Methods and systems for efficient prediction-mode selection

1. A method, in a video coder, for selecting candidate intra prediction modes for a block of samples in a picture, said method comprising:

obtaining directional information in at least one direction for a block of samples, wherein said directional information is derived from at least one of: a first plurality of pairs of samples in said block of samples, wherein a first pair of samples in said first plurality of pairs of samples is not oriented in the same direction as a second pair of samples in said first plurality of pairs of samples, a second plurality of pairs of samples in said block of samples, wherein at least one of said pairs of samples in said second plurality of samples is not adjacent to any other of said pairs of samples in said second plurality of samples, and a third plurality of pairs of samples in said block of samples, wherein at least one of said pairs of samples in said third plurality of pairs of samples comprises a sample from the interior of said block of samples; and wherein any sample used in said obtaining is a sample within said block of samples; and

selecting at least one candidate intra prediction mode for said block of samples based on said directional information.

2. The method as described in claim 1 wherein said obtaining directional information comprises calculating at least one direction cost wherein said at least one direction cost is associated with a direction of an intra prediction mode.

3. A method as described in claim 1 further comprising selecting an intra prediction mode for said block of samples from said at least one candidate intra prediction mode.

4. A method, in a video coder, for selecting candidate intra prediction modes for a block of samples in a picture, said method comprising:

obtaining directional information for a block of samples, wherein said obtaining comprises calculating a plurality of direction costs, wherein:

each of said plurality of direction costs is associated with a direction of an intra prediction mode; and

each of said plurality of direction costs is determined using an associated plurality of sample pairs, wherein each sample in said sample pairs is within said block of samples;

selecting at least one candidate intra prediction mode for said block of samples based on said directional information; and

determining a direction mode for said block of samples wherein said determining comprises:

identifying a maximum direction cost from said plurality of direction costs;

identifying a minimum direction cost from said plurality of direction costs;

calculating a difference between said maximum direction cost and said minimum direction cost; and

determining said direction mode for said block of samples to be a DC direction mode when said difference is less than a threshold.

5. A method as described in claim 4 wherein said calculating a direction cost in said plurality of direction costs further comprises summing an absolute value difference in a said associated plurality of sample pairs.

6. A method as described in claim 5 wherein at least one pair of samples in said associated plurality of sample pairs contains a sample from the interior of said sub-block.

7. A method as described in claim 5 wherein said associated plurality of sample pairs is associated with a prediction mode direction.

8. A method as described in claim 4 wherein said selecting at least one candidate intra prediction mode for said block of samples depends on whether or not said direction mode is said DC direction mode.

9. A method, in a video coder, for selective application of intra prediction mode selection for a block of samples in an encoder, said method comprising:

dividing a block of samples into a plurality of sub-blocks;

obtaining directional information in at least one direction for at least one of said plurality of sub-blocks, wherein said directional information in a first direction is calculated using a first plurality of pairs of samples in said at least one sub-block;

combining said directional information, thereby producing a direction cost for said block of samples;

comparing said direction cost for said block of samples with a cost for inter coding said block of samples, thereby producing a cost comparison result; and

selectively performing intra prediction mode selection for said block of samples based on said cost comparison result.

10. The method as described in claim 9 wherein said obtaining directional information comprises calculating a direction cost for said sub-block, wherein said direction cost for said sub-block is associated with a direction of an intra prediction mode.

11. A method as described in claim 9 further comprising:

adjusting said direction cost for said block of samples by a scale factor, thereby producing a scaled direction cost; and

wherein said comparing comprises comparing said scaled direction cost with a cost for inter coding said block of samples, thereby producing a cost comparison result.

12. A method as described in claim 10 wherein said calculating further comprises summing an absolute value difference in a plurality of sample pairs from said sub-block.

13. A method as described in claim 12 wherein at least one of said plurality of sample pairs contains a sample from the interior of said sub-block.

14. A method as described in claim 11 wherein said scale factor is related to a quantization parameter.

15. A method as described in claim 9 wherein said cost for inter coding said block of samples is a minimum cost of inter prediction.

16. A method as described in claim 11 wherein said selectively performing intra prediction occurs when said cost comparison result indicates said scaled direction cost is less than said cost for inter coding said block of samples.

17. A method, in a video coder, for selecting candidate intra prediction modes for a block of samples in a picture, said method comprising:

dividing a block of samples into a plurality of sub-blocks;

calculating at least one direction cost for at least one of said plurality of sub-blocks, wherein said calculating uses a plurality of samples, wherein each sample in said plurality of samples is within said at least one sub-block; and

selecting at least one candidate intra prediction mode for said block of samples based on said at least one direction cost for at least one of said plurality of sub-blocks.

18. A method as described in claim 17 further comprising determining a direction mode for at least one of said plurality of sub-blocks wherein said determining further comprises:

identifying a maximum direction cost from said at least one direction cost;

identifying a minimum direction cost from said at least one direction cost;

calculating a difference between said maximum direction cost and said minimum direction cost; and

determining said direction mode for said at least one of said plurality of sub-block to be a DC direction mode when said difference is less than a threshold.

19. A method as described in claim 17 further comprising selecting an intra prediction mode for said block of samples from said at least one candidate intra prediction modes.

20. A method as described in claim 17 wherein said calculating further comprises summing an absolute value difference in a plurality of sample pairs from said sub-block of samples.

21. A method as described in claim 20 wherein at least one of said plurality of sample pairs contains a sample from the interior of said sub-block.

22. A method as described in claim 17 wherein said selecting is dependent on a number of said sub-blocks having the same direction mode.

23. A method as described in claim 22 wherein said number is at least three.