APPARATUS AND METHOD FOR PRINTING INFORMATION

Abstract: Apparatus and method for printing information by means of a raster output scanner adapted to form an image on an image-forming medium (405) having fixed imaging sections and transfer the image to a receiving sheet at a repetition frequency corresponding to one machine cycle time (T), in which a front-end processor (10) can postpone a set-up-page signal one or more whole multiples of a machine cycle time (T) by means of a prediction algorithm stored in a memory (12) on the basis of coded signals.

Description: This invention relates to an apparatus for printing information by means of a raster output scanner adapted to form an image on an image-forming medium having fixed imaging sections and to transfer the image to a receiving sheet at a repetition frequency corresponding to one machine cycle time (T), the raster output scanner also comprising a control device which in response to a set-up-page signal starts the formation of a print when an imaging section is situated at a predetermined position and which is connected to a front-end system, being part of the printing system, which can deliver the set-up-page signal, comprising a front-end processor, a memory, a raster-image processor and a page-size bit-map memory, the front-end system being adapted to receive coded information about a complex page and the raster-image processor being adapted to decode the information about the coded complex page and place it in the bit-map memory, the raster-output scanner comprising an exposure device which line by line exposes a fixed imaging section image-wise a fixed time after the set-up-page signal and in response to the electrical signals originating from the bit-map memory. The invention also relates to a method for this apparatus, comprising starting forming a print on a set-up-page signal, storing coded information about a complex page in the front-end system, expanding this coded information, and storing the expanded information in the bit-map memory, reading-out the bit-map memory a fixed time after the set-up-page signal is delivered and line by line exposing a fixed imaging section image-wise by means of the read-out information. An apparatus and method of this kind are described in EP-A-0218287, pre-published as NL-A-8 502 640 on 01-04-86, and can be used in an electronic printing system in which data concerning a page for printing can be input, for example, by means of a connected work station, and be fed to a raster-output-scanner (ROS) via a front-end system containing a word-oriented bit-map memory. The raster ouput scanner is adapted to print a complete page linewise by means of serially presented data, by means of a laser printer in which a light beam is modulated imagewise and in which said modulated light beam is deflected line-wise by means of a polygonal mirror over a light-sensitive surface such as a zinc-oxide binder layer which is applied to a flexible belt. In a known manner a latent image can be written thereon by means of the modulated light beam. This latent image can be developed in a known manner and transferred to a receiving material such as a sheet of paper. By means of the work station it is possible to input text requiring to be printed on a page. This (coded) input text is expanded in the front-end by font data stored in a memory, and the expanded text is stored in a page-size bit-map memory. The work station is provided with a display by means of which the page for printing can be composed. The lay-out of the page can be varied as required via the work station. Many kinds of fonts of variable sizes have to be placed in the bit-map memory at predetermined positions. It accordingly depends on the page complexity how much time is required to decode all the characters occurring and place them in the page-size bit-map memory. When a print order is given, for example to print a number of different pages consecutively, the front-end must be able to perform this in the time available. Since the photo-sensitive layer containing a number of fixed imaging sections must be charged in an early stage in the ROS in the case of continuous use, before image-wise exposure can be started, there is the possibility of a page being so complex that the front-end, on arrival of the charged imaging section at the exposure slit, has not yet completed the filling of the bit-map memory. The charged layer then has to be completely exposed by means of a light source or a number of light sources to prevent the charged image part from being developed completely black. In GB-A-2051679 a printing system is described wherein the actual forming of a latent image is postponed when the information is not yet fully "introduced". However, no details are given how this should be accomplished. This unnecessary charging and exposure will age the photo-sensitive belt more rapidly than is necessary. The object of the invention is to restrict this unnecessary aging as much as possible. According to the invention this object is attained in an apparatus according to the preamble, in that the front-end processor (10) can postpone the set-up-page signal one or more whole multiples of a machine cycle time IT) on the basis of the coded information by means of a prediction algorithm stored in the memory (12), the prediction algorithm determining the individual fine-format times (IFFT) of each character occurring on the complex page and summing all the individual fine-format times occurring, and in a method for an apparatus according to the preamble, by