Printing images by means of a dot matrix printer

Abstract: M different complementary binary masks each having the form of a matrix of points occupying N rows and P columns are defined in such a manner that each of the N x P elements of a mask has a first value or a second value, each point of the matrix corresponding to an element having the first value in one mask and in one mask only. Each portion of the image is printed in M passes of the print head while using the M different masks in succession, such that during each pass, the print head scans a strip of the print medium, the same mask being used repetitively for each group of P columns and N rows, an image dot being released for printing during a pass when the corresponding element of the mask has the first value. The masks used are of sufficiently large dimensions to avoid perceptible repetitive patterns appearing, the elements having the first value being distributed therein in a manner that is pseudo-random and substantially uniform.

Description: The invention relates to printing images in dots by means of a multi-dot print head, in particular a method of printing by ink jet. Ink jet printing is performed by means of a print head that moves relative to a print medium on which an image is to be reproduced. Usually, the print head is displaced in a first scan direction (main scan) while the print medium is displaced in a second scan direction perpendicular to the first (secondary scan). The print head includes a plurality of orifices through which droplets of ink can be ejected in controlled manner, e.g. by electrostatic means or by heater means causing bubbles of vapor to be formed. The ejection orifices are aligned in a transversal direction relative to the main scan direction. Thus, on each scan of the print head, with the print medium being stationary, printing may be performed on a strip of the medium that comprises a plurality of lines, i.e. as many lines as there are ejection orifices. Printing may be one-directional, i.e. with image strips all being printed while the print head moving in the same direction (no printing during head return), or it may be two-directional, i.e. with image strips being printed alternately to and fro along the main scanning direction. When an image strip is printed in a single pass, i.e. in a single scan of the head, and when the print medium is advanced between two passes through a distance equal to the width of one image strip, then print quality is not always good because of the "strip" effect. The image appears to be striped, and this is more visible with two-directional printing (Figure 1) as compared with one-directional printing (Figure 2). In addition, when printing in color, and using a plurality of heads for different primary colors, single-pass printing sometimes gives rise to colors being rendered irregularly. In particular, the droplets form spots that overlap slightly on the print medium, and have an anisotropic environment, and insufficient drying time between the projection of contiguous ink spots can give rise to undesirable color mixing. To overcome these difficulties, proposals have been made to print not in one pass but in two passes, with only half of the image dots (or "pixels") of an image strip being released for printing during the first pass and with the other half being released during the second pass, and with the print medium being advanced through a distance equal to half the width of an image strip between two passes. During each pass, the image dots released for printing are distributed spatially in predetermined complementary patterns or "masks". The two-pass printing method can be generalized to a number of passes greater than two, with an increase in the number of passes having the effect of improving the final quality of the image, but to, the detriment of printing speed. One such method of printing in a plurality of interlaced passes is described, in particular, in document US-A-4 967 203. In document EP-A-0 632 405 another such method is described. In this method drawbacks of repetitive regular masks is aimed to overcome by generating a full image mask without periodicity using a combination of random masks. However using random masks does not allow to control completely the distribution of the dots to print between the different passes and there above it is complex to implement. Although a clear improvement in print quality is achieved by printing in a plurality of passes as proposed in the prior art, the improvement is still insufficient when a very high quality image is desired, and the object of the invention is to propose a method enabling this need to be satisfied. To this end, the invention provides a method of printing images by dots using at least one multi-dot print head that is movable relative to a print medium, in which method : a) a set of M different complementary binary masks is defined in which each mask is in the form of a matrix of points occupying N rows and P columns, where M, N, and P are integers greater than 1, such that: each of the N x P elements of a mask has a first value or a second value; and each point of the matrix corresponds to an element having the first value in one mask and in one mask only; and b) each portion of the image is printed in M passes of the print head, using M different masks in succession, whereby : during each pass, the print head scans