Determining dominant gradient orientation in image processing using double-angle gradients
Methods and image processing systems are provided for determining a dominant gradient orientation for a target region within an image. A plurality of gradient samples are determined for the target region, wherein each of the gradient samples represents a variation in pixel values within the target region. The gradient samples are converted into double-angle gradient vectors, and the double-angle gradient vectors are combined so as to determine a dominant gradient orientation for the target region.
1. A method of determining a dominant gradient orientation for a target region within an image, the method comprising:
converting gradient samples for the target region into double-angle gradient vectors;
combining the double-angle gradient vectors to determine a compound gradient vector for the target region; and
converting the compound gradient vector to a dominant gradient vector, the dominant gradient vector representing the dominant gradient orientation for the target region.
2. The method of claim 1 , wherein the target region is a region surrounding a target pixel.
3. The method of claim 2 , wherein each of the gradient samples is determined by determining a difference between: (i) the pixel value at the target pixel, and (ii) a pixel value of a neighbouring pixel positioned in a respective direction with respect to the target pixel.
4. The method of claim 1 , wherein the gradient samples are in a single-angle domain and the double-angle gradient vectors are in a double-angle domain.
5. The method of claim 1 , wherein converting the gradient samples into double-angle gradient vectors comprises representing the gradient samples in polar coordinates and multiplying their angular components by two.
6. The method of claim 1 , wherein each of the gradient samples represents a variation in pixel values within the target region.
7. The method of claim 1 , wherein:
converting the gradient samples into double-angle gradient vectors comprises encoding each gradient sample with a double-angle gradient vector that has an angle twice that of the gradient sample; and
the dominant gradient vector has an angle half that of the compound gradient vector.
8. The method of claim 1 , wherein the compound gradient vector is in a double-angle domain and converting the compound gradient vector to the dominant gradient vector comprises converting the compound gradient vector into a single-angle domain.
9. The method of claim 1 , wherein converting the compound gradient vector to the dominant gradient vector comprises representing the compound gradient vector in polar coordinates and dividing its angular component by two.
10. The method of claim 1 , wherein combining the double-angle gradient vectors comprises averaging the double-angle gradient vectors.
11. The method of claim 1 , wherein combining the double-angle gradient vectors comprises filtering the double-angle gradient vectors.
12. The method of claim 11 , wherein filtering the double-angle gradient vectors comprises combining the double-angle gradient vectors using a weighted sum.
13. The method of claim 1 , wherein each pixel value comprises one or more characteristics of the respective pixel.
14. The method of claim 13 , wherein each characteristic is one or more of luma, luminance, chrominance, brightness, lightness, hue, saturation, chroma, colourfulness, or any colour component.
15. An image processing system configured to determine a dominant gradient orientation for a target region within an image, the image processing system comprising:
a conversion unit configured to convert gradient samples for the target region into double-angle gradient vectors;
a combining unit configured to combine the double-angle gradient vectors to determine a compound gradient vector for the target region; and
a determining unit configured to convert the compound gradient vector to a dominant gradient vector, the dominant gradient vector representing the dominant gradient orientation for the target region.
16. The image processing system of claim 15 , wherein each of the gradient samples represents a variation in pixel values within the target region.
17. The image processing system of claim 15 , wherein:
the conversion unit is configured to encode each gradient sample with a double-angle gradient vector that has an angle twice that of the gradient sample; and
the dominant gradient vector has an angle half that of the compound gradient vector.
18. A non-transitory computer readable storage medium having stored thereon computer readable instructions that, when executed at a computer system, cause the computer system to perform a method of determining a dominant gradient orientation for a target region within an image, the method comprising:
converting gradient samples for the target region into double-angle gradient vectors;
combining the double-angle gradient vectors to determine a compound gradient vector for the target region; and
converting the compound gradient vector to a dominant gradient vector, the dominant gradient vector representing the dominant gradient orientation for the target region.
19. The non-transitory computer readable storage medium of claim 18 , wherein each of the gradient samples represents a variation in pixel values within the target region.
20. The non-transitory computer readable storage medium of claim 19 , wherein:
converting the gradient samples into double-angle gradient vectors comprises encoding each gradient sample with a double-angle gradient vector that has an angle twice that of the gradient sample; and
the dominant gradient vector has an angle half that of the compound gradient vector.