Device, method, and program for image coding
There is provided with an image coding method including: splitting an image into a plurality of block images each having plural pixels; performing a Discrete Cosine Transform (DCT) processing on the block image to calculate DCT coefficients from the block image; determining whether the block image is in a flat state or non-flat state depending on the DCT coefficients calculated from the block image; specifying as quantization step sizes for first DCT coefficients which are DCT coefficients equivalent to one-dimensional DCT coefficient out of the calculated DCT coefficients, adjusting step sizes equal to or smaller than values of the first DCT coefficients and as quantization step sizes for second DCT coefficients other than the first DCT coefficients out of the calculated DCT coefficients, standard step sizes given beforehand, when the block image is in the flat state; and quantizing the calculated DCT coefficients with respective quantization step sizes specified.
1. An image coding device comprising:
an image splitting circuit configured to split an image into a plurality of block images each having plural pixels;
a Discrete Cosine Transform (DCT) circuit to configured to perform a DCT processing on the block image to calculate DCT coefficients from the block image;
a quantizing circuit configured to quantize the calculated DCT coefficients with respective quantization step sizes specified;
a distortion determining circuit configured to determine whether the block image is in a flat state or non-flat state depending on the calculated DCT coefficients from the block image; and
a step size determination circuit to configured to specify as quantization step sizes for first DCT coefficients which are DCT coefficients equivalent to one-dimensional DCT coefficient out of the calculated DCT coefficients, adjusting step sizes equal to or smaller than values of the first DCT coefficients for the quantizing circuit, and specifying standard step sizes given beforehand as quantization step sizes for second DCT coefficients other than the first DCT coefficients out of the calculated DCT coefficients for the quantizing circuit, when the block image is in the flat state.
2. The device according to claim 1 wherein the distortion determining circuit
counts a first number of the first DCT coefficients which are not zero and a second number of two-dimensional DCT coefficients having pre-determined value or less out of two-dimensional DCT coefficients included in the second DCT coefficients, and
determines that the block image is in the flat state if the first number is a first threshold or more and the second number is a second threshold or more.
3. The device according to claim 2 wherein the distortion determining circuit
counts a third number of first DCT coefficients which are not zero in a y direction and a fourth number of first DCT coefficients which are not zero in a x direction perpendicular to the y direction, as the first number, and
determines that the block image is in the flat state, if the third number is a third threshold or more, the fourth number is a fourth threshold or more, and the second number is the second threshold or more.
4. The device according to claim 1 , wherein the step size determination circuit selects DCT coefficients of a number depending on a compression rate specified beforehand among from the first DCT coefficients, specifies for the selected DCT coefficients the adjusting step sizes, and specifies standard step sizes given beforehand for the DCT coefficients other than the selected DCT coefficients out of the first DCT coefficients.
5. The device according to claim 1 wherein if the standard step size given beforehand for the first DCT coefficient is smaller than value of the first DCT coefficient, the step size determination circuit specifies the standard step size as the adjusting step size of the first DCT coefficient for the quantizing circuit.
6. The device according to claim 1 further comprising a variable length coding circuit configured to perform variable length coding on the DCT coefficients quantized by the quantizing circuit.
7. The device according to claim 1 further comprising a block analysis circuit configured to analyze which of an odd-functional distribution pattern or an even-functional distribution pattern the block image has in each of a mutually-perpendicular x direction and y direction,
wherein the step size determination circuit:
if the block image has the odd-functional distribution pattern in the x or y direction, specifies the adjusting step sizes for DCT coefficients associated with basis functions of an odd function out of the first DCT coefficients in the x or y direction, and specifies standard step sizes given beforehand for DCT coefficients associated with basis functions of an even function out of the first DCT coefficients in the x or y direction; and
if the block image has the even-functional distribution pattern in the x or y direction, specifies the adjusting step sizes for DCT coefficients associated with the basis functions of the even function out of the first DCT coefficients in the x or y direction, and specifies standard step sizes given beforehand for DCT coefficients associated with the basis functions of the odd function out of the first DCT coefficients in the x or y direction.
8. The device according to claim 7 wherein the block analysis circuit analyzes which of the odd-functional distribution pattern or the even-functional distribution pattern the block image has in each of the x direction and y direction, by computation using values of respective pixels in the block image.