a strip of the print medium, with the same mask being used repetitively for each group of P columns and N rows; and the printing of an image dot is allowed during a pass when the corresponding element of the mask has the first value. This method is characterized in particular by the fact that the masks have dimensions N x P that are sufficiently large to avoid perceptible repetitive patterns appearing, and in which the elements having the first value are distributed pseudo-randomly and substantially uniformly. Preferably, each of the masks has substantially the same number of first value elements. In the present case, the term "mask of sufficiently large dimensions" means a mask whose dimensions N and P are such that the probability of spurious repetitive effects (or "artifacts") forming in the eye is greatly reduced. Experience shows that the eye has increased sensitivity for certain pattern repetition pitches, e.g. of vertical lines or horizontal lines. The critical value r for pattern repetition pitch when viewing at 25 cm, i.e. the pitch value for which the eye presents the greatest sensitivity, is about 1 mm, which corresponds to an angular offset between patterns as seen at the focus of the eye, of about 0.2°. This phenomenon is well known; reference may be made in particular by the article by Quian In entitled "Half-tone image quality image based on human vision model" published in "Proceedings SPIE Human Vision, Visual Processing and Digital Display", IV 1-4 February 1993. The width and the height covered by a mask must be considerably greater than this critical pitch, and thus typically considerably greater than r = 1 mm for viewing at 25 cm. The values of N and P are therefore selected accordingly, taking account of the resolution of the image, i.e. of the fundamental pitch p between image dots. If p is expressed in mm, N and P should therefore be chosen so that N x p » r and P x p » r. Using ink jet print heads in which the pitch between ejection orifices is typically 0.07 mm, the values of N and P will therefore be considerably greater than 16, and preferably at least equal to 64 for viewing at 25 cm. The value of P may then be equal to the value of N, but that is not essential. The term "mask in which the elements having the first value are distributed pseudo-randomly and substantially uniformly" means herein a mask which is not constructed by repeating a pattern of smaller dimensions, but in which the first values are distributed in relatively homogenous manner so as to avoid artifacts being formed due to the presence of zones in which the density of dots having the first value is very low or very high. It should be observed here that by using masks of large dimensions with pseudo-random distribution, the method of the invention differs radically from the prior art methods mentioned at the beginning of the present application, since those methods use masks having dimensions N x P = 4 x 4 or 8 x 8, or masks formed by repeating a unit pattern having those dimensions. The method of the invention therefore raises the problem of constructing masks of large dimensions having a distribution that is pseudo-random while nevertheless remaining substantially uniformly distributed. Various implementations of the invention enabling this problem to be solved are described below by way of non-limiting indication, and with reference to the accompanying drawings, in which : Figure 1 is a reproduction of an image obtained by means of an ink jet printer operated in two-directional single-pass mode; Figure 2 is a reproduction of an image obtained by means of an ink jet printer operated in one-directional and single-pass mode; Figure 3 is a highly diagrammatic representation of an ink jet printer; Figure 4 is a highly diagrammatic representation of an ink jet print head; Figures 5A, 5B, and 5C are diagrams showing successive steps in two-pass printing; Figure 6 is a view showing a portion of a pseudo-random thresholding matrix suitable for constructing masks for use in implementing the method of the invention; Figures 7A and 7B are fragmentary views of two complementary masks obtained from the matrix of Figure 6; Figures 8A and 8B are fragmentary views of two other complementary masks obtained from the matrix of Figure 6; Figures 9A, 9B, 10A, and 10B show steps in the construction of masks in a particular implementation of the invention; Figure 11 is a reproduction of an image obtained by means of an ink jet printer operated in two-directional mode with two interlaced passes using masks of the invention; Figure 12 is a reproduction of an image obtained by means of an ink jet printer operated in one-directional mode with two interlaced passes using masks of the invention; and Figures 13A to 13M show the effects of offsets between successive passes with masks in which elements having a first value or a second value are to be found in alternating positions along a line, or are grouped together in pairs. As shown in Figure 3, an ink jet printer comprises a print head 10 movable along a Y direction relative to a print medium 12, e.g. a sheet of paper. The print head is guided along a bar 14 which extends transversely relative to the sheet 12. The sheet is moved in an X direction perpendicular to the Y direction. Typically, the print head 10 comprises at least one column of orifices 20 (Figure 4) through which ink droplets can be