GB2334124A - Printer - Google Patents

Abstract

An apparatus for generating print data comprises means for receiving multi-level image data in the form of pixels; threshold means for selectively thresholding the pixels by determining for each pixel firstly if it has a density above a predetermined threshold value and secondly if it is surrounded by pixels the densities of which are also all above the threshold value; and means for replacing each pixel region the density of which satisfies the above conditions with a pattern of reduced average density.

Description

PRINTING APPARATUS The present invention concerns printers. It is particularly, though not especially, concerned with bubble jet printers. These bubble jet printers are wellknown. They print by ejecting small drops of ink on demand from a jet on a print head that is moved over a print medium such as paper.

Bubble jet printers have become extremely popular because they are silent in operation, reliable and relatively inexpensive. Most designs of bubble jet printer mount the ink reservoir on the printing head carriage so it moves back and forth over the paper as a page is printed.

If the ink cartridge is heavy then the printing head carriage transport mechanisms need to be stronger. If the ink cartridge is bulky then a lot of space in the printer has to be left empty to allow it to move.

Compact low-power bubble jet printers for use with laptops use small, light ink reservoirs to keep the printer small and light. This means the printer cannot print many pages before it runs out of ink.

Bubble jet and other printers often put down more ink than is necessary when printing text. The drop size of a bubble jet printer determines the number of dots which have to be laid down to achieve a solid tint. A printer that can make a solid tint with 360 drops per inch may print text at 720 drops per inch in order to get the fine positioning of the edges of the characters to get the text looking smooth. The printer is putting down more ink than it needs to. Not only do ink cartridges have to be changed more frequently, but the print may take longer to dry, and the ink soaking sideways on the printer may distort the final image.

The present invention concerned with alleviating the above mentioned problems.

In accordance with a first aspect of the present invention there is provided apparatus for generating print data comprising means for receiving multi-level image data in the form of pixels; threshold means for selectively thresholding the pixels of the multi-level image data by determining for each pixel firstly if it has a ! density above a predetermined threshold value and secondly if it is surrounded by pixels the densities of which are also all above said threshold value; and means for replacing each pixel region the density of which satisfies the above conditions with a pattern of reduced average density.

In accordance with a second aspect of the present invention there is provided a method of generating print data comprising receiving multi-level image data; selectively thresholding the pixels of the multilevel image data by determining for each pixel firstly if it has a density above a predetermined threshold value and secondly if it is surrounded by pixels the densities of which are also all above said threshold value; and replacing each pixel region the density of which satisfies the above conditions with a pattern of reduced average density.

In order that the present invention may be more readily understood embodiments thereof will now be described by way of example and with reference to the accompanying drawings, in which; Figure 1 is a perspective view of a host computer and a printer; Figure 2 is a perspective view of the main components of a bubble jet printer; Figures 3A, 3B and 3C illustrating the concept with which the present invention is concerned; Figure 4 illustrates results of printing in accordance with the flow diagram of Figures 6.

Figures 5 and 5A are block diagrams showing a printer driver; Figure 6 is a block diagram illustrating components of the computer/printer arrangement shown in Figure 1; Figures 7A and 7B are flow diagrams showing the operation of the printer shown in Figure 2; and Figure 8 illustrates a halftoning technique.

Referring now to Figure 1 of the drawings, this shows a conventional computer having a monitor screen 1, a conventional keyboard 2 and a processing unit 3 connected to a bubble jet printer 4. It will be appreciated that this is only one arrangement possible amongst a wide range of applications. Printers are used in many other applications and it is entirely feasible for the printer to be capable of receiving image data from an external source and carrying out all the necessary signal processing in the same unit which carries the printer head.

Figure 2 of the accompanying drawings is a perspective view of a printer unit. This unit comprises a head cartridge 10 mounted on a carriage 11 which can be reciprocated in the direction S. The carriage 11 is guided in the direction S along a guide shaft 12 and is reciprocated by a timing belt 13 passing over pulleys 14 and 15 with pulley 15 being driven through a suitable gear train by a carriage motor 16. A transport roller 17 is driven by a transport motor 18 to feed a recording medium such as paper, transparent films for overhead projectors or the like, the recording medium being guided by a paper pan 19 from a feed tray (not shown) to a printing position. When the recording medium is being transported it is biased against the transport roller 17 by feed rollers 20.

