Shading correction circuit and digital camera signal processing circuit using the same

A shading correction circuit is disclosed which prevents unevenness and partial dull of an edge caused by noise which remains when noise is sliced using a fixed value as a threshold level to be used for distinction between noise and an edge component. The shading correction circuit includes a level correction circuit for correcting a level of shading, and a noise removal circuit for receiving a level adjusting coefficient used by the level correction circuit and controlling the slice level of level slice to be used to remove an edge component mixed in a noise component with the level adjusting coefficient.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a shading correction circuit, and more specifically to a shading correction circuit for performing shading correction in digital camera signal processing and a digital camera signal processing circuit which employs the shading correction circuit.

2. Description of the Related Art

Conventionally, shading correction is used only in order to keep the signal level uniform. For example, a circuit which includes a shading level correction circuit and a noise removal circuit independent of each other is disclosed in Japanese Patent Laid-Open No. 003569/1988.

FIG. 7shows an example of a conventional shading correction circuit. Referring toFIG. 7, the shading correction circuit shown has an input terminal34and an output terminal40and includes a multiplication type AD converter35, a noise removal circuit42including a sample hold circuit36and a low-pass filter37, a shading correction coefficient storing memory38, a CPU41, and an address generator39. The address generator39produces an address for outputting a correction coefficient corresponding to a pixel. The address is connected to an address input of the shading correction coefficient storing memory38, and the shading correction coefficient storing memory38outputs a correction coefficient corresponding to a pixel. The correction coefficient is multiplied by an image signal inputted from the input terminal34by the multiplication type AD converter35. A result of the multiplication is inputted to the sample hold circuit36and further inputted to the low-pass filter37, by which noise of the multiplication result is removed. When a shading correction coefficient is produced, a predetermined value is inputted in advance to the shading correction coefficient storing memory38, and a signal obtained by picking up an image of uniform surface lighting is inputted from the input terminal34. Then, the inputted signal is multiplied by a constant by the multiplication type AD converter35, and noise removal of the signal is performed by the sample hold circuit36and the low-pass filter37. Then, a correction coefficient with which a signal to be inputted to the CPU41may be constant is calculated by the CPU41and is written into the shading correction coefficient storing memory38.

However, the conventional shading correction circuit has the following problems.

In shading correction, while one screen is multiplied by different coefficients, since this is simple multiplication, the SN ratio is maintained and the noise level rises. When such simple noise removal means is utilized as in the conventional shading correction circuit, if a filter which can remove amplified noise is applied, then also a band of a video signal is removed. Even if a circuit wherein an edge component is removed by slice processing and a resulting signal which includes only a noise component is subtracted from the original signal is adopted in order to prevent occurrence of such a trouble as just described, since the noise level differs among different portions of an image after the shading correction, if a simple level slice is used, then a noise component and an edge component cannot be separated from each other.

Accordingly, since, in level correction of shading, different coefficients are used for multiplication of pixels at different portions of an image, also the noise level differs among different portions of the image. Therefore, there is a problem that, if a simple noise removal circuit is utilized, then an edge becomes dull and much noise remains at different portions of an image.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a shading correction circuit which prevents unevenness and partial dull of an edge caused by noise which remains when noise is sliced using a fixed value as a threshold level to be used for distinction between noise and an edge component.

In order to achieve the object described above, according to the present invention, the threshold value to be used for distinction between noise and an edge component is controlled with a shading coefficient so that a slice level optimum for each pixel is applied to the noise component which has different magnitudes at different portions of an image as a result of shading correction.

In particular, according to an aspect of the present invention, there is provided a shading correction circuit, comprising a level correction circuit for correcting a level of shading, and a noise removal circuit for receiving a level adjusting coefficient used by the level correction circuit and controlling a slice level of level slice for removing an edge component mixed in a noise component with the level adjusting coefficient.

Preferably, the noise removal circuit receives a shading correction coefficient used in the level control by the level correction circuit and first and second level adjusting coefficients, and performs level adjustment by multiplying the shading correction coefficient by the first level adjusting coefficient, performs offset adjustment by adding the second level adjusting coefficient to the level-adjusted shading correction coefficient, and then performs the level slice.

Further preferably, the noise removal circuit interlocks the slice level with the shading correction level by multiplying the shading correction coefficient after the level adjustment and the offset adjustment by a level slice signal inputted thereto.

The noise removal circuit may include a first multiplication circuit for performing level adjustment by multiplying the inputted shading correction coefficient by the first level adjusting coefficient, and an addition circuit for performing the offset adjustment by adding an output of the first multiplication circuit and the second level adjusting coefficient.