determining on the basis of the coded information by means of a prediction algorithm the decoding time required to fine-format this information and place it in the bit-map memory (16), the prediction algorithm determining the individual fine-format times (IFFT) of each character occurring on the complex page and summing all the individual fine-format times occurring, and postponing the set-up-page signal, on the basis of this determined decoding time, for a time corresponding to one or more whole multiples of a machine cycle time (T). As a result, the printing of complex pages does not mean an unnecessary loading of the photo-sensitive belt while the printing speed is also reduced as little as possible. These and other advantages will be made clear by reference to the description which follows below and by means of Figures, in which: Fig. 1 is a diagrammatic representation of a front-end system, Fig. 2 represents the processes which can be performed by the raster image processor, Fig. 3 is a diagrammatic representation of a raster image processor, Fig. 4 is a diagrammatic representation of a VME-bus interface, Fig. 5 is a diagrammatic representation of a laser scan module interface, Fig. 6 is a diagrammatic representation of a 16-bits processor system of the raster image processor, Fig. 7 is a diagrammatic representation of a raster image bus interface, Fig. 8 is a diagrammatic representation of a VME-master interface of the VME-bus interface, Fig. 9 represents the placing of a character in the bit-map memory, Fig. 10 represents the results of a number of possible operations in the bit-map memory, Fig. 11 is a diagrammatic representation of a raster image memory. Fig. 12 is a cross-section through a laser printer, Fig. 13 represents a time diagram for printing two pages and Fig. 14 represents time diagrams for different functions of an apparatus according to the invention. Fig. 1 is a diagrammatic representation of a front-end system. Here a front-end controller 10 (FEC) is connected to an operating console 19 and also to the control system of a printer 20. Printer 20 is a raster output scanner in which a light beam is modulated imagewise and is deflected line-wise over the surface of a light-sensitive element. The light-sensitive element is driven perpendicular to the direction of deflection of the light beam in order for it to be possible to write a complete image in raster form. An example of a raster output scanner is a laser printer in which a modulated laser beam is driven by means of a rotating multiface mirror over the surface of a charged photoconductor. The photoconductor is exposed imagewise in this process, and the charged image obtained can be developed in a known manner with toner, then transferred to a receiving sheet and fixed by means of heat. The front-end controller 10 contains a 16-bit microprocessor system embodying a Motorola 68000 microprocessor and functions in combination with local ROM and a part of a randomacces memory 12 (RAM) as the control system for the front end. In a font read memory 13 are stored the bit patterns of a number of print types. The front end may be connected via an I/O processor 11, which also contains a 16-bit microprocessor system embodying a Motorola 68000 microprocessor, to a disc memory, work station, computer and/or cluster controller. The FEC 10, I/O processor 11, RAM 12 and font memory 13 are connected to each other via a standard VME bus 14. The font memory 13 can also be constructed as a RAM or form part of RAM 12. The bit patterns of the fonts are then loaded into said RAM from a disc memory or floppy-disc memory. A raster image processor 15 (RIP) is also connected to the VME bus 14. In addition, the raster image processor 15 is connected via a raster image bus 17 (RI bus) to a page-size bit-map memory 16, also termed raster image memory (RIM). The RIP 15 serves to fill the bit-map memory 16 (RIM) imagewise with type characters which are called up from the font memory 13 and deposited in the correct location in the bit-map memory 16. In addition, the RIP 15 can call up graphical information from memory 12 and also set it aside at the desired locations in the bit-map memory 16. If the bit-map memory 16 has been filled, it can be read out again via the RIP 15, the data read out being fed as a serial pixel-bit stream via line 18 to the modulator of the laser printer. The image which is written on the photoconductor is made up of 0.05 × 0.05 mm pixels so that to print an A4-format black/white image approximately 4,000 × 6,000 pixels are necessary. As a consequence of this bit-map memory 16 has a size of approximately 24 megabit or 3 megabyte. During the reading out of the bit-map memory 16 the pixel bit rate to the modulator of the laser printer via line 18 is approximately 25 megapixels/sec., with the result that an A4-format page can be printed in approximately 1 sec. All data relating to a page to be printed are stored in the RAM 12 via the I/O processor 11 from, for example, a work station and under the control of the FEC 10. For this purpose various options are available. Starting from, for example, an A4-page, which has to be printed in the "portrait mode", approximately 4,000 subtables are made corresponding to the approximately 4,000 scan lines which are necessary to write a page. In each subtable type codes of the type characters or graphical characters are stored which have their starting point on a certain scan line. Each type code contains, in addition, data relating to the X position which the character assumes on the scan