, image sharpening processing is not performed in the sagittal direction, and the correction amount only in the meridional direction is It is uniquely determined based on the amount of residual distortion at the time of design.

  The effects of the present invention can provide an imaging apparatus that suppresses a decrease in resolution particularly in the center area of an image and does not cause overall deterioration in image quality even when distortion correction is performed by image processing.

Explanatory drawing which shows the characteristic of the distortion aberration according to 1st Embodiment of this invention Explanatory drawing explaining the concept of distortion correction

<Basic configuration of the embodiment>
Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

  The description about the Example of this invention is described below.

  An embodiment of the present invention will be described with respect to a configuration that can suppress degradation in resolution near the center of the screen due to distortion aberration correction in a photographic lens premised on distortion aberration correction by image processing.

  When the third-order aberration component is dominant, the distortion aberration of the photographing lens is proportional to the cube of the image height (field angle). Therefore, in this case, distortion starts to occur relatively near the center of the screen and increases toward the periphery of the image. When distortion correction by image processing is applied to such a lens, the vicinity of the center of the screen has already been stretched, resulting in degradation of resolution performance.

  In the configuration according to the present invention, firstly, at the time of designing a lens, residual distortion at a low image height is suppressed, and design constraints are relaxed for high-order distortion on the high image height side.

  For this purpose, the use of an aspherical lens is premised, but the design freedom is directed to control of higher-order distortion aberrations of the fifth-order aberration component or more, and the design is made to reduce distortion on the low image height side. On the other hand, the absolute amount of distortion remaining on the high image height side, such as the diagonal end of the screen, may increase somewhat.

  In general, distortion is a function of image height,

It is represented by

  The amount of distortion (dist) is generally a nonlinear function related to image height.

  The distortion with an image height of 50% or less is set to -3% or less (close to 0%). This is shown in FIG. 1 as a longitudinal aberration diagram. In FIG. 1, the horizontal axis represents the distortion amount [%] expressed by the following equation, and the vertical axis represents the image height ratio.

  Since there is no need to design to suppress distortion as a whole, it is not a problem that the residual distortion at 90% of the image height is larger than -10%. However, this is not indefinitely acceptable, and when the degree of barrel distortion becomes greater than −20%, the peripheral image quality deteriorates significantly.

  Distortion correction by image processing, that is, processing for extending the lateral magnification smaller than the center of the screen on the higher image height side due to distortion to the outside of the screen in the meridional direction. In that case, since the low image height region such as an image height of 50% or less remains in a very slight enlargement process, the influence of the reduction in the resolution performance at the center of the screen is reduced.

  On the other hand, a process of enlarging the area outside the screen with a high image height is necessary. In this case, since the resolution performance is reduced due to the enlargement, the resolution performance is compensated by a filter process such as aperture correction / edge enhancement.

  As described above, distortion correction by image processing is a process of stretching and interpolating an image in the meridional direction according to the amount of distortion. As a result, the image is mainly stretched and degradation of the resolution performance occurs in the meridional direction, and the amount stretched in the sagittal direction is considerably smaller than that in the meridional direction.

  As shown in FIG. 2, when 1: point A at the image height r is moved to the position of the image height r ′ for movement, the 1: point A after the movement is set to 3: point A ′. On the screen, consider 1: point B of image height r, which is a pixel adjacent to point A, and 4: point B ′ after enlargement, corresponding to point B: 1: point A and 2: point B The azimuth angle for this is 7: θ. The amount of movement in the meridional direction between 1: point A and 3: point A ′ (and 2: point B and 4: point B ′) before and after stretching is r−r ′. On the other hand, the change between the distance of 1: point A-2: point B in the sagittal direction and the distance of 3: point-A′4: point B ′ after stretching is θ · (r−r ′). 1: Point A-2: Assume that point B is an adjacent pixel. For example, when calculated under the conditions of an actual compact digital camera, 7: θ is about 0.4 to 0.6 [mrad] with an image height of about 80%. In contrast to the stretching in the meridional direction, the sagittal stretching amount is 0.04 to 0.06 [%], which is almost negligible.

  Accordingly, since the influence of stretching occurs only in the meridional direction, it is sufficient to perform aperture correction / edge enhancement processing for correcting the influence of stretching for azimuth only in the meridional direction.

  Furthermore, the resolution performance degradation caused by the enlargement process will be described.

  When barrel distortion is left in the imaging optical system, an image is formed at a higher spatial frequency because the image is formed at a low magnification at a high image height. On the other hand, since the pixel interval of the image sensor is constant, it is equivalent to sampling at a coarser sampling interval after the enlargement process. Therefore, the limit resolution characteristic of the image sensor is deteriorated. Further, the formation of an image with a higher spatial frequency causes a reduction in resolution performance for the MTF characteristics of the optical system.

  In the present invention, the wide-angle residual barrel distortion of the imaging lens system is designed to be in a range that does not require correction up to an image height of 50%, and the enlargement process is performed for an image height of 50% or more. Further, aperture correction / edge emphasis processing is applied only to the meridional direction for an area having an image height of 50% or more. The application amount uses a condition that is uniquely determined from the distortion aberration correction amount.

r Image height

Claims (1)

An imaging optical system in which barrel distortion remains optically, and an image processing apparatus that processes a signal from an imaging apparatus having the imaging optical system,
It has a function of correcting distortion by correcting distortion by image processing,
Distortion as a function of image height,
When expressing
The barrel distortion remaining in the optical system is
-3% ≦ dist, up to 50% of image height
-20% ≦ dist ≦ −10% at 90% of image height
Because
The image subjected to distortion correction by image processing is subjected to image sharpening processing of aperture correction processing or edge enhancement processing only in the meridional direction, and image sharpening processing is not performed in the sagittal direction.
An imaging and image processing apparatus characterized in that the correction amount only in the meridional direction is uniquely determined based on the amount of residual distortion at the time of design.