JP3125256B2 - Digital still video camera - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital still video camera, and more particularly to an autofocus technique for a digital still video camera having a data compression function by orthogonal transformation.

[0002]

2. Description of the Related Art It is common knowledge that recent image pickup apparatuses are equipped with an autofocus function as standard equipment. And
Since an external distance measuring sensor or the like is unnecessary, an autofocus system that determines focusing from a frequency component of a video signal is becoming mainstream. Such a conventional autofocus system will be described with reference to FIG.

A half-pressing operation of a release switch having a two-stage switch structure of switches S1 and S2 (only switch S1 is ON)
) Starts the auto focus control. First, a high-frequency component of a video signal obtained through a lens 1, a CCD 2 serving as an image sensor, and a process circuit 3 for performing Y / C separation processing and the like is extracted by a high-pass filter 4 and integrated by an integration circuit 5, and then integrated. The integrated value is converted to a digital value by the A / D converter 6. At the time of the first video, this value is output to the system controller 9 via the comparator 8, and the lens 1 is moved by the lens driving circuit 10 based on the output from the system controller 9 according to the input value. Also, A
The digital value after the / D conversion is stored in the internal memory 7. Subsequently, the comparator 8 compares the integrated digital value of the high frequency component of the video signal taken in with the movement of the lens 1 with the value stored in the internal memory 7, and the value of the newly taken video signal is increased. For example, the lens is moved in the same direction to continue the focusing operation. The largest value up to that point is stored in the internal memory 7. By repeating this operation, the lens position at which the high-frequency component value of the video signal becomes the maximum is determined to be the focus position. Record the video on 13.

In recent years, in a digital still video camera for recording a digital image in a storage device such as a semiconductor memory, orthogonal transform coding, which is one of data compression techniques, is used to save the capacity of the storage device. Some image data are compressed (for example, Japanese Patent Application Laid-Open No. 1-24).
No. 8785). The orthogonal transform coding divides an entire image represented by digital image data obtained by imaging into a plurality of blocks, and performs an orthogonal transform on each block using, for example, a discrete cosine transform (hereinafter, referred to as DCT). Then, the obtained frequency information (which is called a transform coefficient) is quantized and encoded to compress the data and store it in the storage device.

[0005] Therefore, it has been proposed to perform auto-focusing using frequency information obtained in the process of data compression by the orthogonal transform coding (ITEJ Technical Revision).
po-rt Vol.14, No.32, P31-36, June.1990). This calculates the average value of the high frequency component of each block based on the frequency information of each block obtained by applying the DCT to the taken video signal, and calculates the average value of the average high frequency component value of all the blocks. By doing so, the high-frequency component value of the entire image is obtained, and the lens position at which this value is the maximum is determined as the in-focus position.

[0006]

However, as in the above-described conventional autofocus system using DCT, focusing is performed based on the average value of the high frequency components of the entire image, that is, all blocks on the screen. There are the following problems. That is, since the exposure of the camera is determined in accordance with the main subject, there are portions of the captured image which are too bright or too dark where the exposure is not always optimal. Since such a portion is not the main subject, it is desired that the high-frequency component value does not affect the focus control. However, in the conventional method targeting high-frequency components of all blocks, high-frequency component values of blocks different from the actual main subject are included, and there is a problem that a high-precision autofocus system cannot be obtained. .

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a digital still video camera having a data compression function based on orthogonal transform coding, which provides a more accurate autofocus system than before.

[0008]

SUMMARY OF THE INVENTION For this reason, the present invention has been described with reference to FIG.
Data compression means for digitizing image data taken in through a lens and an image sensor, dividing the digital image data into a plurality of blocks, and performing orthogonal transformation for each block to compress the data as shown in FIG. In the digital still video camera provided with the above, it is determined whether or not the value of the DC component is within a predetermined range set for each block based on the frequency information of each block obtained by the orthogonal transform processing in the data compression means. Means,
High-frequency component value calculating means for calculating the magnitude of high-frequency component values of all blocks whose DC component values are within the predetermined range based on the determination result of the determining means; and a high-frequency component calculated by the high-frequency component value calculating means. Control means for variably controlling the lens position so that the value is maximized.

[0009]

In the above construction, the data compression means digitizes the captured image data, divides the digital image data into a plurality of blocks, and performs an orthogonal transform for each block to obtain a transform coefficient as frequency information for each block. obtain. Then, the data is compressed by quantizing and encoding the transform coefficients.