line, data relating to the font type and also data relating to the height and width of a certain character. The type code also contains data relating to a base address in the font memory 13, where the bit representation of said character is stored in 16-bit words. The set of lists thus formed is termed the list of primitives. Once the data relating to a page to be printed have thus been stored in the RAM 12, the filling of the bit-map memory 16 can be started. For this purpose the letter codes are called up from the RAM 12 one by one by the RIP 15, and together with the associated bit representation from the font memory 13, expanded and deposited at the correct X and Y locations in the bit-map memory 16. In a corresponding manner all the characters are completed one by one until the bit-map memory 16 is completely filled with the pixel representation of the page to be printed. It is not necessary to form these sublists. The data relating to a page to be printed can also be stored in the RAM 12 in a random sequence. During the filling of the bit-map memory 16 the RAM 12, in which the primitives are placed in a random sequence, is successively read out, expanded, and placed at the respective locations in the bit-map memory 16. According to yet another option all the characters occuring on a page are only stored once and provided with data relating to the various positions which each character assumes on the page. In this manner the data relating to a frequently occuring character such as, for example, the letter e, are only stored once in the RAM 12 and all the positions which this letter assumes on the page are inserted in a separate table. Usually the graphical characters which are stored in the RAM 12 or the font memory 13 are first placed in the bit-map memory 16 and only thereafter the type characters. The raster image processor In Fig. 2 the processes are represented which can be carried out by the RIP 15. After starting up the system (Step 24) the RIP 15 is initiated (Step 25) (INIT command) by a system reset or an INIT command from the front-end controller 10, after which the "self test" process (Step 26) is started. This "self test" comprises the testing of various RIP functions and bit-map memory (RIM) functions. The RIP 15 must not have access to the VME bus 14 during the self-test procedure as the FEC 10 requires the VME bus 14 for the testing of the RAM 12 and font memory 13. If the RIP 15 has succesfully carried out the "self-test" program, an interrupt signal is transmitted to the FEC 10, and the RIP 15 goes into the wait mode (Step 27). If the self-test program has detected a fault, the RIP 15 also goes into the wait mode (Step 27), but no interrupt signal is generated for the FEC 10. In this manner the front-end controller 10 knows that a fault has been diagnosed in the RIP 15 "self test". A fault is also indicated to the operator by a LED lighting up. On an "RIP diagnosis" command from the FEC 10 to the RIP 15 the RIP diagnosis process 30 starts. The RIP 15 performs a number of internal tests and also a number of tests on the RIM 16. The results of these tests are stored in the RAM 12 and can be transmitted to, and made visible on, operating console 19. The RIP diagnosis process also tests the VME interfaces. The RIP diagnosis process has a more extensive character while the self-test process performs a more functional hardware test. After the termination of the RIP diagnosis process, the RIP 15 stores status information in the RAM 12, generates an interrupt signal for the FEC 10 and unblocks the VME bus 14. After the RIP 15 has received a "fill bit map" command from the FEC 10, the RIP 15 checks the data transfer on the VME bus 14. The RIP 15 thus acquires access to the RAM 12, which contains the primitives of the page which has to be printed. The RIP 15 expands the list of primitives by means of the pixel representations of the fonts and deposits these in the bit-map memory 16 (RIM). The RIP 15 has access to the RIM 16 via the RI bus 17. The RIM 16 contains, in addition, modification logics which supports the RIP 15 in performing various arithmetical operations on data for the bit-map memory 16 such as, for example, AND, OR and INVERT operations. After the bit map has been filled (Step 28), the RIP 15 stores status information in the RAM 12, generates an interrupt signal for the FEC 10 and unblocks the VME bus 14. The FEC 10 then generates a "read bit map" command and the RIP 15 will wait for a page synchronisation signal which originates from the laser printer via a control interface. After this page synchronisation the RIP 15 starts by reading out the RIM 16 (Step 29) and generates a serial pixel bit stream which is fed to the modulator of the laser printer via a video interface. After the termination of the bit-map reading process (Step 29), the RIP 15 again stores status data in the RAM 12, generates an interrupt signal for the FEC 10 and then unblocks the VME bus 14. The RIP 15 (Fig. 3) is built up around an internal bus system, the raster image processor bus 46 (RIP bus) which is a synchronous bus and is designed to transfer exclusively 16-bit words. The RIP bus 46 contains data lines 47, address and control lines 48 and condition lines 49. The RIP bus is connected via a VME bus interface 41 to the VME bus 14 and via an RI bus interface 45 to the RI bus 17. This RI bus 17 contains inter alia data and address lines 58, a busy line 57, an RI bus address available line 56, a clock line 54 and modification lines. In addition, the RIP bus 46 is connected to a laser scan