Image Forming Apparatus

Abstract

An image forming apparatus prints a first image data rendered at a first number of halftone levels based on a second image data rendered at a second number of halftone levels larger than the first number of halftone levels. A dot forming section forms lines of pixels on a print medium, the lines including a number of groups of sub lines of sub dots. The groups are aligned such that the sub lines of sub dots extend in traverse directions substantially perpendicular to an advance direction. A controller controls the dot forming section such that a selected number of sub dots in the advance direction are combined to form a pixel. Sub dots on the same sub line have the same exposure energy and sub dots on different sub lines in the same pixel have different exposure energies.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein:

FIG. 1 illustrates a general configuration of an image forming apparatus of a first embodiment;

FIG. 2 is a block diagram illustrating sections in a controller;

FIG. 3 is a block diagram illustrating the detail of an LED head;

FIG. 4 illustrates the data structure of image data and how the image data is printed on paper;

FIG. 5 illustrates the principle of correction of the position of dots before printing when the LED head extends in a direction at an angle with the rotational axis of the photoconductive drum due to some positional errors;

FIG. 6 illustrates the positional relation between a Y-coordinate before correction and the Y-coordinate after correction;

FIG. 7 is a timing chart illustrating the process of digitizing the image data;

FIG. 8 is a timing chart illustrating the digitizing process in which the image data in each line is digitized on a pixel-by-pixel basis;

FIG. 9 illustrates the truth table for a data decoder of the first embodiment;

FIG. 10 is a timing chart illustrating a printing operation of the image data;

FIG. 11 illustrates the patterns of sub dots rendered at 7 different halftone levels;

FIG. 12A illustrates a comparative example in which no correction was made;

FIG. 12B illustrates print results of 3-bit image data rendered at 7 halftone levels when the bi-level image data is outputted according to the timing chart in FIG. 10;

FIG. 13A illustrates the correction made in the first embodiment such that the positions of sub dots are moved by a minimum distance;

FIG. 13B illustrates the correction made using a conventional method such that the positions of sub dots are moved by a minimum distance;

FIG. 13C illustrates a comparative example in which the spacing between adjacent sub lines in the advance direction is halved in an attempt to correct positional errors;

FIG. 14 is a block diagram illustrating the sections in a controller of a second embodiment;

FIG. 15 is a timing chart illustrating the process of digitizing the image data;

FIG. 16 is a timing chart illustrating the digitizing process in which the image data in each line is digitized on a pixel-to-pixel basis;

FIG. 17 illustrates the truth table for a data decoder;

FIG. 18 is a timing chart illustrating a printing operation of the image data;

FIG. 19 illustrates the patterns of sub dots rendered at 15 different halftone levels;

FIG. 20A illustrates correction performed in the second embodiment such that the positions of sub dots are moved by a minimum distance;

FIG. 20B illustrates the correction performed by using a conventional method such that the positions of sub dots are moved by a minimum distance;

FIG. 21 is a block diagram illustrating the sections in a controller of a third embodiment;

FIG. 22 is a timing chart illustrating the process of digitizing image data;

FIG. 23 is a timing chart illustrating the digitizing process in which the image data in each line is digitized on a pixel-by-pixel basis;

FIG. 24 illustrates the truth table for the data decoder;

FIG. 25 is a timing chart illustrating a printing operation of the image data;

FIG. 26 illustrates the patterns of sub dots rendered at 10 different halftone levels;

FIG. 27A illustrates correction performed in the third embodiment such that the positions of sub dots are moved by a minimum distance;

FIG. 27B illustrates correction performed by using a conventional method such that the positions of sub dots are moved by a minimum distance;

FIG. 28 is a block diagram illustrating the sections in a controller of a fourth embodiment;

FIG. 29 illustrates the truth table for a data decoder of the fourth embodiment;

FIG. 30 illustrates the patterns of sub dots for the respective halftone levels when printing of data rendered at 4 halftone levels is performed according to the timing chart in FIG. 10;

FIG. 31A illustrates the correction performed in the fourth embodiment such that the positions of sub dots are moved by a minimum distance;

FIG. 31B illustrates a comparative example in which the correction of the position of sub dots is made using a conventional method such that the positions of sub dots are moved by a minimum distance;

FIG. 32 illustrates dots printed on paper in conventional halftoning without correction when an LED head is misaligned so that the LED head extends at an angle with a rotational axis of a photoconductive drum; and

FIG. 33 illustrates one way of correcting the positional errors of sub dots in which the positions of sub-dots indicated by dotted lines are corrected by 3 sub-lines.

Claims

1. An image forming apparatus that prints first image data rendered at a first number of halftone levels, comprising: a dot forming section that forms lines of pixels on a print medium, each of the lines including a number of groups of sub lines, each of the sub lines including sub dots, wherein the number of groups are aligned such that the sub lines extend in first directions substantially perpendicular to a second direction in which the print medium advances; anda controller that controls said dot forming section such that a selected number of sub dots in the second direction are combined to form the pixel based on second image data rendered at a second number of halftone levels, sub dots on the same sub line having the same exposure energy and sub dots on different sub lines in the same pixel having different exposure energies.

2. The image forming apparatus according to claim 1, wherein the second image data includes a third number of lines of pixels for one page of the print medium, each of the third number of lines including a fourth number of pixels that are aligned in the second direction.

3. The image forming apparatus according to claim 1, wherein said dot forming section includes: a halftone level converting section that converts the second image data rendered at the second number of halftone levels into the first image data rendered at the first number of halftone levels;a digitizing section that digitizes the first image data into bi-level image data;a bi-level image memory that stores the bi-level image data;an exposing section that causes light-emitting elements to emit light in accordance with the bi-level image data;a data outputting section that reads the bi-level image data from said bi-level image memory and that produces signals that represents corresponding ones of the different exposure energies, said data outputting section sending the bi-level data and the signals to said exposing section.

4. The image forming apparatus according to claim 3, wherein the second image data includes a third number of lines of pixels for one page of the print medium, each of the third number of lines including a fourth number of pixels that are aligned in the first direction.

5. The image forming apparatus according to claim 4, wherein the third number of lines includes a fifth number of sub lines; wherein each of the sub dots on the sub lines is printed at a position on the print medium given by a first coordinate in the first direction and a second coordinate in the second direction;wherein when said data outputting section reads the bi-level data from said bi-level image memory, the position in the second direction is corrected by a correction value corresponding to the position, the correction value being produced in increments of a spacing between adjacent groups.

6. The image forming apparatus according to claim 1, wherein the first number of halftone levels, the number of groups, and a number of sub lines are related such that M≦(2S−1)×G+1

7. The image forming apparatus according to claim 6, wherein the number of groups is 2, the number of sub lines in each group is 2, and the first number of halftone levels is 7.

8. The mage forming apparatus according to claim 6, wherein the number of groups is 2, the number of sub lines in each group is 3, and the first number of halftone levels is 15.

9. The image forming apparatus according to claim 6, wherein the number of groups is 3, the number of sub lines in each group is 2, and the first number of halftone levels is 10.

10. The image forming apparatus according to claim 6, wherein the number of groups is 2, the number of sub lines in each group is 2 and the first number of halftone levels is 4.