Method and apparatus for post noise reduction black level correction
View Patent ↗Techniques for implementing a Black Level Correction (BLC) processing operation on image data signal pixel values that results in little to no nonlinearity in the dark areas of the image due to black noise clipping, and avoids reducing image quality or adding cost, are provided. Image data signal pixel values are caused to maintain black level while being operated on by image data signal processor circuits that precede a Noise Reduction (NR) processing operation, thus allowing the BLC processing operation to be executed after the NR processing operation. With black noise mostly removed, little to no nonlinearity in the dark areas of the image due to black noise clipping results from the BLC processing operation.
1. A method of processing a signal from an optical image sensor, said optical image sensor producing an optical image sensor signal, wherein the optical image sensor signal has a measured value when no light is impinging on the surface of the optical image sensor, said value being the signal's black level, comprising:
receiving the optical image sensor signal at the input of an analog-to-digital converter, wherein the analog-to-digital converter produces an image data signal therefrom that is composed of positive digital data values and includes a digital representation of the optical image sensor signal black level;
processing the image data signal through a plurality of image data signal processing circuits, wherein at least two of such circuits execute at least a black level processing operation on the image data signal input to the image data signal processing circuit producing a processed image data signal;
executing a noise reduction processing operation on the processed image data signal producing a noise reduced image data signal; and
executing a black level correction processing operation on the noise reduced image data signal producing a black level corrected image data signal.
2. The method of claim 1 , wherein the black level processing operation includes:
minimizing black noise clipping by subtracting the image data signal black level from the image data signal while maintaining positive image data signal values; and
adding a black level to the image data signal, wherein the value of the black level added is substantially equal to the mean value of the image data signal's black noise level.
3. The method of claim 1 wherein the image data signal processing circuits are one or more programmable general purpose processors.
4. The method of claim 1 wherein an image data signal processing circuit which executes at least the black level processing operation on the image data signal received at its input executes two or more other processing operations consecutively.
5. The method of claim 1 wherein a maximum image data signal range at the output of the image data processing circuits which perform at least the black level processing operation on the image data signal is substantially the same as a maximum image data signal range at the input to the image data processing circuits.
6. The method of claim 1 wherein positive image data signal values at the input to the image data signal processing circuits which perform at least the black level processing operation on the image data signal are positive at the output from the image data processing circuits.
7. An image data signal processor that processes signals from an optical image sensor, wherein the optical image sensor signal has a measured value when no light is impinging on the surface of the optical image sensor, said value being the signal's black level, and wherein said signal has been converted to a digital signal therefrom that is composed of positive digital data values and includes a digital representation of the optical image sensor signal black level at its output comprising:
a plurality of image data signal processing circuits, wherein at least two of such circuits are implemented at least partially in hardware and execute at least a black level processing operation on the image data signal input to the image data signal processing circuit producing a processed image data signal;
an image data signal processing circuit executing a noise reduction processing operation on the processed image data signal producing a noise reduced image data signal; and
an image data signal processing circuit executing a black level correction processing operation on the noise reduced image data signal producing a black level corrected image data signal.
8. The image data signal processor of claim 7 wherein the image data signal processing circuits which execute at least the black level processing operation on the image data signal include:
circuitry for subtracting the image data signal black level from the image data signal values while maintaining positive image data signal values, thereby minimizing black noise clipping; and
circuitry for adding a black level to the image data signal, wherein said circuitry adds a black level which is substantially equal to the mean value of the image data signal's black noise level.
9. The image data signal processor of claim 7 wherein the image data signal processing circuits are one or more programmable general purpose processors implemented on at least one integrated circuit chip.
10. The image data signal processor of claim 7 wherein an image data signal processing circuit which executes at least the black level processing operation on the image data signal received at its input executes two or more other processing operations consecutively.
11. The image data signal processor of claim 7 wherein a maximum image data signal range at the output of the image data processing circuits which perform at least the black level processing operation on the image data signal is substantially the same as a maximum image data signal range at the input to the image data processing circuits.
12. The image data signal processor of claim 7 wherein positive image data signal values at the input to the image data signal processing circuits which perform at least the black level processing operation on the image data signal are positive at the output from the image data processing circuits.
13. An image capture apparatus comprising:
an optical image sensor having an analog signal output proportional to a luminance of an optical image incident thereon, wherein the optical image sensor signal output has a measured value when no light is impinging on the surface of the optical image sensor, said value being the signal's black level;
an optical system that casts an image of an object scene onto the optical image sensor;
an amplifier receiving the output of the optical image sensor and connected to the input of an analog-to-digital converter;
an analog-to-digital converter that generates an image data signal from the amplified optical image sensor signal, said image data signal being composed of positive digital data values and a digital representation of the optical image sensor signal black level;
an image data signal processor that receives the image data signal data from the analog-to-digital converter and processes the image data signal through a plurality of image data signal processing circuits;
at least a first and a second image data signal processing circuit, each executing at least a black level processing operation on the image data signal input to the image data signal processing circuit, producing a processed image data signal;
a third image data signal processing circuit executing a noise reduction processing operation on the processed image data signal producing a noise reduced image data signal; and
a fourth image data signal processing circuit executing a black level correction processing operation on the noise reduced image data signal producing a black level corrected image data signal.
14. The image capture apparatus of claim 13 wherein the image data signal processing circuits which execute at least the black level processing operation on the image data signal include:
circuitry for subtracting the image data signal black level from the image data signal values while maintaining positive image data signal values, thereby minimizing black noise clipping; and
circuitry for adding a black level to the image data signal, wherein said circuitry adds a black level which is substantially equal to the mean value of the image data signal's black noise level.
15. The image capture apparatus of claim 13 wherein the image data signal processing circuits are one or more programmable general purpose processors.
16. The image capture apparatus of claim 13 wherein an image data signal processing circuit which executes at least the black level processing operation on the image data signal received at its input executes two or more other processing operations consecutively.
17. The image capture apparatus of claim 13 wherein a maximum image data signal range at the output of the image data processing circuits which perform at least the black level processing operation on the image data signal is substantially the same as a maximum image data signal range at the input to the image data processing circuits.
18. The image capture apparatus of claim 13 wherein positive image data signal values at the input to the image data signal processing circuits which perform at least the black level processing operation on the image data signal are positive at the output from the image data processing circuits.