The printing apparatus so far described is entirely conventional and in a conventional black and white printer the carriage 11 would carry a single print head for black ink. Again, such print heads are well-known and normally include a plurality of outlet orifices each associated with an ink passage which in operation is supplied with ink. The ink is ejected in the well-known manner by the application of short bursts of thermal energy to selected ink passages so as to discharge drops of ink through the orifices onto the recording medium, the discharge being caused by the generation of bubbles in the ink by the applied thermal energy.

In a conventional colour bubble jet printer the head cartridge would include four print heads for respectively printing black, cyan, magenta and yellow. In the embodiment being described the printer is a monochrome printer.

As is well-known in the art, printers such as bubble jet printers are essentially bi-level devices, that is a dot of ink is either deposited or not. In bi-level hard copy devices a range of level intensities is made available by what is known as half-toning. As a simple example, a 2 x 2 pixel area of a bi-level display can be used to produce five different density levels at the cost of halving the spatial resolution along each axis. In general an N * M group of bi-level pixels can provide N * M + 1 density levels whilst bearing in mind there will always be a trade-off between spatial resolution and density resolution to some extent determined by visual acuity the distance from which the image is viewed, and the dots per inch of resolution of the graphics device.

It will also be appreciated that when an N * M pixel pattern is used to approximate half-tones it should be designed not to introduce large scale visual phenomena which would be visible to an observer in an area of similar density values.

However, as discussed in the preamble of the specification, when a bi-level printer such as a bubble jet printer is used to print characters such as text, the dots within the pixels overlap with the result that ink can be wasted along with other, already specified disadvantages.

Referring now to Figures 3A, 3B and 3C of the accompanying drawings these illustrate the concept of which the present application is concerned. Each figure represents a 10 x 10 picture area. It is also assumed that for each figure the original data is represented by the numbers in the individual pixels. In the present example it is assumed that a pixel as defined by the original data can range from white (0) to totally black (100). The intermediate values are of course possible but in the situation shown in Figure 3A the pixels are either white (0) or black (100). Such a situation can be found when printing text. As already described the black pixel (100) will contain a substantial amount of overlapping deposited dots. Thus a lot of ink can be saved if this overlap is avoided or reduced. As will be described this saving can be achieved by replacing the black (100) pixels with halftones and this is in turn achieved by clipping the ink levels at a predetermined threshold value before carrying out halftoning.

However, in order to maintain the definition of the final image, this clipping should not be carried out at the outer edges of an image so that the halftoning does not interfere with the edges. Thus as shown in Figure 3B a pixel value is only clipped when the surrounding 3 x 3 pixels are also greater than or equal to the threshold.

This leaves the edges of the image unchanged, an important feature when considering the printing of text.

The result of such clipping (thresholding) is shown in Figure 3B where it will be seen that the outer edges of the exemplary figure are still at 100% density but a region comprising the inner pixels now have 50% density levels. Effectively the inner region identified by the thresholding has been filtered or mashed to reduce its average density.

If this data is now input to a halftoning process that reduces the original range of 0 to 100 grey levels to just two levels the final result, depending on the particular halftoning process, will be as shown in Figure 3C. In this figure the original zero values have remained 0, the 100 values at the edges have become 1, and the 50 values have been replaced by an alternating 1 and 0 pattern.

The examples shown in Figures 3A, 3B and 3C are purely exemplary. What happens when using the above procedure on a real printer is illustrated in Figure 4.

The magnified examples shown in Figure 4 have been produced by a Canon BJC/620 (RTM) Bubble Jet Printer which prints with a resolution of 720 dots per inch. As with the diagrams of Figure 3 the original data has grey levels ranging from 0 to 100. The column 101 shows bit maps for letters firstly unthresholded, secondly thresholded at 80 and thirdly thresholded at 50. Column 102 shows the same letters as printed by the BJC/620 printer. It will be seen that with the 50% thresholding the ink coverage has been reduced from 11.9% to 8.4%, a saving in ink of over 29%. At a microscopic level it can be seen that the edges of the characters have become slightly rougher as the level of thresholding has increased. However, in real terms the effect to a user would be barely visible and would largely consist in the text looking slightly lighter.

The thresholding algorithm and subsequent halftoning can be carried out either in the actual printer or by an appropriately programmed computer such as a standard PC.

Referring now to Figure 5 of the drawings this shows the thresholding feature incorporated in a printer driver.

