Camera With Automatic Data Compression Factor Selection

1. Field of the Invention

The present invention relates to camera in which the image of a photographic subject is converted into digital data, and the data is compressed and stored on a recording medium. More particularly, the present invention relates to a still camera which automatically selects a data compression factor for data compression processing.

2. Description of the Related Art

Still cameras are know which record the image of a subject in memory as digital image data. For example, FIG. 3 is a block diagram illustrating an example of a prior art imaging system for a still camera which records the image of a subject in memory as digital data. According to the system shown in FIG. 3, an imaging action is begun in response to the half-depression of a release button (not shown). Light from a photographic subject enters a lens 50 and is imaged on an imaging unit 54. The amount of light imaged on the imaging unit 54 is controlled by an electronic shutter (not shown) and a stop 52. The imaging unit 54 photoelectrically converts the light from the photographic subject into analog electrical signals by a built-in charge coupled device (CCD), or similar device. A signal processing unit 56 receives the photoelectrically converted output of the imaging unit 54 and performs white balance correction, amplification, .gamma. correction and like processing on the received signals to produce an image signal. The image signal output from the signal processing unit 56 is converted into a digital image signal by an analog-to-digital (A/D) converter 58, and the output of the A/D converter 58 is temporarily stored in a buffer memory 60. The image signal data stored in the buffer memory 60 is sent to a data compression unit 62 when the release button is fully depressed. The data compression unit 62 performs well-known data compression processing on the image data, and, finally, the compressed image data is stored in memory on a memory card 64. A control unit 66 is also provided which controls each of the above-described circuits.

The prior art imaging system shown in FIG. 3 may operate in two modes: a manual compression mode, and an auto compression mode. The mode of data compression by the data compression unit 62 can be selected by the photographer, and data compression is performed according to the selected data compression mode. When operating in the manual compression mode the photographer may select a data compression factor. However, when performing imaging in the auto compression mode, the action of the data compression unit 62 is carried out according to the properties of the digital image data stored in the buffer memory 60, or by extracting a portion of image data, and from the data length at this time, the camera selects an appropriate compression factor, and data compression is performed according to the selected compression factor.

A system for automatically selecting a compression factor is known. For example, Japanese Laid-Open Patent Publication 2-257780 discloses various methods for performing data compression of digital image data. According to one method disclosed, data compression is performed at a suitable data compression ratio such that the smallest amount of data remains after compression. According to another method disclosed, the compression factor is selected such that a fixed amount of data remains after compression.

However, in the prior art imaging system for a video still camera described above, when a photographer has selected the auto compression mode, compression factor selection is performed by actually performing data compression processing for the digital image data until arriving at the selection of a compression factor. The data compression process, therefore, has to be performed a number of times to select a compression factor, and, as a result, the process is lengthy. Therefore, the time interval from the commencement of the photographic action until the storage on the memory card 64 is long. This time interval is a primary factor preventing the speeding up of continuous photography. Furthermore, a large amount of digital data is necessary for appropriate compression factor selection. The amount of data required for appropriate compression lengthens the time necessary for selecting a compression factor. Further, since, according to the prior art, the data compression unit 62 performs data compression until a compression factor is selected, electrical power is wasted.

Therefore, it is an object of the present invention to provide an image processing system for a camera which performs selection of a data compression factor at a high speed.

It is a further object of the present invention to provide an image processing system for a camera which performs selection of a data compression factor with low electric power consumption and at a high speed.

It is yet a further object of the present invention to provide an image processing system for a camera which automatically selects a compression factor for data compression.

It is another object of the present invention to provide an image processing system for a camera which automatically selects a compression factor and performs the selection of the compression factor with low electric power consumption and at a high speed.

Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.

The foregoing objects of the present invention are achieved by providing a camera which includes an imaging unit which receives light from a photographic subject via a lens and a stop, and which photoelectrically converts the light from the photographic subject into analog electrical image signals. A signal processing unit receives the analog image signals and performs .gamma. correction and like processing of the received signals. The analog image signals are then A/D converted and stored temporarily in a buffer memory. A frequency component extraction unit extracts the frequency components of the analog image signals output by the signal processing unit. The extracted frequency components may be the high frequency components of the brightness signals from the signal processing unit, or other components of the analog image signal. A compression factor selection unit selects a compression factor for performing data compression based upon the extracted frequency components. After the compression factor has been selected, data compression processing is performed on the A/D converted image signals which have been temporarily stored in the buffer memory. The compressed image data is then stored in a recording medium.