The noise removal circuit may include a second multiplication circuit for interlocking the slice level with the shading correction level by multiplying the output of the addition circuit by the level slice signal.

According to another aspect of the present invention, there is provided a digital camera signal processing circuit, comprising a shading correction circuit including a level correction circuit for correcting a level of shading and a noise removal circuit for receiving a level adjusting coefficient used by the level correction circuit and controlling a slice level of level slice for removing an edge component mixed in a noise component with the level adjusting coefficient.

The shading correction circuit and the digital camera signal processing circuit are advantageous in that unevenness and partial dull of an edge of an image caused by noise which remains when noise is sliced using a fixed value as a threshold level to be used for distinction between noise and an edge component.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A shading correction circuit according to the present invention is basically configured such that a noise removal circuit operates in an interlocking relationship with shading correction in digital camera signal processing.

More particularly, a coefficient to be used for correction of the level of shading is inputted also to a noise removal circuit and the slice level of level slice for removing an edge component mixed in a noise component is controlled to remove the edge component using the effective slice level thereby to achieve shading correction interlocked with a noise removal circuit without making an image edge dull and without allowing much noise to remain.

Referring toFIG. 1, there is shown a shading correction circuit to which the present embodiment is applied. The shading correction circuit includes a level correction circuit4and a noise removal circuit6. The level correction circuit4has input terminals1,8and9and output terminals15and16. The noise removal circuit6has input terminals20,24,2,3,10and11and an output terminal7.

A video signal S5(refer toFIG. 4) is inputted to the input terminal1of the level correction circuit4, and a pixel clock S1and a frame synchronization signal S2(both refer toFIG. 4) are inputted through the input terminals8and9, respectively. The level correction circuit4performs level correction for shading, and the video signal after the level correction is inputted from the output terminal16of the level correction circuit4to the input terminal24of the noise removal circuit6. Also a shading correction coefficient S4(refer toFIG. 4) used for the level control is outputted from the output terminal15of the level correction circuit4and inputted to the input terminal20of the noise removal circuit6. Further, separate from the shading correction coefficient S4, three different coefficients for level adjustment and a coefficient of the noise slice level are inputted to the input terminals2,3,10and11of the noise removal circuit6. The noise removal circuit6performs noise removal corresponding to the strength of the shading correction, and the signal after the noise removal is outputted from the output terminal7.

Referring toFIG. 2, the level correction circuit4includes an address counter17, a shading correction coefficient storing memory18, and a multiplication circuit19.

The pixel clock S1inputted through the input terminal8and the frame synchronization signal S2inputted through the input terminal9are inputted to the address counter17. The address counter17counts up in response to the pixel clock S1and is reset in response to the frame synchronization signal S2. An output signal S3(refer toFIG. 4) of the address counter17is inputted to the shading correction coefficient storing memory18. The shading correction coefficient storing memory18outputs the shading correction coefficient S4corresponding to the inputted address, that is, corresponding to a pixel. An output signal of the shading correction coefficient storing memory18is outputted from the output terminal15. The output signal is inputted also to the multiplication circuit19and multiplied by the video signal S5inputted through the input terminal1by the multiplication circuit19. As a result, a shading-corrected signal (video signal S6(refer toFIG. 4)) is outputted from the output terminal16.

Referring now toFIG. 3, the noise removal circuit6is includes multiplication circuits27,29and32, an addition circuit28, a level slice circuit30, a noise component extracting filter31, and a subtraction circuit33. The input terminal20of the noise removal circuit6is connected to the output terminal15of the level correction circuit4. The other input terminal24of the noise removal circuit6is connected to the output terminal16of the level correction circuit4.

The shading correction coefficient S4inputted from the input terminal20is multiplied by the coefficient for level adjustment inputted through the input terminal2so as to adjust the level thereof. Then, a result of the multiplication is added to the level adjustment coefficient inputted through the input terminal3by the addition circuit28to adjust the offset thereof. Further, a result of the addition is multiplied by the level slice signal inputted through the input terminal10by the multiplication circuit29to interlock the slice level with the shading correction level. Furthermore, a result of the multiplication is inputted to a slice level input terminal of the level slice circuit30. A resulting signal calculated by the multiplication circuit29and the noise component calculated from the video signal S6inputted through the input terminal24by the noise component extracting filter31are inputted to the level slice circuit30. Level slice is performed for the inputted noise component by the level slice circuit30. An output signal of the level slice circuit30is multiplied by a coefficient for determination of the strength of the noise removal inputted through from the input terminal11by the multiplication circuit32. Then, a result of the multiplication is inputted to a subtrahend input of the subtraction circuit33. Further, the video signal S6inputted through the input terminal24is inputted to a minuend input terminal of the subtraction circuit33. The subtraction circuit33thus subtracts the subtrahend input from the minuend input. An output of the subtraction circuit33is outputted from the output terminal7.

