Method for compressing image data and image data compressing system

1. A method for compressing image data, comprising:

setting a target data increment of each of N blocks that need to be compressed of an image to be compressed according to a 0 th accumulated target data size and an N th accumulated target data size;

before compressing an n th block among the N blocks, calculating an (n−1) th accumulated compressed data size of an (n−1) th block among the N blocks;

calculating an (n−1) th accumulated target data size of the (n−1) th block according to the 0th accumulated target data size and the target data increment;

removing X least significant bit(s) of a plurality of sets of data in the n th block when a difference between the (n−1) th accumulated compressed data size and the (n−1) th accumulated target data size is smaller than a first predetermined threshold to generate an updated n th block; and

compressing the updated n th block to generate a compressed n th block;

wherein, N and X are finite positive integers, and n is a finite positive integer not greater than N.

2. The method according to claim 1 , further comprising:

before an m th block among the N blocks is compressed, calculating an (m−1) th accumulated compressed data size of an (m−1) th block among the N blocks;

calculating an (m−1) th accumulated target data size of the (m−1) th block according to the 0 th accumulated target data size and the target data increment;

removing Y least significant bit(s) of a plurality of sets of data in the m th block when a difference between the (m−1) th accumulated compressed data size and the (m−1) th accumulated target data size is greater than the first threshold but smaller than a second threshold to generate an updated m th block; and

compressing the updated m th block to generate a compressed m th block;

wherein, the second predetermined threshold is greater than the first predetermined threshold, Y is a finite positive integer smaller than X, and m is a finite positive integer not greater than N and different from n.

3. The method according to claim 2 , wherein X is 2 and Y is 1.

4. The method according to claim 1 , wherein 0 th accumulated target data size is greater than the first predetermined threshold.

5. The method according to claim 1 , further comprising updating the 0 th accumulated target data size according to compression archives of each of the blocks that needs to be compressed.

6. The method according to claim 1 , wherein the N th accumulated target data size is a total data size divided by a target compression rate.

7. An image data compressing system, comprising:

a compressor, compressing an image to be compressed according to a predetermined compression algorithm, wherein data of the image comprises N blocks that need to be compressed; and

a compression rate control circuit, coupled to the compressing module, comprising:

a compressed data accumulator, calculating an (n−1) th accumulated compressed data size of an (n−1) th block among the N blocks before an n th block among the N blocks is compressed according to a block compressed data size that the compressing module generates after compressing the (n−1) th block;

a target data accumulator, calculating an (n−1) th accumulated target data size of the (n−1) th block according to a 0 th accumulated target data size and a target data increment of each of the blocks; and

a comparator, outputting a first compression control signal to the compressing module when a difference between the (n−1) th accumulated compressed data size and the (n−1) th accumulated target data size is smaller than a first predetermined threshold;

wherein, the compressing module further removes X least significant bit(s) of a plurality of sets of data in the n th block to generate an updated n th block when receiving the first compression control signal, and compresses the updated n th block to generate a compressed n th block; and

N and X are finite positive integers, and n is a finite positive integer not greater than N.

8. The image data compressing system according to claim 7 , wherein:

the compressed data accumulator further calculates an (m−1) th accumulated compressed data size of an (m−1) th block among the N blocks before an m th block among the N blocks is compressed;

the target data accumulator further calculates an (m−1) th accumulated target data size of the (m−1) th block according to the 0 th accumulated target data size and the target data increment;

the comparator further outputs a second compression control signal to the compressing module when a difference between the (m−1) th accumulated compressed data size and the (m−1) th accumulated target data size is greater than the first threshold but smaller than a second threshold;

the compressor further removes Y least significant bit(s) of a plurality of sets of data in the m th block when the second compression control signal is received to generate an updated m th block, and compresses the updated m th block to generate an compressed m th block; and

the second predetermined threshold is greater than the first predetermined threshold, m is a finite positive integer not greater than N and different from n, and Y is a finite positive integer smaller than X.

9. The image data compressing system according to claim 8 , wherein X is 2 and Y is 1.

10. The image data compressing system according to claim 7 , wherein 0 th accumulated target data size is greater than the first predetermined threshold.

11. The image data compressing system according to claim 7 , wherein the compression rate control circuit further updates the 0 th accumulated target data size according to compression archives of each of the blocks that needs to be compressed.

12. The image data compressing system according to claim 7 , wherein the N th accumulated target data size is a total data size divided by a target compression rate.