Image processing method according to sort of image and image processing apparatus

An image processing method of applying a multi-level dither process to input gradation image data of M bits per one pixel on the basis of a multi-level dither threshold value arrangement so as to convert into image data having smaller gradation number of N bits (M>N) per one pixel, comprising the steps of:

• preparing a plurality of multi-level dither threshold value arrangements regularly arranged at an incline having an angle in an oblique direction so that an order of the multi-level dither threshold value arrangements between a dither reference threshold value and a dither threshold value plane in a dither matrix rides over a plurality of dither threshold value planes, with differentiating incline angles of the arrangement order;
• discriminating whether an image field is an image field emphasizing a resolution or an image field emphasizing a gradation with respect to the input gradation image data;
• selecting a corresponding multi-level dither threshold value arrangement among the prepared plural number of multi-level dither threshold value arrangements according to an image discrimination result by the discriminating step;
• and
• performing the multi-level dither process on the basis of the selected multi-level dither threshold value arrangement.

An image processing method of applying a multi-level dither process to input gradation image data of M bits per one pixel on the basis of a multi-level dither threshold value arrangement so as to convert into image data having smaller gradation number of N bits (M>N) per one pixel, comprising the steps of:

• preparing a plurality of multi-level dither threshold value arrangements regularly arranged at an incline so that an order of the multi-level dither threshold value arrangements between a dither reference threshold value and a dither threshold value plane in a dither matrix increases a spatial frequency in a low density portion of the input gradation image data and regularly arranged at an incline having an angle in an oblique direction so that it rides over a dither threshold value planes between an intermediate density portion and a high density portion, with differentiating angles of the incline of the arrangement order;
• discriminating whether an image field is an image field emphasizing a resolution or an image field emphasizing a gradation with respect to the input gradation image data;
• selecting a corresponding multi-level dither threshold value arrangement among the prepared plural number of multi-level dither threshold value arrangements according to an image discrimination result by the discriminating step;
• and
• performing the multi-level dither process on the basis of the selected multi-level dither threshold value arrangement.

An image processing apparatus comprising:

• storing means of storing a plurality of multi-level dither threshold value arrangements employed for a multi-level dither process;
• discriminating means of discriminating a given first gradation image data expressing one pixel by a first bit number into an image field emphasizing a resolution and an image field emphasizing a gradation;
• selecting means of selecting an optimum multi-level dither threshold value arrangement from a plurality of multi-level dither threshold value arrangements stored in the storing step on the basis of a discrimination result by the discriminating step;
• and
• converting means of converting the gradation image data into a second gradation image data expressing one pixel by a second bit number less than the first bit number according to a multi-level dither process on the basis of the optimum multi-level dither threshold value arrangement selected by the selecting step.

An image processing apparatus of applying a multi-level dither process to input gradation image data of M bits per one pixel on the basis of a multi-level dither threshold value arrangement so as to convert into image data having smaller gradation number of N bits (M>N) per one pixel, comprising:

• preparing means for preparing a plurality of multi-level dither threshold value arrangements regularly arranged at an incline having an angle in an oblique direction so that an order of the multi-level dither threshold value arrangements between a dither reference threshold value and a dither threshold value plane in a dither matrix rides over a plurality of dither threshold value planes, with differentiating angles of the incline of the arrangement order;
• discriminating means for discriminating whether an image field is an image field emphasizing a resolution or an image field emphasizing a gradation with respect to the input gradation image data;
• selecting means for selecting a corresponding multi-level dither threshold value arrangement among the prepared plural number of multi-level dither threshold value arrangements according to an image discrimination result by the discriminating step;
• and
• multi-level dither process means for performing the multi-level dither process on the basis of the selected multi-level dither threshold value arrangement.

An image processing apparatus of applying a multi-level dither process to input gradation image data of M bits per one pixel on the basis of a multi-level dither threshold value arrangement so as to convert into image data having smaller gradation number of N bits (M>N) per one pixel, comprising:

• preparing means for preparing a plurality of multi-level dither threshold value arrangements regularly arranged at an incline so that an order of the multi-level dither threshold value arrangements between a dither reference threshold value and a dither threshold value plane in a dither matrix increases a spatial frequency in a low density portion of the input gradation image data and regularly arranged at an incline having an angle in an oblique direction so that it rides over a dither threshold value planes between an intermediate density portion and a high density portion, with differentiating angles of the incline of the arrangement order;
• discriminating means for discriminating whether an image field is an image field emphasizing a resolution or an image field emphasizing a gradation with respect to the input gradation image data;
• selecting means for selecting a corresponding multi-level dither threshold value arrangement among the prepared plural number of multi-level dither threshold value arrangements according to an image discrimination result by the discriminating step;
• and
• multi-level dither process means for performing the multi-level dither process on the basis of the selected multi-level dither threshold value arrangement.