The address counter17, shading correction coefficient storing memory18and multiplication circuit19ofFIG. 2and the multiplication circuits27,29and32, addition circuit28, level slice circuit30, noise component extracting filter31and subtraction circuit33ofFIG. 3are well known to those skilled in the art. Therefore, detailed description of the configuration of them is omitted herein.

Operation of the shading correction circuit is described below.FIG. 4illustrates operation of the level correction circuit4ofFIG. 2. The pixel clock S1is inputted through the input terminal8and the frame synchronization signal S2is inputted through the input terminal9.

The address counter17counts up in synchronism with the pixel clock S1, and is reset in response to the frame synchronization signal S2and outputs a counter output signal S3. The counter output signal S3is inputted to the shading correction coefficient storing memory18, by which it is converted into the shading correction coefficient S4. A video signal S5synchronized with the pixel clock S1is inputted through the input terminal1. The video signal S5and the shading correction coefficient S4are multiplied by the multiplication circuit19thereby to form a video signal S6. The video signal S6is outputted from the output terminal16.

An inverse characteristic to the shading of a corresponding pixel is stored in the shading correction coefficient storing memory18and multiplied by the shading correction coefficient S4corresponding to the pixel clock S1which has inputted been through the input terminal8. Consequently, an image signal which does not have unevenness can be obtained.

Where an image sensor which has color filters for the primary colors is used, the shading correction coefficient S4is stored, where the RGB color signal level of a signal obtained by picking up an image of a homogeneous surface light source is represented as IMGn (n=0, 1, 2 . . . ) and the frame average levels of the colors of the pixels are represented as IMG_Ra, IMG_Ga and IMG_Ba, as IMG_Ra/IMGn for a pixel corresponding to the red filter, IMG_Ga/IMGn for a pixel corresponding to the green filter and IMG_Ba/IMGn for a pixel corresponding to the blue filter into the shading correction coefficient storing memory18.

Therefore, the noise removal circuit6ofFIG. 3operates in accordance with the timing chart ofFIG. 5. The level corrected video signal S6is inputted through the input terminal24, and the shading correction coefficient S4is inputted through the input terminal20. Since the gain control has been performed for the video signal S6with the shading correction coefficient S4, also the noise has an amplified level. The noise component extraction filter31extracts the noise component from the video signal S6to obtain a noise signal S9. The shading correction coefficient S4is arithmetically operated with the coefficients inputted through the input terminals2and3by the multiplication circuit27and the addition circuit28so that the level thereof is adjusted. The level adjusted shading correction coefficient is multiplied by the slice level (level slice signal) inputted from the input terminal10, and a resulting signal is inputted to the level slice circuit30. The noise signal S9is level sliced by the level slice circuit30to form a signal S10. The signal S10is multiplied by a coefficient for determination of the strength of noise removal inputted through the input terminal11, and a resulting signal is subtracted from the video signal S6inputted through the input terminal24to form an output signal S11.

Since the shading correction circuit according to the present embodiment is configured in such a manner as described above, the advantages described below are achieved successfully.

With the shading correction circuit, since the slice level of level slice to be used to remove an edge component is made correspond to the shading correction coefficient, the noise level by which only part of an image is emphasized can be uniformed by the shading correction.

The foregoing can be achieved by the fact that the slice level of noise is interlocked with the shading correction coefficient. The shading correction circuit solves the problem that, if the slice level is adjusted to the noise level emphasized without such interlocking as just described, then an edge of a signal which is not noise is removed, but if a normal noise level is used, then noise of an emphasized portion remains.

A modification to the noise removal circuit ofFIG. 3is shown inFIG. 6. Referring toFIG. 6, the modified noise removal circuit is different from the noise removal circuit ofFIG. 3in that it does not have the multiplication circuit29and the input terminal10shown inFIG. 3and the output of the addition circuit28is directly inputted to the level slice circuit30. In the noise removal circuit having the configuration shown inFIG. 6, not the slice level is adjusted using a shading correction coefficient, but level adjustment is performed for the shading correction coefficient to obtain a slice level.

Accordingly, the modified shading correction circuit can achieve an advantage that, since the multiplication circuit can be eliminated from the shading correction circuit, reduction of the circuit scale can be anticipated.