module interface 44 (LSM interface) and the proper central processing unit 43 (CPU) of the RIP 15. The laser scan module LSM interface 44 is connected to lines originating from the printer such as a "start-of-scan" line 52 (SOS), via which a synchronisation signal is supplied to indicate the beginning of a line to be printed, a burst line 53, via which a signal is supplied which corresponds to the desired pixel frequency and a video line 18, via which the serial pixel bit stream is fed to the modulator from the laser printer on reading out the bit-map memory 16. The address and control lines 48 and the condition lines 49 are also further connected to a page synchronisation interface 42. Via line 50 a "page-available" signal (PAV) is fed to the control device of the printer, which signal indicates that a page is fully formatted in the bit-map memory 16 and that the RIP 15 can start reading out the bit-map memory 16 on receiving a "start-of-page" signal (SOP) via line 51 which is derived from the control device of the printer. Raster-output-scanner An endless belt 405 provided with a photo-conductive layer is fed to exposure roller 463 over roller 406 via roller 468 and buffer roller 465 after the belt has been uniformly charged by means of corona device 423. The belt 405 is exposed line by line image-wise on exposure roller 463 by means of a modulated laser beam from laser module 462. The image-wise modulation is achieved by means of a front-end system 461. The part of the belt on which the latent electrostatic charge image is formed then passes via buffer roller 466 and rollers 467 through a developing device 407 where the latent image is converted to a powder image. Between the exposure roller 463 and the developing device 407 there is disposed a frame exposure unit (not shown) which is conventionally used to expose charged belt parts which fall just outside the image-wise exposed image. If a charged image part unexpectedly cannot be exposed image-wise, e.g. because the control device has not received a "page available" signal (PAV), the frame exposing unit then exposes the entire image. The buffer rollers 465 and 466 are interconnected via a fixed arm 464 which is rotatable about shaft 469. Rotation of arm 464 about shaft 469 enables the belt to be taken over exposure roller 463 at a reduced speed during exposure. During the image-wise exposure the circumferential speed of the exposure roller 463, and hence the belt speed locally, is exactly equal to half the belt speed in the other part of the raster-output-scanner. A drive roller 408, which if required is provided with a biasing roller 409, and having an outer surface with a high coefficient of friction with respect to the belt, drives the belt 405 continuously. The belt then runs over a roller 410 which is movable towards and away from the belt, hence up and down as considered in Fig. 12, along a guide 411 in order that the belt may or may not be pressed against a transfer belt 424 trained about a roller 425, during which the powder image can be transferred to the belt 424 as described in the Netherlands Patent Application 7502874. Belt 405 then moves over a roller 412, which may or may not have a biasing roller 413, and then hangs in a loop 414 to a further roller 415. The belt 405 then moves on to a cleaning device 419 for removal of residual powder, as known per se, and is then trained about a roller 420 to the roller 406 and thus past the corona device 423. Roller 425 acts as drive roller for the belt 424 and this belt is taken between the rollers 426, 427 and 428, 429 to a stationary surface 430 which serves to align the belt 424, the belt 424 sagging freely between the rollers 428, 429 and the surface 430. The belt 424 runs from surface 430 to deflecting roller 434 and then to roller 435 and back to drive roller 425. Reference 436 denotes heating devices which by radiant heat render tacky the powder image on belt 424 transferred from belt 405 at rollers 410 and 425, so that this powder image can readily be transferred by belt 424 to the copy paper. This paper is fed from the stack 437 via rollers 438, guide 439, rollers 440 and guide 441, to the nip between the belt 424 and the roller 427, whereafter this copy paper is taken through the guide 442 to the rollers 443 which deposit it on table 444. In the embodiment illustrated the photo-conductive belt 405 is formed by a finite belt which is made endless by means of a seam. The belt is provided with a mark at the seam, said mark being detectable by means of a detector 450 so that the detector 450 generates a signal pulse which is used as one of the input signals to the control circuit 460. Such a mark may, for example, be in the form of a perforation or a small surface having light-reflecting properties deviating from those of the belt. Alternatively, a seamless belt can be used having a mark at any place. Roller 408 is also provided with a so-called pulse disc forms part of a pulse generator as described, for example, in greater detail in US patent 3 912 390. By means of this pulse generator it is possible to generate signal pulses at a frequency proportional to the speed of movement of the belt 405. The signal pulses are also used as an external input signal for the control circuit 460. Belt 405 is divided up into an odd number of imaging sections which are always at a fixed location with respect to the mark. If, for example, a number of prints of one and the same complex page are required, the repetition time at 23 prints per minute is 2.62 seconds. This period of 2.62 seconds