ejected in controlled manner. The orifices 20 are aligned parallel to the X direction, or possibly slightly inclined relative thereto. An image is reproduced by scanning successive strips B of width L on the sheet 12 by moving the head 10 in the Y direction (main scan). During a scan pass, the ejection of ink is controlled so as to reproduce the desired image in the form of dots on a number of lines equal to the number of orifices 20. After each pass, the sheet 12 is advanced in the X direction (secondary scan) and the print head is controlled to perform a new scan pass. Printing may be performed only while the print head is moving in the same direction (one-directional mode), with the head being returned to its starting position each time the sheet is advanced. Strips B may also be printed while displacing the head alternately in one direction and in the opposite direction (two-directional mode). The method of the invention relates to a multi-pass mode of printing, i.e. a print mode in which each strip B is printed in a plurality of passes of the print head, with this being done preferably in interlaced manner, as shown in Figures 5A to 5C for two-pass printing. At the beginning of printing, a half-strip B is printed using the bottom half of the ejection orifices of the head 10 (Figure 5A). The sheet 12 is advanced through a distance U2 and a full strip is printed, using the complete column of ejection orifices (Figure 5B). Thereafter, the sheet is advanced again through a distance L/2 and a full strip is printed using the complete column of ejection orifices (Figure 5C). The process is repeated until the last half-strip of the sheet is reached, which is printed using only the top half of the ejection orifices. On each pass, only one half of the image dots ("pixels") are allowed to print, with the other half being allowed to print during the following pass. The way in which image dots are allowed to print are interlaced is defined by binary masks in the form of matrices of points comprising N rows by P columns. During a printing pass, for successive groups of P image dot columns, image dots are optionally released for printing by controlling the ejection orifices selectively depending on whether the elements in the corresponding locations of the mask have a first value or a second value (e.g. 1 or 0). During the following pass, a complementary mask is used so that all image dots are released for printing in two passes. In the secondary scan direction (X), the masks are repeated for each group of N rows. Print mode can be generalized to a number of passes M greater than 2. The sheet of paper 12 is then advanced a certain number of rows between two passes such that after M passes the total advance is equal to L, and M different masks are used. Each image dot corresponds to an element having the value 1 in one, and only one of the M masks. The 1-value elements are preferably distributed substantially equally between masks. They are distributed in pseudo-random and quasi-uniform manner in each mask so that the tiling of a surface by the mask does not cause a repetitive pattern that is visible to the eye to appear. Naturally, multi-pass printing can be performed in one-directional mode or in two-directional mode. In addition, multi-pass printing may be performed in monochrome mode or in color mode. In color mode, use is made of a plurality of print heads like that shown in Figure 4, with each head being allocated to one color. Usually, four heads are used for the colors cyan, magenta, and yellow, and for the color black. Color printing is then performed in four passes with four different masks and with the masks for the various colors being permutated during successive passes. Naturally, the number of passes with four print heads is not necessarily equal to four. It could be smaller, being at least equal to two or more, using a corresponding number of masks. The masks may be constructed in various ways to satisfy the distribution conditions for the 1 values whereby they are required to be pseudo-random and quasi-uniform. In a first implementation, the masks are made from a matrix of numbers distributed in pseudo-random manner, and for each mask, each element is given the first value or the second value as a function of the value of the number of the corresponding point in the matrix. By way of example, it is possible to use a matrix of the thresholding matrix type as used for converting continuous tone images into binary tone images (i.e. half-tone images). One method of constructing such a thresholding matrix, referred to below as the Minimum Energy Value method or as the MEV method, is described in an article by Pierre Lermant, published in IS & T's 48th Annual Conference Proceedings, May 7-11, 1995, by "The Society for Imaging Science and Technology", Springfield, Virginia, United States of America. Such thresholding matrices created by the MEV method are capable of satisfying the requirements for distribution that is pseudo-random and quasi-uniform. When the number M of passes is equal to 2, both masks are constructed from a thresholding matrix having the same dimensions, as a function of the values of the numbers in the matrix. When the numbers forming the matrix are consecutive integers, one of the two masks may be constructed by allocating the first value (value 1) to elements whose corresponding locations in the