Biasing a noise filter to preserve image texture

Patent metadata

Field	Value
Publication number	US-11074678-B2
Application number	US-201916393892-A
Country	US
Kind code	B2
Filing date	Apr 24, 2019
Priority date	Apr 24, 2019
Publication date	Jul 27, 2021
Grant date	Jul 27, 2021

Abstract

Embodiments relate to biasing an image noise filter to reduce edge and texture blurring of image data. Pixel values used to determine photometric coefficients for a bilateral filter are modified by offset values. The offset value for a pixel value is determined by applying a high pass filter to the pixel (referred to as the center pixel) and neighboring pixels of the center pixel. By adding the offset value to the center pixel value, the pixel value difference between the neighboring pixels and the center pixel becomes smaller for pixels on the same side of an edge as the center pixel. Thus, pixels on the same side of the edge get more weight in the bilateral noise filter. Conversely, pixels on the opposite side of the edge as the center pixel get less weight in the bilateral filter. As a result, the biased bilateral filter reduces blurring of edges and increases preservation of texture in the image data.

First claim

The invention claimed is:

1. An image processor comprising:
an offset calculator circuit configured to:
receive pixel values for a block of original pixels in image data, the block of original pixels comprising a center pixel and neighboring pixels within a predetermined distance from the center pixel, and
apply a high pass filter to pixel values of the block of original pixels to generate an offset value for the center pixel;

a center pixel modifier circuit coupled to the offset calculator circuit and configured to adjust a pixel value of the center pixel by the offset value to generate a modified pixel value for a modified pixel that becomes part of a modified pixel block, the modified pixel block comprising the modified pixel value and the neighboring pixel values; and
a photometric distance calculator circuit coupled to a pixel block assembler circuit to receive the modified pixel block, the photometric distance calculator circuit configured to determine photometric distances of pixel values in the modified pixel block and determine photometric coefficients by processing the photometric distances for generating a bilateral filter that preserves texture in the image data.

2. The image processor of claim 1 , further comprising:
the pixel block assembler circuit coupled to the center pixel modifier circuit to receive the modified pixel value, the pixel block assembler circuit configured to assemble the modified pixel block by including at least the modified pixel value as a pixel value for one of the pixels in the block of modified pixels.

3. The image processor of claim 2 , wherein the image processor further comprises a bilateral filter circuit coupled to the photometric distance calculator circuit and configured to perform bilateral filtering by at least multiplying one of the photometric coefficients, a corresponding spatial coefficient of the block of original pixels, and a pixel value of a corresponding pixel in the block of original pixels.

4. The image processor of claim 1 , wherein the offset calculator circuit comprises a clipping circuit configured to set the offset value as a first value if the offset value is above the first value and set the offset value as a second value if the offset value is below the second value.

5. The image processor of claim 4 , wherein the first value and the second value are programmed into a look up table.

6. The image processor of claim 1 , wherein the block of original pixels is a 5×5 block of pixels.

7. The image processor of claim 1 , wherein the high pass filter is a Laplacian high pass filter.

8. The image processor of claim 1 , wherein the high pass filter is a Laplacian of Gaussian high pass filter.

9. The image processor of claim 1 , wherein the image data is luminance image data.

10. The image processor of claim 1 , wherein the image data is one of red, blue, or green image data.

11. A method comprising:
receiving, by an offset calculator circuit, pixel values for a block of original pixels in image data, the block of original pixels comprising a center pixel and neighboring pixels within a predetermined distance from the center pixel;
applying, by the offset calculator circuit, a high pass filter to pixel values of the block of original pixels to generate an offset value for the center pixel;
adjusting, by a center pixel modifier circuit coupled to the offset calculator, a pixel value of the center pixel by the offset value to generate a modified pixel value for a modified pixel that becomes part of a modified pixel block, the modified pixel block comprising the modified pixel value and the neighboring pixel values;
determining, by a photometric distance calculator circuit, photometric distances of pixel values in the modified pixel block; and
determining, by the photometric distance calculator circuit, photometric coefficients by processing the photometric distances.

12. The method of claim 11 , further comprising:
assembling the block of modified pixels by including at least the modified pixel value as a pixel value for one of pixels in the block of modified pixels.

13. The method of claim 12 further comprising:
performing, by a bilateral filter circuit coupled to the photometric distance calculator circuit, bilateral filtering by at least multiplying one of the photometric coefficients, a corresponding spatial coefficient of the block of original pixels, and a pixel value of a corresponding pixel in the block of original pixels.

14. The method of claim 11 , wherein the offset calculator circuit comprises a clipping circuit configured to set the offset value as a first value if the offset value is above the first value and set the offset value as a second value if the offset value is below the second value.

15. The method of claim 14 , wherein the high pass filer is a Laplacian high pass filter or a Laplacian of Gaussian high pass filter.

16. A system comprising:
an image sensor configured to capture image data;
an image processor comprising:
an offset calculator circuit configured to:
receive pixel values for a block of original pixels in a version of the captured image data, the block of original pixels comprising a center pixel and neighboring pixels within a predetermined distance from the center pixel, and
apply a high pass filter to pixel values of the block of original pixels to generate an offset value for the center pixel;

a center pixel modifier circuit coupled to the offset calculator and configured to adjust a pixel value of the center pixel by the offset value to generate a modified pixel value for a modified pixel that becomes part of a modified pixel block, the modified pixel block comprising the modified pixel value and the neighboring pixel values; and
a photometric distance calculator circuit coupled to a pixel block assembler circuit to receive the modified pixel block, the photometric distance calculator configured to determine photometric distances of pixel values in the modified pixel block and determine photometric coefficients by processing the photometric distances.

17. The system of claim 16 , wherein the image processor further comprises:
the pixel block assembler circuit coupled to the center pixel modifier to receive the modified pixel value, the pixel block assembler circuit configured to assemble the block of modified pixels by including at least the modified pixel value as a pixel value for one of the pixels in the block of modified pixels.

18. The system of claim 17 , wherein the image processor further comprises:
a bilateral filter circuit coupled to the photometric distance calculator circuit and configured to perform bilateral filtering by at least multiplying one of the photometric coefficients, a corresponding spatial coefficient of the block of original pixels, and a pixel value of a corresponding pixel in the block of original pixels.

19. The system of claim 16 , wherein the offset calculator circuit comprises a clipping circuit configured to set the offset value as a first value if the offset value is above the first value and set the offset value as a second value if the offset value is below the second value.

20. The system of claim 16 , wherein the high pass filter is a Laplacian high pass filter or a Laplacian of Gaussian high pass filter.

Assignees

• Apple Inc

Inventors

• Lin Sheng
• Cheng Wu
• Smirnov Maxim

Classifications

• G06T5/20 Primary

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• G06T2207/20192

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• G06T2207/10024

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• G06T2207/20028

  Bilateral filtering · CPC title

• G06T2207/20021

  Dividing image into blocks, subimages or windows · CPC title