Image processing apparatus and image processing method
There is provided an image processing apparatus in which in a case of tone-expressing one pixel corresponding to a plurality of areas (printing resolution) in a multi-pass print, it is possible to output a uniform image without graininess or density unevenness over all-tone regions from a low-density region to a high-density region. To this end, by referring to dot patterns in which a print or a non-print of dots onto each pixel (area) corresponding to a printing resolution are in advance defined, quantization data is converted into binary data having the higher printing resolution. At this time, the dot patterns are prepared to be different from each other for M times of scans in a multi-pass print of M passes. In consequence, it is possible to restrict a variation of a coverage ratio due to a printing position displacement in each printing scan.
1. An image processing apparatus for executing an image process for printing an image on a printing medium by causing a print head which prints dots to scan relative to a predetermined region on the printing medium plural times resulting in a plurality of scans comprising:
a control unit which comprises:
a central processing unit;
a distribution unit programmed to distribute multi-valued image data corresponding to the predetermined region into at least a first multi-valued data corresponding to the predetermined region for a first scan and a second multi-valued data corresponding to the predetermined region for a second scan;
a quantization unit programmed to quantize each of the first multi-valued data and the second multi-valued data to generate a first quantization data and a second quantization data; and
a conversion unit programmed to convert the first quantization data into a first print data used for printing dots by the first scan to the predetermined region by using a first dot pattern which indicates number and positions of dots printed to pixel areas in the predetermined region in accordance with a value of the first quantization data and convert the second quantization data into a second print data used for printing dots by the second scan to the predetermined region by using a second dot pattern which indicates number and positions of dots printed to pixel areas in the predetermined region in accordance with a value of the second quantization data,
wherein (i) at least one pixel area for printing dot indicated by the second dot pattern corresponding to the second quantization data having a predetermined value is positioned at a same position as a pixel area for printing dot indicated by the first dot pattern corresponding to the first quantization data having the predetermined value and (ii) at least one pixel area for printing dot indicated by the second dot pattern corresponding to the second quantization data having the predetermined value is positioned at a same position as a pixel area for not printing dot indicated by the first dot pattern corresponding to the first quantization data having the predetermined value.
2. The image processing apparatus according to claim 1 , wherein the quantization unit quantizes each of the first multi-valued data and the second multi-valued data by an error diffusion method.
3. The image processing apparatus according to claim 1 , wherein the first dot pattern corresponding to the first quantization data having the predetermined value and the second dot pattern corresponding to the second quantization data having the predetermined value are dot patterns by which a coverage area of the dots is kept to be substantially constant regardless of a printing position displacement between the first scan and the second scan.
4. A storage medium for storing programs for operating a computer as an image processing apparatus according to claim 1 .
5. The image processing apparatus according to claim 1 , further comprising:
a printing unit configured to cause the print head to scan the printing medium printing dots.
6. The image processing apparatus according to claim 5 , wherein the printing unit causes the print head to scan to each of a first scan region on the printing medium and a second scan region which adjoins the first scan region in a printing medium conveying direction on the printing medium so that the printing order of the first and second dot patterns is constant in the first and second regions on the printing medium.
7. The image processing apparatus according to claim 1 , wherein (i) about half of pixel areas for printing dot indicated by the second dot pattern corresponding to the second quantization data having a predetermined value are positioned at same positions as pixel areas for printing dot indicated by the first dot pattern corresponding to the first quantization data having the predetermined value and (ii) remaining pixel areas for printing dot indicated by the second dot pattern corresponding to the second quantization data having the predetermined value are positioned at same position as a pixel areas for not printing dot indicated by the first dot pattern corresponding to the first quantization data having the predetermined value.
8. An image processing method for printing an image on a printing medium by causing a print head which prints dots to scan relative to a predetermined region on the printing medium plural times resulting in a plurality of scans comprising the steps of:
a distribution step for distributing multi-valued image data corresponding to the predetermined region into at least a first multi-valued data corresponding to the predetermined region for a first scan and a second multi-valued data corresponding to the predetermined region for a second scan;
a quantization step for quantizing each of the first multi-valued data and the second multi-valued data to generate a first quantization data and a second quantization data; and
a conversion step for converting the first quantization data into a first print data used for printing dots by the first scan to the predetermined region by using a first dot pattern which indicates number and positions of dots printed to pixel areas in the predetermined region in accordance with a value of the first quantization data and convert the second quantization data into a second print data used for printing dots by the second scan to the predetermined region by using a second dot pattern which indicates number and positions of dots printed to pixel areas in the predetermined region in accordance with a value of the second quantization data,
wherein (i) at least one pixel area for printing dot indicated by the second dot pattern corresponding to the second quantization data having a predetermined value is positioned at a same position as a pixel area for printing dot indicated by the first dot pattern corresponding to the first quantization data having the predetermined value and (ii) at least one pixel area for printing dot indicated by the second dot pattern corresponding to the second quantization data having the predetermined value is positioned at a same position as a pixel area for not printing dot indicated by the first dot pattern corresponding to the first quantization data having the predetermined value.