The above objects of the present invention may also be achieved with an image processing system having a signal processing unit which outputs analog image signals which are color analyzed into component signals of the three primary colors, red, green and blue. The frequency component extraction unit extracts the frequency components of the green component signal of the analog image signals in order to select a compression factor .

These and other objects and advantages of the invention will become apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, of which:

FIG. 1 is a block diagram of an image processing system in a camera in accordance with a preferred embodiment of the present invention.

FIG. 2 is a block diagram showing a circuit for performing compression factor selection in accordance with the preferred embodiment of the present invention.

FIG. 3 is a block diagram showing prior art still camera circuitry for digitally storing an image.

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Like reference numerals refer to like elements throughout.

FIG. 1 is a block diagram illustrating an image processing system for processing image data in a camera in accordance with a preferred embodiment of the present invention. The present invention is applicable to video still cameras, movie cameras, television cameras and like types of cameras. The image processing system includes: a lens 10; a stop 12; an imaging unit 14 which images light from a photographic subject; a signal processing unit 16 which performs .gamma. correction and like signal processing; an A/D converter 18; a buffer memory 20 which temporarily stores image data; a data compression processing unit 22; a memory card 24 on which image data is recorded; and a control unit 26 equipped with a microprocessor, CPU or similar control circuitry which performs control of each of the circuits within the image processing system.

The imaging unit 14 includes a CCD and like imaging elements, and an electronic shutter. Light from a subject imaged on the imaging elements is converted, at predetermined intervals, into electrical signals, and image plane (frame) units of analog image data are output. The memory card 24 may be a semiconductor memory, a magnetic tape, magnetic disk, or other similar type of recording medium.

The image processing system also includes a high pass filter 28, an integrator 30, and a compression factor selection unit 32. According to the preferred embodiment, the high pass filter 28 receives only the brightness signals from the analog image data which are output from the signal processing unit 16. The high pass filter 28 passes only those brightness signals having frequency components at or above a constant K frequency (for example, 4 MHz).

The integrator 30 integrates the output signals from the high pass filter 28 for each image plane during predetermined intervals based upon instructions from the control unit 26, and the integrator 30 outputs a result which is a voltage value.

The compression factor selection unit 32 receives the voltage value output frown integrator 30 and outputs a signal, based upon the voltage value output from the integrator 30, for use in data compression factor selection in the data compression processing unit 22.

The data compression processing unit 22 may be set to an auto compression mode or a manual compression mode by a changeover switch 34. When the changeover switch 34 is set to the auto compression mode, the data compression processing unit performs data compression based upon the state of two input ports P.sub.1 and P.sub.0 (described below with respect to FIG. 2). When the changeover switch 34 is set to the manual compression mode, data compression is performed according to a compression factor set by compression factor selection switches 36a and 36b. The selected compression factor is displayed by the display unit 38.

As seen in FIG. 2, the compression factor selection unit 32 includes two comparators 40a and 40b, and two standard voltage generators 42a and 42b. The standard voltage generators 42a and 42b generate standard voltages A(V) and B(V), respectively, where A<B, from the standard voltage V.sub.CC of the camera. The inputs to the comparator 40a are the added output voltage V.sub.Y of the integrator 30 and the subtracted standard voltage A(V) from the standard voltage generator 42a. Similarly, the inputs to the comparator 40b are the added output voltage V.sub.Y of the integrator 30 and the subtracted standard voltage B(V) from the standard voltage generator 42b. The outputs from the comparators 40a and 40b are input to the input ports P.sub.1 and P.sub.0, respectively, of the data compression processing unit 22.

As described above, a camera equipped with the image processing system in accordance with the present invention is capable of operating in both a manual compression mode and an auto compression mode. The mode changeover is instructed by the changeover switch 34. When the mode changeover switch is set to the manual compression mode, a selection from three kinds of compression factors (i.e., low compression factor L, medium compression factor M, and high compression factor H) is made according to whether the compression factor selection switches 36a and 36b are ON or OFF. When the mode changeover switch 34 is in the auto compression mode, the compression factor is selected based upon the output signals from the compression factor selection unit 32, and the selected compression factor is displayed by the display unit 38.