9. The device according to claim 7 wherein if the block image has the even-functional pattern in the x or y direction, the step size determination circuit specifies the adjusting step size for a “01” or “10” coefficient out of the DCT coefficients associated with the basis functions of the odd function in the x or y direction.
10. An image coding method comprising:
splitting an image into a plurality of block images each having plural pixels;
performing a Discrete Cosine Transform (DCT) processing on the block image to calculate DCT coefficients from the block image;
determining whether the block image is in a flat state or non-flat state depending on the calculated DCT coefficients from the block image;
specifying as quantization step sizes for first DCT coefficients which are DCT coefficients equivalent to one-dimensional DCT coefficient out of the calculated DCT coefficients and adjusting step sizes equal to or smaller than values of the first DCT coefficients and as quantization step sizes for second DCT coefficients other than the first DCT coefficients out of the calculated DCT coefficients, standard step sizes given beforehand, when the block image is in the flat state; and quantizing the calculated DCT coefficients with respective quantization step sizes specified.
11. The method according to claim 10 wherein the determining includes
counting a first number of the first DCT coefficients which are not zero and a second number of two-dimensional DCT coefficients having pre-determined value or less out of two-dimensional DCT coefficients included in the second DCT coefficients, and
determining that the block image is in the flat state if the first number is a first threshold or more and the second number is a second threshold or more.
12. The method according to claim 11 wherein the counting includes counting a third number of first DCT coefficients which are not zero in a y direction and a fourth number of first DCT coefficients which are not zero in a x direction perpendicular to the y direction, as the first number, and
the determining includes determining that the block image is in the flat state, if the third number is a third threshold or more, the fourth number is a fourth threshold or more, and the second number is the second threshold or more.
13. The method according to claim 10 , further comprising inputting a compression rate, wherein
the specifying includes selecting DCT coefficients of a number depending on the compression rate among from the first DCT coefficients, specifying for the selected DCT coefficients the adjusting step sizes, and specifying standard step sizes given beforehand for the DCT coefficients other than the selected DCT coefficients out of the first DCT coefficients.
14. The method according to claim 10 wherein if the standard step size given beforehand for the first DCT coefficient is smaller than value of this first DCT coefficient, the specifying includes specifying the standard step size as the adjusting step size of this first DCT coefficient.
15. The method according to claim 10 further comprising performing variable length coding on the DCT coefficients quantized.
16. The method according to claim 10 further comprising analyzing which of an odd-functional distribution pattern or an even-functional distribution pattern the block image has in each of a mutually-perpendicular x direction and y direction,
wherein the specifying includes:
if the block image has the odd-functional distribution pattern in the x or y direction, specifying the adjusting step sizes for DCT coefficients associated with basis functions of an odd function out of the first DCT coefficients in the x or y direction, and specifying standard step sizes given beforehand for DCT coefficients associated with basis functions of an even function out of the first DCT coefficients in the x or y direction; and
if the block image has the even-functional distribution pattern in the x or y direction, specifying the adjusting step sizes for DCT coefficients associated with the basis functions of the even function out of the first DCT coefficients in the x or y direction, and specifying standard step sizes given beforehand for DCT coefficients associated with the basis functions of the odd function out of the first DCT coefficients in the x or y direction.
17. The method according to claim 16 wherein the analyzing includes analyzing which of the odd-functional distribution pattern or the even-functional distribution pattern the block image has in each of the x direction and y direction, by computation using values of respective pixels in the block image.
18. The method according to claim 16 wherein if the block image has the even-functional pattern in the x or y direction, the specifying includes specifying the adjusting step size for a “01” or “10” coefficient out of the DCT coefficients associated with the basis functions of the odd function in the x or y direction.
19. A computer readable medium storing program instructions which when executed by a computer results in performance of steps comprising:
splitting an image into a plurality of block images each having plural pixels;
performing a Discrete Cosine Transform (DCT) processing on the block image to calculate DCT coefficients from the block image;
determining whether the block image is in a flat state or non-flat state depending on the calculated DCT coefficients from the block image;
specifying as quantization step sizes for first DCT coefficients which are DCT coefficients equivalent to one-dimensional DCT coefficient out of the calculated DCT coefficients and adjusting step sizes equal to or smaller than values of the first DCT coefficients and as quantization step sizes for second DCT coefficients other than the first DCT coefficients out of the calculated DCT coefficients, standard step sizes given beforehand, when the block image is in the flat state; and
quantizing the calculated DCT coefficients with respective quantization step sizes specified.