is termed one machine cycle time T. This corresponds to a belt length part of about 708 millimetres. During the continuous printing one imaging section is always passed over at a time so that imaging sections n, n+2, n+4 etc. are printed. Since the belt 405 contains a number of odd imaging sections, all the imaging sections are always used on the consecutive belt runs. After a starting signal has been given, the front-end 461 delivers a set-up-page signal (SUP) to the control circuit 460 if the front edge of a fixed imaging section has reached a fixed position 470. Corona 423 is then switched on and the belt passing through is uniformly charged. After the belt has covered approximately 220 mm, the corona 423 is switched off. Exposure on roller 463 cannot be started unless the front edge of the stored imaging section has just reached the exposure slit on roller 463. From the start of the charging process this takes about 2.6 seconds. Hence the exposure of the first imaging section starts 2.6 seconds after the set-up-page signal. To this end, the control circuit 460 delivers a start-of-page signal (SOP) to the front-end 461 to start reading out the bit-map memory 16. In making more of the same prints from one complex page the corona 423 will be switched on after every 2.6 seconds and then switched off after each imaging section, while the exposure also re-starts every 2.6 seconds. For the printing of a page the FE controller 10 must first ensure that the bit-representation of a complex page is put in the bit-map memory 16. To this end the FEC 10 starts the so-called fine-formatting process. Each character fed to the FEC 10 in coded form undergoes a number of processes for this purpose. The FEC 10 must retrieve the character from a memory, then retrieve the associated position data, then calculate the precise position of that character and put the result of all this in an instruction table in the RAM 12. After the fine-formatting process, the "fill bit-map" order is given. From the instruction table a first coded character is retrieved and expanded, with the data concerning the bit-representation of the associated character, and placed at the correct location in the page-size bit-map memory 16. This memory 16 can then be read out line-by-line and fed to the modulator of the laser printer in the correct tempo. If different pages have to be printed consecutively, 1.6 seconds maximum are available for the fine-formatting process, 1 second for filling the bit map and 1.6 seconds are always availabe for reading-out. A page which, for example, contains a very large number of small characters, may require to be printed in some cases. The fine-formatting process then requires more than 1.6 seconds and the bit-map filling process more than 1 second. The charged imaging section leading edge then reaches the exposure slit on exposure roller 463 before the page-size bit-map memory 16 is filled. Image-wise exposure cannot then be started and the charged imaging section will be completely developed in the developing system 407 and a completely black print will be the result. Also, with this method the belt 405 becomes inadmissibly dirty. To obviate this, a prediction algorithm is used to calculate how long the fine-formatting processing will take. This takes place before the time that the leading edge of an imaging section has reached the starting position 470. If the prediction algorithm calculation shows that the fine-formatting process will last longer than 1.6 seconds, the front-end 461 does not deliver a SUP signal to the control circuit 460 so that this control system does not start the cycle charging, etc. Depending upon the calculated fine-formatting process time and the corresponding bit-map filing process time, the SUP signal is delivered exactly one or more whole multiples of one machine cycle time T later. Fig. 13 represents the available times for printing two consecutive pages, the horizontal column 550 belonging to a first page, horizontal column 560 to a following page, and line 566 representing a time axis. At the time t₀ the FEC 10 starts the prediction algorithm 551 to deliver or not a SUP signal at the end thereof (t₁). At time t₁ the fine-formatting process (552) then starts, and is also carried out by the FEC 10, and this process must have ended at time t₂. The RIP 15 then starts to fill the bit-map memory 16 (553). When it is completely filled, the front-end 461 delivers a PAV signal to the control circuit 460. At time t₄, after the control circuit 460 has delivered a SOP signal to the front-end 461, the RIP 15 starts to read out line-by-line the page-size bit-map memory 16 (554). A full stored page is read out at time t₅. At time t₃ the FEC 10 again starts the prediction algorithm (561) for the next page. At time t₄ a SUP signal is again delivered and the fine-formatting process (562) by the FEC 10 takes place until time t₅. RIP 15 then fills (563) the bit-map memory (until time t₆) and the second page is read out (564) from the bit-map memory 16 between times t₇ and t₈. The way in which more pages are printed if the fine-formatting process takes up more than the time available in one machine cycle time T, will now be described with reference to Fig. 14. The curves 490 to 495 diagrammatically indicate when and how long an associated function is switched on. Thus curve 490 represents the operation of the predictor, curve 491 the operation of the fine-formatting process, curve 492 the set-up-page signal, curve 493 the operation of the corona, curve 494 the operation of the bit-map