Method for re-positioning significant coefficient in image encoding/decoding process and device therefor

1. A method for encoding an image by an encoding device, the method comprising:

dividing a quantized coefficient block into coefficient groups having a predetermined size;

searching a first coefficient group which is a last coefficient group including a non-zero coefficient according to a predetermined inverse-scanning order within the quantized coefficient block;

searching a last non-zero coefficient according to the inverse-scanning order within a second coefficient group which is a preceding coefficient group of the first coefficient group according to the inverse-scanning order;

counting a number of non-zero coefficients within the first coefficient group and a number of zero coefficients at positions starting from the last non-zero coefficient within the second coefficient group;

when the number of non-zero coefficients within the first coefficient group is smaller than the number of zero coefficients at positions starting from the last non-zero coefficient within the second coefficient group, re-positioning the non-zero coefficient of the first coefficient group to a position next to the last non-zero coefficient within the second coefficient group; and

performing entropy-encoding by an encoder according to a predetermined scanning order.

2. A method for decoding an image by a decoding device, the method comprising:

deriving a quantized transform coefficient by entropy-decoding a bit stream output from an encoder;

parsing information related to a number of moved quantized transform coefficient which have been re-positioned by the encoder, from the bit stream;

positioning the quantized transform coefficient according to a predetermined scanning order within a quantized coefficient block divided into coefficient groups having a predetermined size;

searching a second coefficient group which is a last coefficient group including a last non-zero coefficient and a position of the last non-zero coefficient according to a predetermined inverse-scanning order within the quantized coefficient block;

re-positioning the moved quantized transform coefficient within the second coefficient group to a first coefficient group which is a next coefficient group of the second coefficient group according to the scanning order, wherein a coefficient positioned at a position preceding by the number −1 from the last coefficient to a position of the last coefficient according to the inverse-scanning order within the second coefficient group is re-positioned to the first coefficient group; and

inverse-quantizing the re-positioned quantized transform coefficient to derive a transform coefficient,

wherein the moved quantized transform coefficients are re-positioned by the encoder when the number of non-zero coefficients within the first coefficient group is smaller than the number of zero coefficients at positions starting from the last non-zero coefficient within the second coefficient group.

3. The method of claim 2 , wherein

the moved quantized transform coefficient is sequentially re-positioned, starting from a first coefficient position of the first coefficient group according to the inverse-scanning order.

4. The method of claim 2 , further comprising:

parsing information indicating whether a position of the quantized transform coefficient has been moved from the bit stream,

wherein when the information indicates that the position of the quantized transform coefficient has been moved, the position of the moved quantized transform coefficient is re-positioned to its original position.

5. An encoding device for encoding an image, the encoding device comprising:

a memory configured to store the image; and

a processor configured to:

divide a quantized coefficient block into coefficient groups having a predetermined size;

search a first coefficient group which is a last coefficient group including a non-zero coefficient according to a predetermined inverse-scanning order within the quantized coefficient block;

search a last non-zero coefficient according to the inverse-scanning order within a second coefficient group which is a preceding coefficient group of the first coefficient group according to the inverse-scanning order;

count a number of non-zero coefficients within the first coefficient group and a number of zero coefficients at positions starting from the last non-zero coefficient within the second coefficient group;

when the number of non-zero coefficients within the first coefficient group is smaller than the number of zero coefficients at positions starting from the last non-zero coefficient within the second coefficient group, re-position the non-zero coefficient of the first coefficient group to a position next to the last non-zero coefficient within the second coefficient group; and

perform entropy according to a predetermined scanning order.

6. A decoding device for decoding an image, the decoding device comprising:

a processor configured to:

derive a quantized transform coefficient by entropy-decoding a bit stream output from an encoder;

parse information related to a number of moved quantized transform coefficient which have been re-positioned by the encoder, from the bit stream;

position the quantized transform coefficient according to a predetermined scanning order within a quantized coefficient block divided into coefficient groups having a predetermined size;

search a second coefficient group which is a last coefficient group including a last non-zero coefficient and a position of the last non-zero coefficient according to a predetermined inverse-scanning order within the quantized coefficient block; and

re-position the moved quantized transform coefficient within the second coefficient block to a first coefficient group which is a next coefficient group of the second coefficient group according to the scanning order, wherein a coefficient positioned at a position preceding by the number −1 from the last coefficient to a position of the last coefficient according to the inverse-scanning order within the second coefficient group is re-positioned to the first coefficient group; and

an inverse-quantization performing unit configured to derive a transform coefficient by inverse-quantizing the re-positioned quantized transform coefficient,

wherein the moved quantized transform coefficients are re-positioned by the encoder when a number of non-zero coefficients within the first coefficient group is smaller than a number of zero coefficients at positions starting from the last non-zero coefficients within the second coefficient group.