The printer driver comprises an input port 110 for receiving printed data from, for example, a computer; a 3-line buffer 111 storing the received printed data, a circuit 112 for carrying out the thresholding algorithm which has just been described, a halftoning circuit 113, a printer driver 114 and a bubble jet printer 115. The buffer 111 is shown in greater detail in Figure SA.

In order to reduce hardware costs in the highly competitive bubble jet printer market it is likely that thresholding and halftoning will actually be carried out in a PC and not on board the printer. This is the arrangement shown in Figure 6. In this figure a PC is indicated at 200. A scanner of any suitable type is shown at 201 and is connected to a computer 200 by a suitable input port 202. The computer 200 also has an additional input port 202' via which it can receive image data from an external source (not shown) which can be a local area network or other suitable data link.

Within computer 200 all the necessary signal processing, thresholding and halftoning would be carried out in software. The binary output of the computer is connected via a line 208 to the line buffer 209 of a printer 210.

Printer 210 is, in this embodiment, a bubble jet printer similar to that shown in Figure 2 and includes in addition to the line buffer 209 a printer head driver 211 and a print head 212.

The operation of the system shown in Figure 5 and 6 will now be described with reference to the single flow diagrams of Figures 7A and 7B.

Referring now to Figures 7A and 7B the flow diagram starts with step S1. In this step the line buffer 111 shown in Figure 5, and in greater detail in Figure SA, is reset. It will be appreciated that this line buffer, cr its equivalent, is also found in the thresholding circuit 206 shown in Figure 6 and that in both of the embodiments of the Figures 5 and 6 thresholding is carried out in a similar manner. Also in step S1 the first two lines of image data are read into lines A and B of the three line buffer. At the same time a flag is set to the last line read. Normally this flag is set to zero and the reason for this will be described hereinafter. In step S2 line thresholding is started and the first pixel in the line B is copied from the line buffer. It will be appreciated that pixels in line A will never be thresholded or clipped when thresholding is started. As explained with regard to Figures 3A, 3B and 3C even if the pixels of line A were all at 100% density they would not be thresholded in the interests of maintaining a high resolution edge. Thus in terms of Figure 5A, pixel B (1) becomes a pixel of interest D (1).

In step S3 n is set to 2 and so on as the pixel of interest is moved down the line. Step S4 is the step in which it is decided whether or not the pixel of interest is to be thresholded in accordance with the values of the nine pixels surrounded it. If the answer is YES, D(n) is thresholded at step S5, and if NO, no thresholding is carried out so that in Step S6 D(n) is set to equal B (n) and the pixel of interest retains its original value.

In step S7 the decision is made as to whether the pixel of interest is the last but one of line B. This is necessary because even if the last pixel is black (100) it will not be clipped that because it must lie on an edge. If the pixel of interest is not the last the thresholding algorithm is continued via the feedback loop and if the pixel is the last it is transferred in step S8 without the thresholding. In step S9 the output line D is halftoned and sent for output through the printer hardware.

In step S10 the lines of the three line buffer are scrolled so that the thresholding algorithm can be applied again to line B which of course now contains the image data originally contained in line A. In Step Sll the decision is made whether or not the end of input data has been reached. If the decision is NO the next line of data is read into line A of the buffer at step S12.

If the answer is YES the last line flag is checked at step S13. If it is '0" it is set to "1" at step S14 and if it is "1" the process terminates as all the lines of data which can be thresholded have been thresholded. If the answer is NO then the thresholding algorithm proceeds but no pixels will be thresholded as the condition set out in step S5 will not be met. This ensures the integrity of the potential final outer edge and prevents the clipping of these pixels.

Referring now to Figure 8, this shows the process by means of which the thresholded image is halftoned. It will be appreciated that there are many halftone algorithms available for binarising multi-level image data so as to convert into binary data suitable for printer output.

Figure 8 shows a simple algorithm by means of which binary halftoning can be carried out. This error diffusion algorithm was first introduced by Floyd and Steingerg in 1975. In the algorithm illustrated in Figure 8 the threshold is fixed at "h" where the input D(n), obtained from the flow diagram of Figures 7A and 7B, varies as usual from G = O(white) to G = 1 (black).

The threshold is indicated at 300 and with the result in binary output value of 0 or 1 is compared with the original grey level value at 301. The difference is suitably called the "error" for location n. The signal consisting of past error values is passed through an error filter 302 to produce a correction factor to be added at 303 to future input values. Errors are thus diffused over a weighted neighbourhood determined by E(n).