filler and curve 495 the reading out of the bit-map memory. T denotes one machine cycle time. After a start signal the predictor calculates (501) whether the fine-formatting process (502) falls within the available time. If so, a set-up-page signal 503 is delivered whereupon the corona is switched on (504). After the fine-formatting process (502) is completed, the bit-map memory is filled (505). The bit-map memory is then read out (506). In the meantime, the predictor has again calculated (511) whether the new page can be fine-formatted within the available time. If so, fine-formatting takes place (512) and a set-up-page signal 513 is delivered. The corona is then started (514) and the bit-map filling process (515) takes place, whereupon the bit-map memory is again read out (516) and fed line-by-line to the modulator of the laser module. In the meantime the predictor has again calculated whether the fine-formatting process for a subsequent complex page falls within the available time. Assuming that this is not the case, then after prediction (521) the fine-formatting process (522) will be started but the set-up-page signal 523 will not be delivered, so that the corona is not started and hence the photo-sensitive layer is not charged. After the completion of the fine-formatting process (522) the bit-map is re-filled (525). In the meantime a set-up-page signal 527 is again delivered at the correct time whereupon the corona is switched on (524) and the bit-map is read-out (526) at the correct time. In the meantime a new cycle has again started with prediction (531), fine-formatting (532), set-up-page signal 533, charging by means of the corona (534), filling of the bit-map (535), and its reading out (536). An example of the preparation of a prediction algorithm will be explained with reference to a complex page containing only characters. The time is calculated that the front-end processor requires to process a complex page (the decoding time). The characters are stored in a page contents queue of the RAM 12. After fine-formatting of the contents of this page contents queue, the result is put into an instruction table, This instruction table is also known as the "character block". Thus this contains all the characters which occur on one comples page, with data concerning type, place, size etc. It also contains details of occurring spaces, line stoppers (carriage return + line feed, CRLF), control characters such as: end medium (EM), line feed (LF), horizontal tabulation (HT), vertical tabulation (VT), partial line up (PLU) and partial line down (PLD). The character block also reacts to a number of control functions. As an example, 75 lines each of 80 characters, hence about 6 000 characters, is normally taken as the basis. Of these usually some 20% are space characters (1200) leaving about 4800 graphic characters. There will also be 75 CRLF's, which again adds 150 extra characters per page. The process times for the fine-formatting of characters, control characters, space characters, CRLF's etc., which are known as the individual fine-format times (IFFT) are stored in the memory in a system table in tenths of microseconds. The total time for the fine-formatting (FFT) wil therefore have to be the summation of all the individual fine-format times occurring. The bit-map filling time for a given character or a mirrored character depends on its width, height, orientation and the total number of identical characters occurring on the page. The process time is based on the time required to place an individual character in the bit-map memory. This average process time (ACPT) is calculated for an average character by means of the following formula: ACPT= C1 × ACH + C2 × ACW + C3 where ACH represents the average character height and ACW the average character width, C1 the process time of words within a scan line, C2 the process time of a scan line and C3 an overhead time. The character size or font size is also stored in the character block for each letter type. From this the average character height and average character width can be calculated in accordance with: ACH = 0.6 × font size and ACW = 0.4 × font size ACPT is also known as the character time. For each specific font the process time (FT) of all the corresponding characters occurring on a complex page is calculated as follows: FT = number of characters occurring × character time. The time for filling the bit-map as calculated with the prediction algorithm will thus correspond to a summation of all the separate process times FT. The total decoding time calculated with the prediction algorithm will thus be the sum of FFT and the summation of all the separate process times. VME-bus interface In Fig. 4 the VME-bus interface 41 is diagrammatically represented in more detail. A master interface 100, a slave interface 101 and an interruptor 102 are connected to the VME bus 14. The data lines 47 from the RIP bus 46 are connected to the master interface 100. The address and control lines 48 are connected, as are the condition lines 49 from the RIP bus 46, to the master interface 100, the slave interface 101 and the interruptor 102. The task of the VME-bus interface 41 is to shield the RIP 15 from the asynchronous VME bus 14. The VME master interface 100 (VME-MI) contains an internal control system by means of which the buffers and registers present are controlled and via this interface access cycles can be performed on the VME bus 14. In the control system programmable logic is used. For the controlling the slave interface 101 and the interruptor 