Alias avoidance in image processing

1. An image processing apparatus in which output pixel values are generated with respect to pixels of an input image selected in accordance with an image feature direction in the input image, the apparatus comprising:

a processor programmed to

compare blocks of pixels of the input image, the blocks being disposed with respect to a pixel position under test so that a correlation between the blocks indicates an image feature direction applicable to that pixel position; and

detect whether the image feature direction may be used to interpolate an output pixel value at the pixel position under test by performing at least:

(i) a monotonicity test to determine whether at least one block of the block of pixels exhibits a monotonic variation in a pixel property in a horizontal direction;

(ii) a direction test to detect whether a detected image feature direction for the pixel position under test meets predetermined directional criteria including at least a first directional criterion; and

(iii) in at least cases where the direction test fails, a pixel value test to detect whether pixel values in image regions disposed with respect to the pixel position under test in accordance with possible image feature directions meet one or more predetermined pixel value criteria,

wherein if the direction test fails, an interpolation direction is selected for the pixel position under test as that image feature direction having the greatest block correlation which passes the pixel value test.

2. The image processing apparatus according to claim 1 , wherein the first directional criterion is whether, for the pixel position under test, the detected image feature direction is similar to a detected image feature direction for at least one other pixel position disposed with respect to the pixel position under test.

3. The image processing apparatus according to claim 2 , wherein the other pixel position is directly above the pixel position under test.

4. The image processing apparatus according to claim 3 , further comprising:

a memory configured to buffer information defining detected image feature directions for at least a most-recently processed line of pixels.

5. The image processing apparatus according to claim 2 , wherein the processor detects a similarity of image feature direction by detecting whether the image feature direction applicable to the other pixel is within a threshold gradient amount of the image feature direction for the pixel position under test.

6. The image processing apparatus according to claim 5 , wherein the threshold is dependent on the gradient of the image feature direction detected in respect of the pixel position under test.

7. The image processing apparatus according to claim 1 , wherein the first directional criterion is whether the correlation between blocks relating to the detected image feature direction is better than any other block correlation for that pixel position by at least a threshold amount.

8. The image processing apparatus according to claim 2 , wherein

the predetermined directional criteria include at least a second directional criterion;

the second directional criterion is whether the correlation between blocks relating to the detected image feature direction is better than any other block correlation for that pixel position by at least a threshold amount; and

both the first and second directional criteria must be fulfilled in order to pass the direction test.

9. The image processing apparatus according to claim 1 , wherein the processor applies one or more other tests to detect whether an image feature direction detected in respect of a pixel position under test may be used in interpolation of an output pixel value at that pixel position, whereby a failure of any one test of the group carried out by the tester and the detector is sufficient to prevent that image feature direction being used.

10. The image processing apparatus according to claim 1 , wherein the processor interpolates an output pixel with respect to pixels of the input image selected in accordance with the detected image feature direction in the input image.

11. The image processing apparatus according to claim 1 , the processor carries out at least some of the block comparisons with respect to blocks containing pixels of the input image and up-sampled pixels derived from the pixels of the input image.

12. The image processing apparatus according to claim 11 , wherein the up-sampled pixels are derived so as to interleave horizontally with pixels of the input image.

13. The image processing apparatus according to claim 1 , wherein

the input image is an interlaced image; and

the output image is a progressive scan image.

14. The image processing apparatus according to claim 1 , wherein the processor is further programmed to detect whether the image feature direction may be used to interpolate an output pixel value at the pixel position under test by performing a vertical transition test between scan lines in the at least one block.

15. A vision mixing apparatus comprising:

an image processing apparatus in which output pixel values are generated with respect to pixels of an input image selected in accordance with an image feature direction in the input image, the apparatus including:

a processor programmed to

compare blocks of pixels of the input image, the blocks being disposed with respect to a pixel position under test so that a correlation between the blocks indicates an image feature direction applicable to that pixel position, and

detect whether the image feature direction may be used to interpolate an output pixel value at the pixel position under test by performing at least:

(i) a monotonicity test to determine whether at least one block of the block of pixels exhibits a monotonic variation in a pixel property in a horizontal direction;

(ii) a direction test to detect whether a detected image feature direction for the pixel position under test meets one or more predetermined directional criteria including at least a first directional criterion, and

(iii) in at least cases where the direction test fails, a pixel value test to detect whether pixel values in image regions disposed with respect to the pixel position under test in accordance with possible image feature directions meet one or more predetermined pixel value criteria,

wherein if the direction test fails, an interpolation direction is selected for the pixel position under test as that image feature direction having the greatest block correlation which passes the pixel value test.

16. A video display apparatus comprising:

an image processing apparatus in which output pixel values are generated with respect to pixels of an input image selected in accordance with an image feature direction in the input image, the apparatus including:

a processor programmed to

compare blocks of pixels of the input image, the blocks being disposed with respect to a pixel position under test so that a correlation between the blocks indicates an image feature direction applicable to that pixel position, and

detect whether the image feature direction may be used to interpolate an output pixel value at the pixel position under test by performing at least:

(i) a monotonicity test to determine whether at least one block of the block of pixels exhibits a monotonic variation in a pixel property in a horizontal direction;

(ii) a direction test to detect whether a detected image feature direction for the pixel position under test meets one or more predetermined directional criteria including at least a first directional criterion, and

(iii) in at least cases where the direction test fails, a pixel value test to detect whether pixel values in image regions disposed with respect to the pixel position under test in accordance with possible image feature directions meet one or more predetermined pixel value criteria,

wherein if the direction test fails, an interpolation direction is selected for the pixel position under test as that image feature direction having the greatest block correlation which passes the pixel value test.

17. An image processing method in which output pixel values are generated with respect to pixels of an input image selected in accordance with an image feature direction in the input image, the method comprising:

comparing blocks of pixels of the input image, the blocks being disposed with respect to a pixel position under test so that a correlation between the blocks indicates an image feature direction applicable to that pixel position; and

detecting whether the image feature direction may be used to interpolate an output pixel value at the pixel position under test, the detecting step including at least:

(i) a monotonicity test to determine whether at least one block of the block of pixels exhibits a monotonic variation in a pixel property in a horizontal direction;

(ii) performing a direction test to detect whether a detected image feature direction for the pixel position under test meets one or more predetermined directional criteria including at least a first directional criterion; and

(iii) in at least cases where the direction test fails, performing a pixel value test to detect whether pixel values in image regions disposed with respect to the pixel position under test in accordance with possible image feature directions meet one or more predetermined pixel value criteria,

wherein if the direction test fails, an interpolation direction is selected for the pixel position under test as that image feature direction having the greatest block correlation which passes the pixel value test.

18. A non-transitory computer readable storage medium having instructions stored therein, which when executed by a computer, causes the computer to perform a method according to claim 17 .