Method for controlling the amount of compressed data

1. A method for controlling the amount of compressed data that is used to represent an image when the image is compressed as a plurality of frequency subbands and subsequently decompressed and viewed on a display by a human observer, comprising the steps of:

(a) specifying one or more viewing condition parameters;

(b) determining a quantizer step size for each frequency subband that minimizes an amount of data that must be encoded for a compressed representation of the image by using the one or more viewing condition parameters and a model that characterizes the human visual system;

(c) using an image processor to compress the image using the quantizer step size for each frequency subband to produce an amount of compressed data;

(d) comparing a pre-specified maximum amount of compressed data to the amount of data produced by using the quantizer step size; and

(e) if the amount of compressed data exceeds the pre-specified maximum amount, reducing the amount of compressed data by discarding least visually relevant compressed data so that the amount of compressed data is less than or equal to the pre-specified maximum amount.

2. The method according to claim 1 , wherein the one or more viewing condition parameters include a threshold viewing distance.

3. The method according to claim 2 , wherein the threshold viewing distance is specified in screen heights.

4. The method according to claim 2 , wherein the threshold viewing distance is specified in picture heights.

5. The method according to claim 2 , wherein the threshold viewing distance is specified in displayed pixels.

6. The method according to claim 1 , wherein the step of reducing the amount of compressed data comprises applying post-compression rate-distortion optimization.

7. The method according to claim 6 , wherein the post-compression rate-distortion optimization uses distortion weighting factors that are calculated using the model of the human visual system and one or more viewing condition parameters.

8. The method according to claim 7 , wherein the one or more viewing condition parameters that are used to calculate the distortion weighting factors are the same as the one or more viewing condition parameters that are used to determine the quantizer step sizes.

9. The method according to claim 7 , wherein the one or more viewing condition parameters that are used to calculate the distortion weighting factors include at least one viewing condition parameter that is different from the one or more viewing condition parameters that are used to determine the quantizer step sizes.

10. The method according to claim 1 , wherein the quantizer step size for each frequency subband is a threshold step size for the specified viewing condition parameters.

11. The method according to claim 1 , wherein the quantizer step size for each frequency subband is less than a corresponding threshold step size for the specified viewing condition parameters.

12. The method according to claim 11 , wherein the ratio of the quantizer step size for each frequency subband to the corresponding threshold step size is a constant value less than or equal to 1.0.

13. The method according to claim 1 , wherein the original image has three or more color channels and the step of determining a quantizer step size for each frequency subband further includes determining a separate quantizer step size for each frequency subband of the three or more ecilor channels.

14. The method according to claim 13 , wherein the step of determining a quantizer step size for each frequency subband further includes determining a single quantizer step size for corresponding frequency subbands across the three or more color channels by selecting the minimum value of the separate quantizer step sizes for corresponding frequency subbands.

15. A computer readable medium useful in association with a computer system that includes a processor and a memory, the computer readable medium including computer instructions for controlling the amount of compressed data that is used to represent an image when the image is compressed as a plurality of frequency subbands and subsequently decompressed and viewed on a display by a human observer by:

(a) specifying one or more viewing condition parameters;

(b) determining a quantizer step size for each frequency subband that minimizes an amount of data that must be encoded for a compressed representation of the image by using the one or more viewing condition parameters and a model that characterizes the human visual system;

(c) compressing the image using the quantizer step size for each frequency subband to produce an amount of compressed data;

(d) comparing a pre-specified maximum amount of compressed data to the amount of compressed data produced by using the quantizer step size; and

(e) if the amount of compressed data exceeds the pre-specified maximum amount of compressed data, reducing the amount of compressed data by discarding least visually relevant compressed data so that the amount of compressed data is less than or equal to the pre-specified maximum amount of compressed data.

16. The computer readable medium according to claim 15 , wherein the one or more viewing condition parameters include a threshold viewing distance.

17. The computer readable medium according to claim 16 , wherein the threshold viewing distance is specified in screen heights.

18. The computer readable medium according to claim 16 , wherein the threshold viewing distance is specified in picture heights.

19. The computer readable medium according to claim 16 , wherein the threshold viewing distance is specified in displayed pixels.

20. The computer readable medium according to claim 15 , wherein the step of reducing the acceptable amount of compressed data includes applying post-compression rate-distortion optimization.

21. The method claimed in claim 1 , wherein the model that characterizes the human visual system is a contrast sensitivity function.

22. The method claimed in claim 1 , wherein the model that characterizes the human visual system is used to determine both the quantizer step size, and distortion weighting factors that enable reduction of the amount of compressed data when it initially exceeds the pre-specified maximum amount of compressed data.

23. The computer readable medium claimed in claim 15 , wherein the model that characterizes the human visual system is a contrast sensitivity function.

24. The computer readable medium claimed in claim 15 , wherein the model that characterizes the human visual system is used to determine both the quantizer step size, and distortion weighting factors that enable reduction of the amount of compressed data when it initially exceeds the pre-specified maximum amount of compressed data.