102 are also equipped with programmable logic. The VME-MI 100 (Fig. 8) also comprises data transfer functions such as data base-master (DTB-master) and data base-requestor (DTB-requestor). To achieve the desired speed in the data transfer some additional functions have been added in said VME-MI 100. The first function is an address up/down counter formed by an address high counter 132 and an address low counter 133. During the loading of the RIM 16 with the bit representations of the various type characters or graphical characters which are each stored separately with consecutive addresses in the font memory 13 or the RAM 12, the counters 132 and 133 are preset for each character with the base address of said character in, for example, the RAM 12. Via buffer 134 and the VME address bus 141 of VME bus 14 said base address is supplied to the RAM 12, and the first 16-bit word at the respective memory location is supplied via the VME data bus 142 of the VME bus 14 to a bidirectional buffer 135 and then placed at the correct location in the RIM 16. The subsequent address for the RAM 12 is generated by increasing the counter 133 by 1 and the second 16-bit word is supplied via the VME-MI 100 to the RIM 16. In a corresponding manner all the addresses associated with a given character are generated until such time as the character has completely been written into the RIM 16. In this manner the result is achieved that the CPU 43 needs to generate a base address only once per character so that during the loading other functions can be performed, for example pixel processing operations, RIM address determination, etc. After a character has been finished, a new base address for a subsequent character is fed to the counters 132 and 133 and the cycle described above is repeated. The second function, the mirror function, is performed with a mirror circuit 136 which is constructed with programmable logic such as FPLAs or PALs and which can be used if characters have to be set rotated through 180° in the RIM 16. The mirror circuit 136 interchanges bit 0 of a 16 bit word with bit 15, bit 1 with bit 14, bit 2 with bit 13, etc. The CPU 43 does not now generate the base address, but calculates, from data relating to the length and width and base address of a character, the highest occurring address for said given character in the RAM 12. This highest address is loaded into the counters 132 and 133 while the counters are also switched over to down-counters by the VME-MI controller 130. After each memory access of RAM 12 the content of the counter 133 is reduced by 1 and the 16-bit words from RAM 12 are mirrored in the mirror cicuit 136 and placed in the RIM 16 via the data-in register 137. These cycles are continued until the original base address of the character is reached. In the VME-MI 100 a data-out register 138 is also connected via data lines 47 to the RIP bus 46 in order to feed data in this manner to, for example, the FEC 10 or to RAM 12. The VME-MI controller 130 is connected via control lines 48 and condition lines 49 to the RIP bus 46 and, in addition, via a buffer 131 to address, data and control lines 139 and bus arbitration lines 140 of the VME bus 14. The CPU 43 can call up various modes in the VME-MI 100 such as "release bus", "multiple access", "single access" and "change". Before the VME-MI 100 can switch to the single- or multiple-access mode, the following data must first also be specified: read or write, normal or mirrored, the desired address and the data to be processed. These specifications can only be altered during the "release bus" mode and during "change" mode. The data to be processed may, however, always be changed. This is also indicated by a "CHANGE ACKNOWLEDGE" line. The register which contains the read data from the VME bus 14 can always be read out if a "REGISTER FULL" line is active. After calling up a "release bus" mode, the VME-MI 100 will unblock the VME bus 14. This means that the VME-bus drivers are disabled and a BUSY signal from the VME bus is rendered inactive. The unblocking of the VME bus 14 can only take place if the last access cycle is completely finished. A "CHANGE ACKNOWLEDGE" signal indicates that the "release bus" mode is established. In this mode of the interface no acccess to the VME bus 14 can take place. After a "change" request the VME-MI 100 is instructed to seize VME bus 14 if this was not yet the case. This is achieved with the bus arbitration lines 140. The assumption of the "change"mode is indicated via the "CHANGE ACKNOWLEDGE" line. Access to the VME bus 14 can then take place. The contents of the address and data registers can also be changed in the "change" mode. The "change" mode provides a possibility to stop accesses to the VME bus temporarily without the VME bus being unblocked. A single access to the VME bus can be initiated by calling up a "single cycle" mode. If the preceding mode was a "release bus" mode the VME bus is only seized via a corresponding active signal from the arbitration logic. Thereafter only a word access to the VME bus can be performed. A read/write indicator decides whether a read or write cycle has to be performed. A read cycle means that data from the VME bus 14 is clocked into the data-in register 137 via the mirror circuit 136, which last can be activated by means of a normal/mirrored indicator. When data is being clocked into the data-in register 137, a REGISTER-FULL flag is set to indicate to CPU 43 that the transfer of data is complete and that the data have entered the said register. The REGISTER-FULL flag is set