matrix are occupied by even numbers, and the other mask is complementary thereto. Figure 6 shows a portion of a random thresholding matrix comprising numbers in the range 0 to 255, and Figures 7A and 7B show the corresponding portions of two complementary masks constructed in this way. In a variant, one of the two masks may be constructed by allocating the first value (1) to elements whose corresponding locations in the matrix are occupied by a number situated on one side or the other of a threshold value, with the other mask being complementary thereto. By way of example, the threshold value may be the mean value of the numbers in the matrix. Figures 8A and 8B show portions of two complementary masks constructed in this way on the basis of the matrix of Figure 6, with the mean value being 128. Uniformity appears to be better ensured with this second variant than with the first. It may also be observed that this second variant does not require the numbers constituting the matrix to be consecutive integers. The threshold value is chosen as a function of the numbers constituting the matrix so that each mask contains a desired number of elements having the first value. When the number M of masks is greater than 2, it would also be possible to consider using a matrix of numbers distributed in random manner such as a thresholding matrix obtained by the MEV method. By way of example, each mask can then be constructed by allocating the first value to elements whose locations correspond to numbers in the matrix lying within a predetermined range of values, with the ranges allocated to different masks being contiguous, without mutual overlap, and preferably being of the same width. Nevertheless, in a second implementation of the method of the invention, which is more particularly suitable when the number of masks is greater than 2, masks are constructed directly by a sequential positioning method, the location of an element having the first value in one mask being determined without taking account of the locations of elements having the first value and allocated to other masks, but while nevertheless preventing a location that is already occupied by an element having the first value in another mask being occupied again. By way of example, principles are used similar to those of the MEV method described in the above-mentioned article by Pierre Lermant. Starting from an empty N x P matrix, that algorithm consists in successively selecting points having minimum energy value, or MEV points. The M masks may be constructed one after another, or else in parallel. When they are constructed one after the other, a first mask is made starting from an empty matrix and allocating the value 1 to elements whose locations correspond to the MEV points which are identified in succession, while taking into account the points that have already been placed. Figure 9A uses circles to represent MEV points placed in succession in the matrix, the point A being the last-placed point, taking account of locations already allocated. Figure 9B is a translation of the MEV point locations into 1-value elements in the first mask M A . This procedure is repeated until the desired number of 1-value elements has been placed in the first mask, e.g. N x P/M elements (or the closest integer) when it is desired that all of the masks should comprise substantially the same number of 1-value elements. A second mask is made in the same way, i.e. not taking account of the MEV points identified for making the first mask, but preventing MEV points being placed at locations already occupied by MEV points in the first mask. This is illustrated in Figure 10A when solid line circles show the locations in the matrix of MEV points placed during the process of constructing the second mask, while dashed-line circles show the locations of MEV points for the first mask. These first mask points are considered as being "virtual" for the process of creating the second mask. That is to say, the theoretical locations of MEV points for the second mask are determined without taking account of the MEV points corresponding to the first mask, but each MEV point for the second mask is actually placed so as to avoid occupying in the matrix a location corresponding to an MEV point of the first mask. Thus, the MEV point of the second mask given reference B, whose theoretical location coincides with that of point A, is, in practice, placed beside point A. Figure 10B shows the portion of the second mask M B that corresponds to Figure 10A. By taking account of the locations of the MEV points in the first mask, it is possible to ensure that the 1 value is not given to an element having the same location in both the first mask and the second mask. The procedure continues in similar manner for the following masks, taking no account of the MEV points for the preceding masks when looking for MEV point locations, but ensuring that points are not placed in locations that have already been occupied when drawing up the preceding masks. The number of elements having the value 1 in the first mask is determined so that the total number of 1-value elements in all of the masks comes to the number N x P. In the second case, masks are constructed in parallel as follows : a) a search is made for a first MEV point in an empty matrix and the element of the first mask whose location corresponds to that of the MEV point is