EP1418053B1

EP1418053B1 - Method of printing to improve the quality of image edges

Info

Publication number: EP1418053B1
Application number: EP20030078432
Authority: EP
Inventors: André Van Doorn; Marwjin T.P. Christiaans
Original assignee: Oce Technologies BV
Current assignee: Canon Production Printing Netherlands BV
Priority date: 2002-11-06
Filing date: 2003-11-03
Publication date: 2007-10-03
Anticipated expiration: 2023-11-03
Also published as: EP1418053A1

Description

The invention relates to a method of printing, in particular to a method of bi-directional printing. More in particular, the invention relates to a method of printing using an ink-jet or laserjet printer having a resolution which is twice or more than twice the resolution of the input image.
Such methods are known for allowing e.g. a 600 dpi printer to print a 300 dpi bitmap.
In printing, a droplet discharged from a print head nozzle to form a pixel has a ball-shaped leading portion and an elongated tail portion. During falling this tail portion may divide itself into two or more droplet parts. Due to the fact that the print head performs a continuous movement during a printing stroke, the ball portion will reach the printing surface, in particular paper, before the tail or the droplet parts will do. The ball portion hits the paper at the desired location according to a grid based on the higher resolution of the printer to form a pixel. The remaining portions of the droplet, however, will hit the paper at locations spaced from the centre of the pixel in the direction of movement of the print head and form small satellite pixels.
According to a known method of printing, double pixels for e.g. a 600 dpi resolution are printed using a reciprocating print head having two rows of nozzles arranged perpendicular to the direction of travel, in which -as seen in the direction of travel- the leading row of nozzles is activated for discharging ink droplets to form the pixel located at the trailing side of the double-pixel to be formed. As a result, satellites will hit the paper at the downstream sides of the pixels. The images resulting from this process have blurred or vague edges which is unacceptable for 600 dpi printing.
This effect is even more noticeable when printing is performed according to a two pass bi-directional mode, in which in a first swath a first part of the leading row of nozzles is activated and in the adjacent, second swath an adjacent, second part of the trailing row of nozzles. The satellites will appear along the edges of the row of double-pixels in the first swath only, resulting in more or less step-like edges which intensifies the irregular appearance of the edges of the image.
In an attempt to find a solution to this problem it has been proposed to lower the speed of travel of the print head so as to decrease the distance between the location of the centre of the pixel formed by the ball portion of the droplet and the location where the tail portion(s) of the droplets hit(s) the paper. In this solution, however, the printing time would increase resulting in less printing efficiency.
US 5,992,968 discloses an ink jet printing method wherein reciprocal printing is performed for a printing medium by reciprocally scanning an ink jet head including a plurality of ink ejecting ports for ejecting ink therefrom and an opening surface having said ink ejecting ports formed therethrough, the ink ejecting direction relative to said opening surface being inclined toward either of a forward scanning direction and a return scanning direction of said ink jet head. The disclosed method comprises the step of scanning said ink jet head in a direction toward inclination of the ink ejecting direction at a first scanning speed to effect printing for the printing medium, said ink jet head driving at a first driving frequency during scanning at the first scanning speed. The disclosed method further comprises the step of scanning said ink jet head in a reverse direction at a second scanning speed which is lower than the first scanning speed to effect printing for the printing medium, said ink jet head driving at a second driving frequency, which is lower than the first driving frequency, during scanning at the second scanning speed. Reciprocable printing can be realized at a high speed without any reduction of a printing quality by receiving each satellite in a main dot not only in forward scanning but also in return scanning. However, this method requires a cumbersome calibration procedure in order to tune the scanning speeds and the driving frequency.
