JP3176650B2 - Auto focus method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an auto-focus system, and more particularly to an auto-focus system in which any object image is always detected with high accuracy and contrast information is automatically controlled.

[0002]

2. Description of the Related Art In an autofocus system, a subject image obtained through an optical system such as a lens is converted into an electric signal, and contrast information is detected from the electric signal so that the detected contrast information value is maximized. There is one that performs feedback control of the position of an optical system. Autofocus control by such a method is used for a video camera or the like.

FIG. 4 shows a configuration of a conventional still video camera. The position of the lens 2 is controlled by the lens driving circuit 1, and the image of the subject is captured by the CCD 3 serving as an image sensor.
Image. The image signal converted into an electric signal by the CCD 3 is subjected to processing such as γ correction and color separation in an imaging process circuit 4, and is converted into digital data by an A / D converter (ADC) 5. Will be recorded. As is well known, block data of N pixels (for example, 8 pixels) is read from the buffer memory 6 and subjected to orthogonal transform processing in a DCT (discrete cosine transform) circuit 8. The orthogonal transform is performed by the DCT circuit 8,
The obtained direct current (DC) conversion coefficient is supplied to the quantization circuit 9, while the alternating current (AC) conversion coefficient is supplied to the quantization circuit 10.
And the lens AF control circuit 40. Lens AF
The control circuit 40 receives the AC conversion coefficient, controls the lens driving circuit 1 to move the lens 2, controls the position of the lens 2 so that the AC conversion coefficient obtained by the DCT circuit 8 becomes maximum, and focuses ( Focus) to control. The pulse generation (SSG) circuit 7 includes a CCD 3, an imaging process circuit 4,
A / D converter 5, buffer memory 6, and lens A
Various pulses such as horizontal and vertical synchronization signals for controlling the F control circuit 40 are generated.

The DC conversion coefficient quantized by the quantization circuit 9 is supplied to a delay circuit 11 and a subtraction circuit 12, and a prediction error is obtained as an output of the subtraction circuit 12. This prediction error is encoded by the encoding circuit 14 and output to the synthesizing circuit 16. On the other hand, the AC conversion coefficients quantized by the quantization circuit 10 are subjected to coefficient rearrangement processing by a so-called zigzag scanning circuit 13, then encoded by an encoding circuit 15, and output to a synthesis circuit 16. The prediction error of the DC conversion coefficient and the AC conversion coefficient synthesized by the synthesis circuit 16 are stored in the recording device 1.
7 recorded. The entire sequence of these components is controlled by the system control circuit 18. The DCT circuit 8 is often used at the time of image encoding, and if contrast information is detected by the DCT circuit,
Since the DCT circuit can be shared, the configuration is simplified and the cost is reduced.

[0005]

As described above, the detection of contrast information in a conventional still video camera or the like is based on an AC conversion coefficient obtained by orthogonally transforming image data. Autofocus control is performed by controlling the lens position so as to maximize the conversion coefficient. However, in the conventional method, as described below, effective contrast information cannot be obtained depending on the type and pattern shape of a subject image, and therefore, high-accuracy and stable autofocus control cannot be realized. That is, for example, when the image pattern as shown in FIG. 5 is a subject, the horizontal signal of the obtained image signal changes in level as shown in FIG. As can be seen from FIG. 8, there is a signal level change in the block when out of focus than when in focus. In other words, the AC conversion coefficient is obtained when the subject is out of focus. Also, the level of the AC conversion coefficient is also 0 because the level change in the block becomes 0 at the time of focusing. Then, just before focusing, the level change in the block becomes sharp, so that the level of the AC conversion coefficient becomes maximum.

The relationship between the lens position and the level of the AC conversion coefficient, that is, the contrast, when the subject image is an image pattern having a pattern corresponding to the divided block as shown in FIGS. 6 and 7 including FIG. Is as shown in FIG. As is apparent from FIG. 9, no effective contrast peak value can be obtained at the focal point, so that focusing cannot be performed.
In addition, since the peak value suddenly drops to 0 and rapidly shifts to the peak value, autofocus control becomes unstable, and high-precision and stable autofocus becomes impossible.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an autofocus system capable of always detecting contrast information with high accuracy and performing autofocus control for a subject image having any pattern. Another object of the present invention is to provide a subject image having any pattern,
An object of the present invention is to provide an autofocus method capable of always performing highly accurate and stable autofocus.

[0008]

In order to solve the above-mentioned problems, an autofocus system according to the present invention divides image data into a plurality of block data and obtains a first block obtained by orthogonally transforming each divided block data. The DC conversion coefficients obtained by the orthogonal transformation are collectively referred to as one block data, and the AC conversion coefficient obtained by orthogonally transforming the one block data again is referred to as a second AC conversion coefficient. Detecting the first AC conversion coefficient and the second
And performs autofocus control using the selected AC conversion coefficient as contrast information of the image.

[0009]

According to the present invention, DC conversion coefficients obtained by orthogonally transforming a plurality of block data of image data are combined into one block data, and an AC conversion coefficient obtained by orthogonally transforming this one block data is obtained. Detected as contrast information. Further, a first AC conversion coefficient obtained for each block data is detected in advance, and a second AC conversion coefficient obtained by performing orthogonal transformation on one block data in which the DC conversion coefficients are collected is detected.
Autofocus control is performed by selecting one of the first AC conversion coefficient and the first AC conversion coefficient and using it as contrast information.

[0010]

Next, the present invention will be described with reference to the drawings. In the case of an image pattern as shown in FIGS.
The DC conversion coefficient of a block obtained by orthogonal transformation has a large level difference between adjacent blocks. Therefore, the DC conversion coefficients of each block are detected, these are collected, a new block is generated, and the AC conversion coefficient obtained by orthogonally transforming the generated block is represented by the image patterns shown in FIGS. Even if it is, it is not 0. The present invention uses the AC conversion coefficient thus obtained as contrast information. For example, in the autofocus method, the above-described AC conversion coefficient becomes the maximum value at the time of focusing, and if this is used, more reliable autofocus control can be performed.