[0010] Based on the frequency information of each block obtained in the data compression process, the determination means determines, for each block, whether or not the DC component of the block is within a predetermined range set in advance. The high frequency component value calculating means is
Except for blocks where the DC component value is extremely large or small, the magnitude of the high frequency component value is calculated only for all the blocks for which the DC component is determined to be within the predetermined range by the determination means. The control means controls the lens position based on the calculated high-frequency component value so that the high-frequency component value is maximized, and sets this position as a focus position.

[0011]

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 2 showing the configuration of the present embodiment, an optical image of a subject obtained through a lens 21 of an optical system is formed on an image sensor, for example, a CCD 22. CCD22,
The optical image is converted into an analog electric image signal by a transfer pulse from the CCD drive circuit 23 and output to the process circuit 24. The process circuit 24 converts the input analog image signal into a video signal of a luminance signal (Y) and a color signal (C) and outputs the video signal to the A / D converter 25. The A / D converter 25 converts an analog image signal into a digital image signal. This digital image signal is temporarily stored in a small-capacity buffer memory 26. A compression circuit 27 as a compression unit divides the entire image into blocks of predetermined pixels (for example, 8 × 8 pixels), performs orthogonal transform by DCT for each block, and obtains frequency information (transform coefficients) for each block. The data is compressed by performing quantization and encoding. Each of these operation timings is controlled by a timing signal from a system controller 28 containing a microcomputer. Further, the system controller 28 performs the auto focus control based on the DCT frequency information, which is a feature of the present embodiment, by turning on the switch S1 of the release switch 29. That is, frequency information obtained in the compression process of the compression circuit 27 is input, and it is determined whether or not the DC component of each block is within a predetermined range. Only the value obtained by adding the high-frequency component values for each block calculated by the DCT calculation is divided by the number of blocks to calculate the average value of the high-frequency components of all the blocks whose DC component values are within a predetermined range. Lens through the lens drive circuit 30 to maximize the value
Focus is achieved by moving 21. The memory 31 stores the maximum value of the high frequency component in the focusing process. Here, the system controller 28 corresponds to the determination means and the high frequency component value calculation means,
28 and the lens drive circuit 30 constitute control means.

When the in-focus state is determined, the image data compressed by the compression circuit 27 is recorded in an external memory such as a card memory 32 by turning on the switch S2 of the release switch 29. Next, the autofocus control operation of the present embodiment will be described in detail with reference to the flowchart of FIG.

When the ON signal of the switch S1 is input to the system controller 28 by operating the release switch 29, the CCD 22 is driven by the CCD drive circuit 23 to start capturing images, and the auto focus control operates. First, in step 1 (referred to as S1 in the figure, the same applies hereinafter), the captured video signal is converted into digital image data by the A / D converter 25 and temporarily stored in the buffer memory 26.

In step 2, the compression circuit 27 compresses the digital data in the buffer memory 26. Such compression processing divides the entire image represented by the image data into a plurality of blocks each having a block size of N × N pixels (for example, 8 × 8 pixels), and then orthogonally transforms each block by DCT. Conversion is performed, thereby obtaining a conversion coefficient as frequency information from the pixel data.
Further, data compression is performed by sequentially performing data quantization and encoding.

In step 3, the DC component value indicating the average luminance of each block is compared with a preset lower limit value and upper limit value based on the frequency information for each block obtained in the compression process of step 2. Thus, it is determined whether or not the DC component value of each block is within a predetermined range. In this determination, a block having a DC component value within a predetermined range is determined to be an image having an appropriate exposure, and a block having a DC component value larger than an upper limit value or a block having a DC component value smaller than a lower limit value is determined as an image having an inappropriate exposure. This is to judge.

In step 4, only the blocks for which the DC component value is determined to be within the predetermined range in step 3 are extracted from each block for the value of the high-frequency component in a specific frequency range. In step 5, the average of the high-frequency component values of each block obtained in step 4 is calculated. For example, if there are M blocks whose DC component values are within a predetermined range, the average high-frequency component value of each block is represented by A ₁
ＡA _M , the average value is (A ₁ + A ₂ +...
+ A _M ) / M.

In step 6, it is determined whether or not the flag F is 0. If F = 0, the image is determined to be the first image, and step 7
Proceed to. In step 7, the lens 21 is moved by a small amount according to the average value of the high-frequency components calculated in step 5.
In step 8, the average value of step 5 is stored in the memory 31.