given the value 1; b) a search is made for a first MEV point in an empty matrix, without taking account of the first MEV point corresponding to the first mask, and the value 1 is allocated to the element of the second mask whose location corresponds to that of the first MEV point searched in this way, while ensuring that it does not occupy the location of the element having the value 1 in the first mask; c) successively, and for each remaining mask, a search is made for the first MEV point without taking account of the locations of the first MEV points for the preceding masks, and the value 1 is allocated to the location corresponding to that of the searched-for MEV points, however it is prevented from occupying the location of any element having the value 1 in the preceding masks; d) once all of the first elements in the M masks have been placed, a search is made for the second MEV point of the first mask, taking account of the first MEV point already placed in the first mask, but considering the first MEV points for the other masks as being virtual points, i.e. not taking them into account while searching for the second MEV point or the first mask, but nevertheless ensuring that locations already occupied by MEV points for the other masks are not selected again, and the value 1 is allocated to the location of the first mask corresponding to the location of the second MEV point as determined in this manner; e) placing the second 1-value element of each remaining mask in the same manner, i.e. taking account of the first MEV point for the same mask while searching for the second MEV point, and considering that the MEV points for the other masks are virtual points; and f) proceeding in similar manner for the following 1-value elements until all M searched-for complementary masks have been obtained, i.e. until N x P 1-value elements have been placed. The process of constructing masks in series or in parallel, as described above, can naturally be used with methods of constructing matrices other than the MEV method. References to other methods of constructing thresholding matrices can be found, for example, in the above-mentioned article by Pierre Lermant. A two-pass mode print method has been implemented using an ink jet printer having a print head with 64 ejection orifices disposed at a pitch of about 0.07 mm. Two complementary masks of dimensions 64 x 64 were used, after being constructed in the manner illustrated by Figures 6, 7A, and 7B. Figures 11 and 12 show the results obtained in two-directional and one-directional modes, respectively. The improvement in image quality compared with Figures 1 and 2 is considerable. In a multi-pass print method, the positions of image dots printed during each pass are interrelated and form a partial image. The positions relating to these partial images depend on the mechanical accuracy of the machine. If each mask is made up of a majority of isolated points, excessive shift between two passes will cause points that are theoretically separate to become superposed and will cause gaps to be left, thereby considerably degrading the quality of the image. This effect is not visible so long as the error E in the positioning pitch of the head is such that E < D-p, where D is the diameter of the ink spot formed on the print medium and p is the fundamental pitch between image dots, with D being selected to be greater than p so as to ensure that there is overlap between two spots projected for two adjacent image dots on the same line. When D - p < E < p, portions of the image that ought to be covered in ink run the risk of not being covered because of two spots corresponding to two contiguous image dots being spaced apart excessively. Under such circumstances, defects can be attenuated by ensuring that each row of the mask is formed by groups of at least two consecutive elements having the same value. To guarantee this result, complementary masks are constructed having dimensions of N x P/2 and each element of each line is duplicated so as to obtain masks of dimensions N x P that are used during printing. It would even be possible to use complementary masks of dimensions N x P obtained as described above, omitting the same P consecutive columns in each mask after duplicating the elements, e.g. the first P columns or the last P columns. The above is illustrated in Figures 13A to 13M. Figure 13A shows a line of an image that is to be printed continuously in black, and Figures 13B and 13C show the first two rows of two complementary masks in which the 1-value elements are isolated. When the shift or slip between two passes is zero or almost zero, then the image line is printed as shown in Figure 13D, which shows the result obtained after the first pass using the first mask, and by Figure 13E which shows the final result after the second pass using the second mask. When a shift E occurs, the first and the second masks are applied as shown by Figures 13F and 13G. Figures 13H and 13I show the results obtained after one pass and then after two passes. Gaps, or omissions, I are visible, with E being such that E > D - p. If the elements of the masks are duplicated with the same shift E (Figures 13J and 13K), the results obtained after one pass and then after two passes are shown by Figures 13L and 13M. In Figure 13M, it can be seen that defects are considerably attenuated in comparison with Figure 13I.

