Image processing capable of handling multi-level image data without deterioration of image quality in highlight areas

An image processing method of quantizing multi-value image data to binary data, comprising:

• the first step of generating a random number, and binary-converting lower-bit data of an image signal of a pixel of interest on the basis of the generated random number;
• the second step of adding the lower-bit data of the pixel of interest binary-converted in the first step to upper-bit data of the image signal of the pixel of interest;
• and
• the third step of calculating a plurality of weighted average values of a plurality of pixels near the pixel of interest on the basis of binary data of pixels, which have already been binary-converted before the pixel of interest, and binary-converting the upper-bit data of the image signal of the pixel of interest on the basis of the plurality of calculated weighted average values.

An image processing apparatus for quantizing multi-value image data to binary data, comprising:

• binary data memory means for storing binary data of pixels, which have already been binary-converted before a pixel of interest;
• average value calculation means for calculating a weighted average value near the pixel of interest on the basis of the binary data stored in said binary data memory means;
• random number generation means for generating a random number;
• lower-bit binary-conversion means for binary-converting lower-bit data of a multi-value image signal on the basis of the random number value generated by said random number generation means;
• adder means for adding the lower-bit data binary-converted by said lower-bit binary-conversion means to upper-bit data of the multi-value image signal;
• upper-bit binary-conversion means for binary-converting the sum data from said adder means on the basis of the weighted average value calculated by said average value calculation means;
• error memory means for storing a density error generated upon binary conversion by said upper-bit binary-conversion means;
• correction means for preserving a density by distributing the density error stored in said error memory means to non-binary-converted pixels near the pixel of interest;
• and
• setting means for sharing storage areas of said binary data memory means and said error memory means, and setting a ratio of the storage areas of said binary data memory means and said error memory means according to processing.

An image processing method of quantizing multi-value image data to binary data, comprising:

• the first step of generating a random number, and binary-converting lower-bit data of an image signal of a pixel of interest on the basis of the generated random number;
• the second step of adding the lower-bit data of the pixel of interest binary-converted in the first step to upper-bit data of the image signal of the pixel of interest;
• the third step of calculating a plurality of weighted average values of a plurality of pixels near the pixel of interest on the basis of stored binary data of pixels which have already been binary-converted before the pixel of interest, and binary-converting the upper-bit data of the image signal of the pixel of interest on the basis of the plurality of calculated weighted average values;
• the fourth step of storing a density error generated upon binary conversion in the third step, and preserving a density by distributing the stored density error to non-binary-converted pixels near the pixel of interest;
• and
• the setting step of sharing a storage area of the binary data of the pixels which have already been binary-converted before the pixel of interest, and a storage area of the density error generated upon binary conversion in the third step, and setting a ratio of the two storage areas according to processing.

An image processing apparatus capable of binary-converting multi-value image data expressed by an n-bit signal by at least two different binary conversion methods in units of pixels, comprising:

• selection means for selecting one of the binary conversion methods on the basis of a value of upper l-bit data (l<n) of the multi-value image data;
• first binary-conversion means for binary-converting lower m-bit data (m<n) of the multi-value image data by the binary conversion method selected by said selection means;
• and
• second binary-conversion means for binary-converting the multi-value image data quantized by binary-converting the lower m-bit data by a binary conversion method different from the binary conversion method for the lower m-bit data.

An image processing method in an image processing apparatus capable of binary-converting multi-value image data expressed by an n-bit signal by at least two different binary conversion methods in units of pixels, comprising:

• the selection step of selecting one of the binary conversion methods on the basis of a value of upper l-bit data (l<n) of the multi-value image data;
• the first binary-conversion step of binary-converting lower m-bit data (m<n) of the multi-value image data by the binary conversion method selected in the selection step;
• and
• the second binary-conversion step of binary converting the multi-value image data quantized by binary-converting the lower m-bit data by a binary conversion method different from the binary conversion method for the lower m-bit data.

An image processing apparatus for quantizing multi-value image data to n-value data, comprising:

• calculation means for calculating a weighted average value near a pixel of interest on the basis of n-value data of pixels which have already been n-value-converted before the pixel of interest;
• random number generation means for generating a normalized uniform random number;
• second random number generation means for dividing a density area into (n-2) areas, and generating (n-2) normalized second random numbers by normalizing the uniform random number generated by said random number generation means according to the divided areas;
• n-value conversion means for n-value-converting the pixel of interest on the basis of the weighted average value calculated by said calculation means and the (n-2) random number values generated by said second random number generation means;
• and
• correction means for preserving a density by distributing a density error generated upon n-value conversion of the pixel of interest to non-n-value-converted pixels near the pixel of interest.

An image processing method in an image processing apparatus for quantizing multi-value image data to n-value data, comprising:

• the calculation step of calculating a weighted average value near a pixel of interest on the basis of n-value data of pixels which have already been n-value-converted before the pixel of interest;
• the random number generation step of dividing a density area into (n-2) areas, generating a normalized uniform random number, and generating (n-2) normalized second random numbers by normalizing the generated uniform random number according to the divided areas;
• the n-value conversion step of n-value-converting the pixel of interest on the basis of the weighted average value calculated in the calculation step and the (n-2) random number values generated in the random number generation step;
• and
• the correction step of preserving a density by distributing a density error generated upon n-value conversion of the pixel of interest to non-n-value-converted pixels near the pixel of interest.

An image processing apparatus comprising:

• first generation means for generating a random number;
• second generation means for generating a dither threshold value;
• selection means for selecting either one of the random number and the dither threshold value, on the basis of a value of upper l-bit data (l<n) of the multi-value image data;
• first binary conversion means for binary-converting lower m-bit data (m<n) of the multi-value image data by the random number or the dither threshold value selected by said selection means;
• and
• second binary-conversion means for binary-converting the multi-value image data quantized by binary-converting the lower m-bit data.