At step 9, the flag F is set to "1". Next, a new image is captured, and the operations from Step 1 to Step 5 are performed again. And
Since the flag F is 1 in the second and subsequent image capturing, the determination in step 6 is NO and the process proceeds to step 10.

In step 10, the new high-frequency component value calculated in step 5 is compared with the previous high-frequency component value stored in the memory 31. In step 11,
The peak determination of the high-frequency component value is performed based on the comparison result of step 10. That is, the magnitude of the previously stored high-frequency component value is compared with the value calculated this time, and if the current value is large, it is determined that the peak is not reached, and the process proceeds to step 7, where the lens is moved by a small amount and this value is stored. Then, the image capturing operation is repeated again, and the value obtained again is compared with the stored maximum value up to the previous time. When the newly calculated high-frequency component value becomes smaller than the previous value, the previous value is determined to be the maximum value, and peak detection is performed. When a peak is detected, the process proceeds to step 12.

In step 12, the focus is determined based on the detection of the peak value of the high frequency component value. In step 13, the release switch 29 based on the focus determination in step 12
When the switch S2 is turned on, the compressed image data is written to the card memory 32.

As described above, first, it is determined whether or not the DC component value of each block in the compression process is within a predetermined range. Except for blocks having extremely large and small DC component values, blocks in the predetermined range are excluded. If the focus determination is performed based on only the high-frequency component values, the focusing operation is performed based on the data of a sharply exposed image portion, and the change in the high-frequency component value when the focus deviates from the focal point is large. The focus can be easily detected, and the accuracy of the obtained high-frequency component value is high, and the accuracy of the autofocus is improved.

In such a still video camera having a data compression function by orthogonal transform encoding, when only the switch S1 of the release switch is pressed,
Sequences such as autofocus and exposure control, storage in an internal memory, and orthogonal transformation processing in a compression process are performed. Simultaneously, when a switch S2 is pressed, compression processing such as quantization and encoding is executed to store an image in an external memory. Is recorded, the response from when the photographer presses the release switch to when the image is recorded in the external memory is quickened, and the power consumption can be suppressed.

[0023]

As described above, according to the present invention, when focusing is performed by controlling the lens position so that the high-frequency component of the compressed data obtained by the orthogonal transform coding is maximized, the DC component in the compressed data is reduced. Since high-frequency components of blocks that are extremely large or small and are not in a predetermined range are excluded, the reliability of data used for focus determination can be increased, and the control accuracy of autofocus can be improved. In addition, since focus determination is performed using only clear image data with proper exposure, a change in high-frequency component data when the focus is deviated from the focus is large, and the focus position can be easily detected.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of the present invention.

FIG. 2 is a schematic configuration diagram showing an embodiment of the present invention.

FIG. 3 is a flowchart illustrating a focusing operation of the embodiment.

FIG. 4 is a schematic configuration diagram showing an example of a conventional autofocus system.

[Explanation of symbols]

21 Lens 22 CCD 25 A / D converter 27 Compression circuit 28 System controller 30 Lens drive circuit 31 Memory

Continued on the front page (72) Inventor Yuki Sakai 2970 Ishikawacho, Hachioji-shi, Tokyo Konica Corporation (72) Inventor Katsuya Nagaishi 2970 Ishikawacho, Hachioji-shi, Tokyo Konica Corporation (72) Inventor Tadaaki Yoneda 2970 Ishikawacho, Hachioji-shi, Tokyo Konica Corporation (72) Inventor Shuji Hayashi 2970 Ishikawacho, Hachioji-shi, Tokyo Konica Corporation (56) References JP-A-4-151981 (JP, A) (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/232 G02B 7/28

Claims (1)

(57) [Claims] A digital still video camera having data compression means for digitizing captured image data, dividing the digital image data into a plurality of blocks, and performing orthogonal transformation for each block to compress the data. A determination unit that determines whether a value of a DC component is within a predetermined range set in advance for each block based on frequency information of each block obtained by the orthogonal transformation process in the data compression unit; and High-frequency component value calculating means for calculating the magnitude of the high-frequency component values of all blocks whose DC component values are within the predetermined range based on the determination result;
A digital still video camera, comprising: control means for variably controlling a lens position so that a high-frequency component value calculated by said high-frequency component value calculation means is maximized.