With reference to FIGS. 1 and 2, the operation of a camera in accordance with the preferred embodiment of the present invention will now be described. A release button (not shown) is half-depressed causing preparatory actions for photography to begin. During the preparatory actions for photography, light from a photographic subject is imaged on the imaging elements of the imaging unit 14 via the lens 10 and the stop 12. The light is photoelectrically converted into analog electrical image signals by the imaging unit 14, and the analog image signals are input to the signal processing unit 16 where .gamma. correction and like processing are performed. Next, when the release button is fully depressed after having been half-depressed, the analog image signals from the signal processing unit 16 are A/D converted by the A/D converter 18, and temporarily stored in the buffer memory 20. The brightness signals of the image signals output from the signal processing unit 16 are input to the high pass filter 28 which passes only the signals having frequency components above K MHz, where K may be, for example, 4 MHz. The integrator 30 receives the brightness signals which have passed the high pass filter 28, and integrates these signals for every image plane frame during a predetermined time period. The output of the integrator 30 is a voltage value. The comparators 40a and 40b of the compression factor selection unit 32 determine the greater relative magnitude of the output voltage V.sub.Y from the integrator 30 and the standard voltages A(V) and B(V) generated by the standard voltage generators 42a and 42b, respectively. The results of the comparison by the comparators 40a and 40b are output to the input ports P.sub.1 and P.sub.0, respectively, of the data compression processing unit 22.

As described above, the data compression processing unit 22, when set to the auto compression mode by the changeover switch 34, performs data compression based upon the state of the input ports P.sub.1 and P.sub.0. When the changeover switch 34 is set to the manual compression mode, data compression by the data compression processing unit 22 is performed according to a compression factor set by compression factor selection switches 36a and 36b. When the auto compression mode is selected, the output voltage V.sub.Y from the integrator 30, the state of the input ports P.sub.1 and P.sub.0, and the relationship between the selected compression factors are as shown below in Table 1.

As seen in Table 1, the selected compression factor becomes lower to the extent that the output voltage V.sub.Y of the integrator 30 becomes higher due to a greater number of high frequency components in the brightness signal of the analog image data. Since an image with a large number of high frequency components is a detailed image, it is necessary to take a low data compression factor in comparison with a picture having a small number of high frequency components, such as a monotonous picture.

Upon full depression of the release button, the digital image data temporarily stored in the buffer memory 20 are output to the data compression unit 22 which performs image data compression based upon the selected compression factor. A single frame sequence ends with the recording of the compressed image data on the memory card 24.

Therefore, in accordance with the present invention, a data compression factor is automatically selected for performing data compression by the data compression unit 22 based upon the high frequency components of the brightness signals of analog image data output from the signal processing unit 16. Accordingly, the time from the commencement of a photographic action until storage of image data on the memory card 24 can be shortened in comparison to prior art imaging systems where data compression is performed based upon the properties of the digital image data, or based upon a data compression processing unit extracting partial image data, and high speed continuous photography becomes possible. Furthermore, since it is not necessary to actuate the A/D converter 18, buffer memory 20 and data compression processing unit 22 for the selection of the compression factor, no waste of electric power takes place.

Although the image processing system according to the preferred embodiments has been described with respect to using the high frequency components of the brightness signals as an index for data compression factor selection, the invention is not limited in this manner, and the analog image signals themselves may be used for compression factor selection, or various signals comprising the analog image signals may be used.

For example, the color component signals (R signal, G signal, B signal) of the three primary colors (red, green and blue, respectively) may be used for compression factor selection. In this regard, color component signals from the imaging unit 14 are analyzed and output, and the G signal may be input to the high pass filter 28. It is understood from the known equation

where E.sub.Y is the brightness signal value and E.sub.R, E.sub.G, E.sub.B are values of the R signal, G signal, and B signal, respectively, that the G signal is the largest component constituting the brightness signal value. If the compression factor is selected based on the value of the G signal, results are obtained which are similar to those obtained if the compression factor is selected based on the brightness signal. Thus, the invention is not limited to using high frequency components of the analog image signal for selection of the compression factor, and the compression factor may be selected based upon optional frequency components.

Although preferred embodiments of the invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.