FIG. 1 is a block diagram showing an embodiment of a still video camera to which an autofocus system according to the present invention is applied. In FIG. 1, components denoted by the same reference numerals as those in FIG. 4 have configurations having the same functions, and thus detailed description will be omitted. During a period in which a valid signal (valid scanning selection) is output from the CCD 3, a switch 19 is used to perform orthogonal transformation processing on input image data (block data).
Is set to the terminal b. The block data read from the first buffer memory 6 in block units is supplied to the DCT circuit 8 via the terminal b of the switch 19. DC
The T circuit 8 orthogonally transforms the block data from the switch 19 and sends the obtained DC conversion coefficient to the quantization circuit 9 and also sends it to the second buffer memory 21. The DC conversion coefficient read from the buffer memory 21 is supplied to the terminal a of the switch 19, and is supplied again to the DCT circuit 8 via the switch 19. The DC conversion coefficient obtained by the DCT circuit 8 is supplied to the quantization circuit 9 and the buffer memory 21, and the AC conversion coefficient is supplied to the quantization circuit 10 and the lens AF control circuit 20. The above period ends,
In the vertical blanking period, the switch 19 is set to be switched to the a side. At this time, the buffer memory 21
Thereafter, the DC conversion coefficient of each block is supplied to the DCT circuit 8 via the terminal a of the switch 19. DC
The T circuit 8 further performs an orthogonal transform process on the DC transform coefficients of these blocks. Here, the obtained AC conversion coefficient indicates a value corresponding to the level change between blocks as described above.

FIG. 2 shows a timing chart of the operation processing. During the effective scanning line period indicated by the signal VBLK, the orthogonal transform process of the input image by the DCT circuit 8 and the process of writing the DC transform coefficient into the buffer memory 21 are performed.
An orthogonal transformation process is performed on the data obtained by summing up the DC conversion coefficients read from. The DCT circuit 8 supplies the lens AF control circuit 20 with two types of AC conversion coefficients (a first AC conversion coefficient K1 obtained from input image block data, and a data obtained by summing up DC conversion coefficients of each block). The obtained second AC conversion coefficient K2) is output with a time lag.

FIG. 3 shows the relationship between the lens position and the contrast of the first AC conversion coefficient K1 and the second AC conversion coefficient K2 obtained for the image patterns shown in FIGS. I have. Of the AC conversion coefficients input at a time lag as described above, first, the first AC conversion coefficient K1 is treated as contrast information, and focus control of the lens is performed. In FIG. 3, the lens shifts from the initial position A to the point B according to the movement of the lens. In the next step, the lens position is E
Move to the point and become the minimum point. However, at this lens position, the second AC conversion coefficient K2 shows a peak value. When the first AC conversion coefficient K1 shifts from the point B to the point E, and when the second AC conversion coefficient K2 reaches the point C, the second AC conversion coefficient K2 is treated as contrast information, and is determined to be in focus. To stop lens movement. The second buffer memory 2
1 only needs to store the DC conversion coefficient for one screen,
When the orthogonal transformation processing is performed on a block of 8 pixels in the vertical and horizontal directions, the capacity of the order of the order of the orthogonal transformation processing for the frame memory for one screen is reduced.
Only one-half the capacity is required.

[0014]

As described above, in the present invention, the DC conversion coefficients obtained for each block data are collectively subjected to the orthogonal transformation processing, and the obtained AC conversion coefficients are also used as contrast information. Optimal contrast information can be always obtained for an image pattern, and reliable auto focus control can be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of a still video camera to which an autofocus method according to the present invention is applied.

FIG. 2 is a timing chart of the operation in the embodiment of FIG.

FIG. 3 is a diagram showing a relationship between a lens position and contrast in the embodiment of FIG.

FIG. 4 is a configuration block diagram including autofocus control of a conventional still video camera.

FIG. 5 is a diagram showing a specific image pattern that causes a problem when detecting contrast information by a conventional method in autofocus control based on contrast information.

FIG. 6 is a diagram illustrating another specific image pattern that causes a problem when detecting contrast information according to a conventional method in autofocus control based on contrast information.

FIG. 7 is a diagram showing still another specific image pattern that causes a problem when detecting contrast information according to a conventional method in autofocus control based on contrast information.

8 is a diagram showing a level change of a horizontal signal of the image pattern signal of FIG. 5;

FIG. 9 is a diagram illustrating a relationship between a lens position and contrast when a subject image is an image pattern that matches a divided block.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Lens drive circuit 2 Lens 3 CCD 4 Imaging process circuit 5 A / D converter 6, 21 Buffer memory 7 Pulse generation circuit 8 D
CT circuit 9, 10 Quantization circuit 11 Delay circuit 12 Subtraction circuit 13 Zigzag scanning circuit 14, 15 Encoding circuit 16 Synthesizing circuit 17 Recording device 18 System control circuit 19 Switch 20, 40 Lens AF control circuit

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/22-5/257

Claims (1)

(57) [Claims] An image data is divided into a plurality of block data.
Divided data obtained by orthogonally transforming each divided block data.
1 is obtained from the orthogonal transform.
The DC conversion coefficients are combined into one block data.
Data obtained by orthogonally transforming one block data of
Current conversion coefficient as a second AC conversion coefficient,
One of the first AC conversion coefficient and the second AC conversion coefficient
Is selected, and the selected AC conversion coefficient is
An autofocus method wherein autofocus control is performed using last information .