Method for encoding images, and an image coder

1. A method for encoding a digital image in a block-based manner, in which a spatial prediction for a block is performed to reduce amount of information to be transmitted, wherein the method comprises:

determining a classification for at least two neighboring blocks of a block to be predicted according to the image contents of said neighboring blocks;

selecting a sub-set of prediction methods from a set of available prediction methods on the basis of the classifications of said at least two neighboring blocks;

defining an arrangement for the list of said sub-set of prediction methods in a rank order, said rank order determined on the basis of the classifications of said at least two neighboring blocks, wherein each prediction method in the said list has a unique rank with respect to each of the other prediction methods;

selecting a prediction method for the block to be predicted from said sub-set of prediction methods;

forming a spatial prediction for the block to be predicted using the selected prediction method; and

providing a signal representative of rank of the selected prediction method.

2. A method according to claim 1 , comprising calculating a cost function representative of an error incurred when using a particular prediction method to form a spatial prediction for the block to be predicted and selecting the prediction method for the block to be predicted from said sub-set of prediction methods that yields the smallest value for the cost function.

3. A method according to claim 2 , wherein the cost function includes a measure of an error incurred when using a particular prediction method to form a spatial prediction for the block to be predicted and a measure of an amount of information required to be transmitted to a corresponding decoder when said particular prediction method is selected.

4. A method according to claim 1 , wherein the classification for a neighboring block is determined on the basis of directionality in the image contents of the neighboring block.

5. A method for decoding an encoded digital image in a block-based manner, in which a spatial prediction for a block is performed to reduce amount of information to be transmitted, wherein the method comprises:

determining a classification for at least two neighboring blocks of a block to be predicted according to the image contents of said neighboring blocks;

selecting a sub-set of prediction methods from a set of available prediction methods on the basis of the classifications of said at least two neighboring blocks;

defining an arrangement for the list of said sub-set of prediction methods in a rank order, said rank order determined on the basis of the classifications of said at least two neighboring blocks, wherein each prediction method in the said list has a unique rank with respect to each of the other prediction methods;

receiving a signal indicative of rank order prediction method in said sub-set of prediction methods;

selecting a prediction method for the block to be predicted from said list of said sub-set of prediction methods, said prediction method having said rank order; and

forming a spatial prediction for the block to be predicted using the selected prediction method.

6. A method according to claim 5 , wherein the classification for a neighboring block is determined on the basis of directionality in the image contents of the neighboring block.

7. An encoder for encoding a digital image in a block-based manner, the encoder being arranged to perform a spatial prediction for a block to reduce an amount of information to be transmitted, wherein the encoder comprises:

a block classifier arranged to determine a classification for at least two neighboring blocks of a block to be predicted according to the image contents of said neighboring blocks;

a prediction method selector arranged to:

select a sub-set of prediction methods from a set of available prediction methods on the basis of the classifications of said at least two neighboring blocks;

define an arrangement for the list of said sub-set of prediction methods in a rank order, said rank order determined on the basis of the classifications of said at least two neighboring blocks, wherein each prediction method in the said list has a unique rank with respect to each of the other prediction methods; and

select a prediction method for the block to be predicted from said sub-set of prediction methods;

a prediction estimator to form a spatial prediction for the block to be predicted using the selected prediction method; and

a multiplexing unit to provide a signal representative of rank of the selected prediction method.

8. An encoder according to claim 7 , comprising a cost function calculator for calculating a cost function representative of an error incurred when using a particular prediction method for forming a spatial prediction for the block to be predicted and the prediction method selector is arranged to select the prediction method for the block to be predicted from said sub-set of prediction methods that yields the smallest value for the cost function.

9. An encoder according to claim 8 , wherein the cost function includes a measure of an error incurred when using a particular prediction method to form a spatial prediction for the block to be predicted and a measure of an amount of information required to be transmitted to a corresponding decoder when said particular prediction method is selected.

10. An encoder according to claim 7 , wherein the classification for a neighboring block is determined on the basis of directionality in the image contents of the neighboring block.

11. A decoder for decoding an encoded digital image in a block-based manner, the decoder being arranged to perform a spatial prediction for a block to reduce an amount of information to be transmitted, wherein the decoder comprises:

a block classifier arranged to determine a classification for at least two neighboring blocks of a block to be predicted according to the image contents of said neighboring blocks;

a prediction method selector for:

selecting a sub-set of prediction methods from a set of available prediction methods on the basis of the classifications of said at least two neighboring blocks; and

defining an arrangement for the list of said sub-set of prediction methods in a rank order, said rank order determined on the basis of the classifications of said at least two neighboring blocks, wherein each prediction method in the said list has a unique rank with respect to each of the other prediction methods;

a demultiplexing unit for receiving a signal indicative of rank order prediction method in said sub-set of prediction methods;

the prediction method selector further arranged to select a prediction method for the block to be predicted from said list of said sub-set of prediction methods, said prediction method having said rank order; and

a prediction estimator to form a spatial prediction for the block to be predicted using the selected prediction method.

12. A method according to claim 11 , wherein the classification for a neighboring block is determined on the basis of directionality in the image contents of the neighboring block.