Image capturing apparatus provided with image processor

Abstract

An image capturing apparatus capable of performing a suitable illuminance nonuniformity correction by setting a suitable γ-characteristic every block even in the case that the size of a character image projected on a sensing surface changes according to an image capturing magnification, and capable of extracting a boundary area between a white board portion and a background portion and applying a suitable image processing to this boundary area during the illuminance nonuniformity correction for a picked image, and capable of image capturing a representation such as characters drawn on a white board in such a manner that an obtained image is clear and easy to see by suitably performing an illuminance nonuniformity correction even in the case of color image capturing, and capable of detecting a regularly reflected light with high accuracy and can thereby securely prevent an error of obtaining an image of low quality by image capturing, and capable of preventing an error in flash-image capturing a representation such as characters drawn on a white board under insufficient illumination light and effectively perform an illuminance nonuniformity correction.

Description

[0001] This application is based on patent application Nos. 9-12999, 9-13000, 9-13001, 9-13002, 9-13003, 9-13004, 9-13005, 9-13006, 9-13019, 9-13020, and 9-13021 filed in Japan, the contents of which is hereby incorporated by reference. BACKGROUND OF THE INVENTION [0002] This invention relates to an image capturing apparatus capable of picking up a light image of an object by an image pickup device such as CCD (Charge Coupled Device) by photoelectrically converting it into an electric image and storing it in a storage medium such as a hard disk card after applying a specified image processing thereto. [0003] There have been known electronic cameras as image capturing apparatus. Electronic cameras have, as compared with conventional cameras which record light images on silver halide film, an advantage of picking up images of satisfactory quality by suitably applying an image quality processing to the picked up images according to im age capturing purposes and types of objects since t...

AI Technical Summary

Benefits of technology

[0055] According to another aspect of the present invention, a method for controlling an image capturing apparatus provided with an illuminance nonuniformity corrector for performing an illuminance nonuniformity correction to obtained image data, and a flash device for emitting flash light to an object, the method comprising the step of prohibiting flash light emission of the flash device when the illuminance nonuniformity correction is designated.

Problems solved by technology

Specifically, if the white board is assumed to be illuminated by ceiling lights of the room and sunlight coming through the windows, illuminance nonuniformity occurs due to a nonuniform illumination light.

If the block size in relation to the character size is improper, a suitable γ-characteristic cannot be obtained.

The convex portion corresponding to the white portion of the histogram of level-frequency distribution generated using the pixel data in the block becomes small and it is difficult to determine the white saturation level based on this convex portion.

Thus, it is difficult to stably set the white saturation level based on this convex portion.

Thus, there is a likelihood that the white level is erroneously detected based on the convex portion corresponding to the background portion.

This leads to a disadvantage that the white board portion of the block including the background portion turns black.

In the case that the white board is captured together with its background, the image quality is considerably reduced, making the image hard to be seen if the coloring phenomenon occurs in the white board portion in a boundary area between the white board portion and the background portion during the image processing, namely the illuminance nonuniformity correction.

Accordingly, the illuminance nonuniformity correction may not be performed even if the conventional γ-correction technique is applied to digital cameras.

Thus, even if this technique is applied to a picture image where the illuminance nonuniformity occurs in a two-dimensional manner, it is difficult to effectively correct the illuminance nonuniformity along main scanning direction.

However, since the histogram of level-frequency distribution of the pixel data is generated every block and the threshold value of the binary processing is set by neural network using the histogram of level-frequency distribution data, a complicated and cumbersome calculation is disadvantageously required to set the threshold value.

If the block size is set too small, the histogram of level-frequency distribution of the pixel data is improper and a suitable threshold value cannot be set.

Further, since a long time is disadvantageously required for the calculation due to a huge number of blocks, there should be a limit in the number of blocks.

Further, a calculation made to avoid the discontinuity of the image quality resulting from a difference between the processings applied to the blocks using different γ-characteristics is not easy.

Thus, a problem of the coloring phenomenon in the boundary area between the white board portion and the background portion is not considered at all and, hence, there is no indication or disclosure concerning this problem.

As a result, the illuminance nonuniformity correction causes a problem of coloring the white portion.

They neither disclose nor indicate the illuminance nonuniformity correction technique for a color image and the problem in the illuminance nonuniformity correction of the color image.

When characters and / or figures drawn on a white board in a conference hall are to be captured by an electronic camera provided with a built-in flash, the built-in flash is often automatically fired because only an insufficient amount of illumination light is normally available, thereby resulting in flash image capturing.

Thus, this case also leads to a similar reduction in the image quality.

Therefore, the illuminance nonuniformity correction cannot be effectively performed, and the regularly reflected light adversely affects the blocks which are around this block, but do not include the regularly reflected light.

In known image forming apparatuses such as digital copying machines, if illumination light is regularly reflected by a document, the density of characters or the like written on the document is considerably reduced by this regularly reflected light and a document image cannot be accurately picked up.

However, with electronic cameras, the illumination condition of the illumination light is not constant and an external light such as a sunlight is incident on the white board as a spot light and regularly reflected.

Thus, if the detection is made every line as with the known method for detecting the regularly reflected light in the digital copying machines, it is difficult to securely detect a spot regularly reflected light and a sufficiently satisfactory detection accuracy cannot be ensured.

The binary processing technique disclosed in Japanese Unexamined Patent Publication No. 6-113139 also mainly concerns a binary processing in copying machines and facsimile machines, and does not at all disclose the aforementioned problem of the regularly reflected light peculiar to the image capturing of the digital camera and the method for avoiding such a problem.

Such a distortion reduces the readability of the representation.

Thus, the white level becomes discontinuous due to a difference of the γ-characteristics between neighboring blocks, making it difficult to perform a proper illuminance nonuniformity correction and, depending on a case, leading to the creation of a pseudo line at the boundary of the blocks.

On the other hand, there are some cases where the illuminance nonuniformity correction cannot be properly performed even if the illuminance nonuniformity correction is performed after the oblique image correction.

Accordingly, if the histogram of level-frequency distribution of the block including the dummy data is generated during the illuminance nonuniformity correction, the obtained histogram of level-frequency distribution cannot be accurate because of the presence of the dummy data.

This leads to an improper γ-characteristic for the block including the dummy data.

Thus, the γ-correction cannot be properly applied to the image in this block, and the white level becomes discontinuous between neighboring blocks, thereby creating a pseudo line at the boundary of the blocks.

However, this method has a disadvantage that an effective γ-characteristic cannot be obtained for a block having a small number of effective pixel data despite a complicated processing of extracting the pixel data.

The above problem occurs not only in the case of correcting the geometric distortion of the image obtained by image capturing the object in the oblique direction, but also in the case of correcting a geometric distortion resulting from the characteristic of an image pickup optical system.

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