An object of the invention is to improve on this.
A further object of the invention is to provide a method of printing double or multiple pixels having sharp edges on both sides.
Yet another object of the invention is to provide a method of two pass bi-directional printing at a high frequency resulting in a high resolution image with sharp edged double or multiple pixels.
From one aspect according to the invention, these and other objects are realized by a method of printing pixel images on a sheet or web of paper using an ink-jet or laserjet printer, said printer having means for storing an input image corresponding to the pixel image in the form of a first grid of imaginary lines according to a first resolution, said printer being adapted for printing said pixel image in the form of a second grid of imaginary lines according to a second resolution which is a plurality of the first resolution, said printer having a print head with at least one nozzle, wherein said printer is provided with control means for operating said print head in accordance with the input image recorded in said storing means, wherein said print head reciprocates across the sheet or web of paper along a first path in a first direction and along a second path in a second, opposite direction, wherein said pixel image has first pixels located at a first edge which forms an upstream edge with respect to the movement of the print head in the first direction and said pixel image has second pixels located at a second edge which forms an upstream edge with respect to the movement of the print head in the second direction, wherein all first pixels at said first edge are printed through activation of the print head nozzle by the control means to discharge an ink droplet at a line of the second grid which coincides with the first edge at that location when moving in the first direction, and wherein all second pixels at said second edge are printed through activation of the print head nozzle by the control means to discharge an ink droplet at a line of the second grid which coincides with the second edge at that location when moving in the second direction.
From a second aspect, the invention provides a method of printing pixel images on a sheet or web of paper using an ink-jet or laserjet printer, wherein said printer has a resolution which is a plurality of the resolution of the input image, said printer having a print head which reciprocates across the sheet or web of paper along a first path in a first direction and along a second path in a second, opposite direction, wherein said pixel images consist of a plurality of pixels located adjacent to each other as viewed in said first and second directions, including first and second pixels to be placed at the leading or upstream edges of the image in question as considered in the first and second directions, wherein the print head is controlled to discharge an ink droplet for forming the first and second pixels only when moving in the first and second directions, respectively.
In the method according to the invention, the satellites will always hit the paper at a location which is within the intended area of the double or multiple pixel, in other words in the area which is indeed to be covered by ink. As a result, the edges of the printed image will be sharp, without the printing speed having to be decelerated.
In a further embodiment, the second or printer resolution is twice the first or input image resolution, in particular 600 dpi.
In an advantageous embodiment of the method according to the invention, use is being made of a print head having a first and a second row of nozzles arranged according to lines transverse to said first and second directions, wherein said first row of nozzles is located at the downstream side of the print head when moving in the first direction, wherein said second row of nozzles is located at the downstream side of the print head when moving in the second direction, wherein said first pixels are printed by activating the first row of nozzles and the second pixels are printed by the second row of nozzles.
Below, the invention will be elucidated on the basis of the accompanying drawings, in which:

figure 1 A: schematically shows the discharge of an ink droplet from a nozzle of a print head;
figure 1 B: schematically shows the droplet just before hitting a sheet of paper;
figures 2A and 2B: diagrammatically show the application of double-pixels in two steps;
figure 3: shows a portion of a line printed by using a method according to the invention; and
figure 4: shows another image that can be obtained by a method according to the invention.

In figure 1A the ink-jet or laserjet printer head 1 is shown during discharge of an ink droplet 2 from nozzle 10. The droplet 2 has a lower, ball-shaped portion 3 and an elongated tail portion 4.
After having travelled downwardly for a short period of time, the droplet 2, in particular the tail portion 4, will break up in two or more tail parts, as shown in figure 1 B with tail part 4a and 4b.
The printer head 1 will travel in the direction A with respect to the papersheet 5, as result of which the ball portion 3 and the tail parts 4a, 4b will hit the papersheet 5 at locations spaced from each other in the direction A, as shown in figure 1C.
In figure 2A the printer head 1 is schematically shown having two rows h₁, h₂ of nozzles 10a-h and 11a-h, which rows are spaced at a distance d from each other.
Also in figure 2A is shown the paper sheet 5 from above, as well as an imaginary grid 6 of gridlines 7 and gridlines 8, which are spaced from each other at a distance d. In this example, which corresponds to a 600 dpi grid, d is 1/600 inch.
Figures 2A and 2B show the process in so called two path bidirectional printing. According to this process, printing of images is carried out in two overlapping swaths. A first swath in a direction from left to right (A) and in the second overlapping swath in the opposite direction (B).
Accordingly nozzles 10a-h and 11a-h are active in the first swath and nozzles 10a-h and 11a-h are active in the second swath.
When the printer head 1 is moved in the direction A, the -not shown-control system of the printer activates the nozzles 10a-h to discharge ink droplets 2 such that the ball portion 3 of the ink droplets will hit the paper sheet 5 at line 7a of the grid 6. The tail parts 4a, 4b or satellites will hit the paper sheet 5 at the down stream side of the ball portions 3.
In the return stroke, depictured in figure 2B, the printer head 1 is moved back in the direction B, and in that case the printer head 1 is activated to have the nozzles 11 a-h discharged ink droplets 2 such that the ball portions 3 of the ink droplets hit the paper sheet 5 at the grid line 7b. The satellites 4a, 4b will hit the paper in area between lines 7a and 7b.
In this way a series of double pixels 9 is formed, forming -in this example- a line forming image as if it were a series of 300 dpi pixels.
Due to the fact that the satellite will always hit the paper sheet 5 within the double pixels, the outer edges of each line will be sharp and an image containing a number of lines will be sharp too.
This is shown in figure 3 by means of line 12 having sharp edges 12a and 12b.
In figure 4 it is shown that any image can be printed using the method according to the invention. Every image may have a dimension in moving direction of the printer head corresponding to a double pixel. Every image (600 dpi) is converted into an image of double pixels, the double pixels forming an image of 300 dpi. Even without knowing the exact outlines of an image part the invention ensures that the satellite pixels all will fall within the double pixel boundaries. In all these cases the printer head is activated such that the ink droplets that are required for forming the boundary lines 13a, 13b, of image 13 are discharged when the printer head moves in a direction A and B, respectively, with a result that the satellites 4a, 4b always hit the paper sheet in an area between the lines 13a and 13b.
The invention is described with an inkjet head having two rows of nozzles (10a-h; 11a-h) but also a head having a single row can be used for this invention.

Claims

Method of printing an image on a recording medium (5) using an ink jet printer having a print head (1) with at least one nozzle (10) for discharging ink droplets (2), wherein first pixels are printed by selectively discharging ink droplets (2) in first swaths in a first direction (A) and wherein second pixels are printed by selectively discharging ink droplets (2) in second swaths in a second direction (B) opposite to the first direction (A), characterised in that, if an ink droplet (2) is discharged in a first swath, giving rise to a first ball portion (3), then an ink droplet (2) is discharged in a second swath overlapping the first swath, giving rise to a second ball portion adjacent to and placed upstream from the first ball portion (3), when considered in the second direction (B), for forming a double pixel (9).
Method of printing an image as claimed in claim 1, the ink droplet (2) discharged in the first swath giving rise to satellite first pixels (4a, 4b) placed downstream from the first ball portion (3) when considered in the first direction (A), the ink droplet (2) discharged in the second swath giving rise to satellite second pixels placed downstream from the second ball portion when considered in the second direction (B), said satellite first and second pixels being placed within the external edges of the formed double pixel (9).
Method of printing an image as claimed in claim 1 or 2, using a print head (1) with at least one row of nozzles (10a-h) arranged according to a line orthogonal to the first direction (A), a group of first pixels being printed in a first swath according to a first grid of first imaginary lines (7a) orthogonal to the first direction (A) and equidistantly spaced by a distance 2d, a group of second pixels being printed in a second swath according to a second grid of second imaginary lines (7b) equidistantly spaced by a distance 2d and parallel to the first imaginary lines (7a), the first and second grids being interlaced such that the distance between each first imaginary line (7a) and a closest neighboured second imaginary line (7b) is d.
Method of printing an image as claimed in claim 3, the print head having a first row of nozzles (10a-h) and a second row of nozzles (11 a-h) parallel to each other and spaced by a distance d, the group of first pixels being printed by ejecting ink droplets from the first row of nozzles (10a-h) and the group of second pixels being printed by ejecting ink droplets from the second row of nozzles (11a-h).
Method of printing an image according to claim 4, the printer having means for storing a bitmap input image having an input image resolution, and control means for operating the print head in accordance with the bitmap input image stored in the storing means, the printer resolution along a line parallel to the first direction (A) being twice the input image resolution.