In the preceding description the data being processed was monochrome. It is also possible to apply the thresholding and clipping concept to colour signals. In such a case care will have to be taken that colour values are not adversely affected in the final image. In order to prevent, or at least reduce, this problem the gamma correction tables usually employed in the processing of colour signals could be used in association with additional look up tables stored in ROM containing correction values for clipped colour pixels.

A colour printer using the previously described scheme might replace the cyan and magenta clipped regions with a 50% chequer pattern after halftoning. The density of this cyan-magenta overprint would depend on whether the cyan and magenta chequer patterns were in-phase or outof-phase.

To get consistent results it may be necessary to replace the thresholded regions with a fixed pattern in each separation instead of clipping and halftoning. Or the thresholded regions could be replaced with a pattern of "on" and "off" values that would not be changed by the subsequent halftoning.

Thus with a colour printer it may be desirable to impose a fixed pattern on selected colour separations over those regions where the pixel values have been identified by thresholding as requiring clipping.

Claims (18)

1. CLAIMS: 1. Apparatus for generating print data comprising means for receiving multi-level image data in the form of pixels; threshold means for selectively thresholding the pixels of the multi-level image data by determining for each pixel firstly if it has a density above a predetermined threshold value and secondly if it is surrounded by pixels the densities of which are also all above said threshold value; and means for replacing each pixel region the density of which satisfies the above conditions with a pattern of reduced average density.
2. 2. Apparatus according to claim 1 and including means for generating said pattern of reduced density by halftoning the clipped image data.
3. 3. Apparatus according to claim 1 or claim 2 and including means for varying said threshold.
4. 4. Apparatus according to any preceding claim, wherein threshold means operates in response to an algorithm which operates on the eight pixels immediately adjacent to each pixel of interest.
5. 5. Apparatus according to claim 4, wherein the threshold means includes a three line buffer through which successive lines of image data are scrolled, the contents of the buffer being represented by A(1) A(n), B(1) ... B(n), C(1) ... C(n) where n is the number of pixels in a line, and A, B and C are successive lines of image data, and wherein for each successive pixel of interest as represented by B(n) it is determined if A(n1), A(n), A(n+1), B(n-l), B(n), B(n+l), C(n-1), C(n), C(n+1) are greater than said threshold.
6. 6. Apparatus according to any preceding claim, wherein the image data is colour image data, and wherein for each pixel region the density of which satisfies said conditions, a pattern of "on" and 'off" values is imposed on the region.
7. 7. Apparatus according to any one of the preceding claims and including print means for printing the thresholded and halftoned image data.
8. 8. Apparatus according to claim 7, wherein the print means include a bubble jet ink head and driver circuit for driving the print head in accordance with the threshold and halftoned image data.
9. 9. Apparatus according to claim 7 or claim 8 and comprising a computer containing said threshold means, and a printer containing said printer driver and said print head.
10. 10. A method of generating print data comprising means receiving multi-level image data in the form of pixels; selectively thresholding the pixels of the multilevel image data by determining for each pixel firstly if it has a density above a predetermined threshold value and secondly if it is surrounded by pixels the densities of which are also all above said threshold value; and replacing each pixel region the density of which satisfies the above conditions with a pattern of reduced average density.
11. 11. A method according to claim 10 and including halftoning the clipped pixels.
12. 12. A method according to claim 10 or 11, wherein said thresholding incorporates an algorithm which utilises eight pixels immediately adjacent to each pixel of interest.
13. 13. A method according to claim 12, wherein the thresholding includes scrolling the image data through a three line buffer, A(1) ... A(n), B(1) ... B(n), C(1) ... C(n) where n is the number of pixels in a line, and A, B and C are successive lines of image data, and wherein for each successive pixel of interest as represented by B(n) it is determined if A(n-l), A(n), A(n+1), B(n-l), B(n), B(n+1), C(n-1), C(n), C(n+l) are greater than said threshold.
14. 14. A method according to any one of claims 10 to 13, wherein the image data is colour data and wherein for each pixel region the density of which satisfies said conditions, a pattern of "off" and "on" values is imposed on the region.
15. 15. A method according to any of claims 11, 12, 13 or 14 and including printing the thresholded and halftoned image data.
16. 16. A method according to claim 15, wherein the printing is carried out by a bubble jet printer.
17. 17. Apparatus for generating print data substantially as hereinbefore described with respect to any one of the figures of the accompanying drawings.
18. 18. A method of generating print data substantially as hereinbefore described with reference to and any one of the accompanying drawings.