Claims: A method of printing images by dots using at least one multi-dot print head that is movable relative to a print medium, in which method : a) a set of M different complementary binary masks is defined in which each mask is in the form of a matrix of points occupying N rows and P columns, where M, N, and P are integers greater than 1, such that: each of the N x P elements of a mask has a first value or a second value; and each point of the matrix corresponds to an element having the first value in one mask and in one mask only; and b) each portion of the image is printed in M passes of the print head, using M different masks in succession, whereby: during each pass, the print head scans a strip of the print medium, with the same mask being used repetitively for each group of P columns and N rows; and the printing of an image dot is allowed during a pass when the corresponding element of the mask has the first value; characterized in that the masks have dimensions N x P that are sufficiently large to avoid perceptible repetitive patterns appearing by choosing the numbers N and P so that N x p >> r and P x p >> r where p is the fundamental pitch between two image dots, and r is the critical pitch for pattern repetition at which the eye presents maximum sensitivity, and in which the elements having the first value are distributed pseudo-randomly and substantially uniformly. A method according to claim 1, characterized in that each of the masks has substantially the same number of first value elements. A method according to any one of claims 1 to 2, for ink jet printing by means of at least one print head including at least one column of ejection orifices for ejecting ink droplets. A method according to claim 3, characterized in that the number N is equal to the number of ejection orifices. A method according to claim 3 or 4, characterized in that the numbers N and P are at least equal to 16. A method according to claim 5, characterized in that the numbers N and P are at least equal to 64. A method according to any one of claims 1 to 6, characterized in that the numbers N and P are equal. A method according to any one of claims 1 to 7, characterized in that to construct the M masks, an N x P matrix is made of numbers placed in pseudo-random manner, and for each mask, each element has the first or the second value allocated thereto as a function of the value of the number at the corresponding point of the matrix. A method according to claim 8, characterized in that when the value M is equal to 2, a matrix is made constituted by N x P consecutive integers placed in pseudo-random manner, and each element of a first of two masks is allocated the first value or the second value depending on whether the corresponding point of the matrix is even or odd, the second mask being complementary to the first. A method according to claim 8, characterized in that for each mask, the first value is allocated to elements whose locations correspond to numbers in the matrix lying in a predetermined range of values, the ranges allocated to the various masks being contiguous without mutual overlap. A method according to any one of claims 1 to 7, characterized in that the set of masks is constructed by a sequential positioning method, the location of an element having the first value in a mask being determined without taking account of the locations of elements having the first value and allocated to the other masks, but while ensuring that a location which has already been occupied by an element having the first value in another mask is not occupied again. A method according to claim 11, characterized in that the masks are constructed by a sequential positioning method comprising seeking locations with minimum energy values. A method according to any one of claims 1 to 12, characterized in that masks are used in which each row is made up of groups comprising at least two consecutive points of the same value. A method according to any one of claims 1 to 13, characterized in that color printing is performed by means of a plurality of different print heads each corresponding to one color, and allocation of masks amongst the various heads is permutated during successive passes.

Inventors: BERCHE STEPHANE, LERMANT PIERRE L, WU YIFENG, BERCHE, STEPHANE, LERMANT, PIERRE L., WU, YIFENG, Berche, Stéphane
Applicants: OCE GRAPHICS FRANCE, OCE GRAPHICS FRANCE S.A.
Application year: 1996
Main classification: B41J2
IPC codes: B41J2, G06K15, H04N1