at the instant at which the data is read into data-in register 137 and after this access the content of the address counter is incresed by 1. If a mirror function has been enabled, the content of the address counter is decreased by 1. If the REGISTER-FULL flag is still activated and data is being read from the VME-bus, the normal VME cycle is prolonged until the data-in register 137 has been completely read out and new data have been read into the data-in register 137. A write cycle is in principle the same as a read cycle. The only difference is the direction of the data stream. In a write cycle the data which are in the data-out register 138 are transferred to the VME bus 14. The mirror circuit 136 does not alter anything in the written data. The data-in register 137 must already have been read for it to be possible to clear the REGISTER-FULL flag. The "multiple-access" mode exhibits many similarities to the "single-acces" mode. A "single-access" mode is intended for the reading and writing of commands from, and status information to, the FEC. A "multiple-access" mode is primarily intended for reading graphical and font data, a subsequent access being automatically initiated by the VME-MI 100. The new address is generated by the address counter. The only action which has to be performed in this case is the reading of the data-in register 137. The various modes described above are selected with the VME-MODE lines which are connected to some of the signal lines of the CPU 43. The read/write selector and the normal/mirrored selector are also connected to such signal lines. The CHANGE-ACKNOWLEDGE and VME register-full signals originate from the WAIT lines of the CPU 43. The VME address is stored in 24-bit counters 132 and 133, the input and output data in two 16-bit registers 137 en 138. "Address-high" and "address-low" counters 132 and 133 and the data-out register 138 are loaded by means of register clock lines. The data-in register 137 can be read by means of a register enable line originating from the CPU 43. The LSM interface In Fig. 5 the LSM interface 44 is represented diagrammatically in more detail. On reading out the RIM 16 the RIP 15 fetches a 16-bit word from said memory and transmits it via datalines 47 of the RIP bus 46 to register 111. The control block 110 sends a "load" signal via line 115 to shift register 112 and the content of register 111 is loaded in parallel into the shift register 112. The laser printer emits burst pulses with a frequency of approximately 24 MHz which are supplied via line 53 and via I/O buffer 113 to the shift register 112 and the control block 110. The content of the shift register 112 is serially shifted out by means of said pulses and is supplied via I/O buffer 113 via line 18 to the modulator of the ROS. The burst pulses are supplied to a 16 counter in the control block 110 and once 15 pulses have been counted or during the 16th counting pulse, a word which has now been freshly set in register 111 is transmitted in parallel to shift register 112 and shifted out. Before the shifting-out operation of said 16-bit word is started, the register 111 is loaded with a new 16-bit word. An "EMPTY" flag is set once data have been set in the shift register 112 and new data can be written into the register 111. The "EMPTY" flag is connected to a "wait line" of the CPU 43 of the RIP 15. In this manner a whole scan line is transmitted in sequence to the ROS. The control block 110 emits condition signals to the CPU 43 via condition lines 49 of the RIP bus 46. After a scan line has been completed and before a SOS signal is supplied via line 52 from the ROS to the control block 110, the fetching of data from the RIM 16 is briefly suspended by the RIP 15 (wait condition). During this time the register 111 is cleared via line 114. In response to the SOS signal the cycle of filling register 111, transmission to shift register 112, shifting out etc., described earlier is repeated again for a subsequent scan line. After the loading of a word into register 111 the "full" status is likewise transmitted via condition lines 49 to the CPU 43, the latter delaying the fetching up of a new word until the content of the register 111 has again been loaded into the shift register 112. After a PAV signal the number of SOS pulses is counted by means of a counter in the CPU 43, and by this means it is possible to ascertain when a page has been transmitted completely to the ROS. The Central Processing Unit In Fig. 6 the CPU 43 of the RIP 15 is represented diagrammatically in more detail. Said CPU is built up around a microprogrammable microprocessor, processor 74, type Am29116, and an associated address sequencer 70, type 2910A, both manufactured by Advanced Micro Devices. In response to each clock cycle the microinstruction which has to be performed is set in the microinstruction register 72. Said microinstruction originates from the micro-PROM 71 and the latter is again addressed by means of the address sequencer 70. In the processor 74 every function is controlled by some of the microinstruction bits. Said microinstructions can be divided into bits for the address sequencer 70, the processor 74, the branch-address control unit 79, the condition selector 75, the wait selector 77 and the enable block 78. The sequence in which the microinstructions which are stored in the micro-PROM 71 are performed is likewise controlled by the address sequencer 70. In addition to the possibility of consecutive access to the addresses, conditional branch instructions to every microinstruction in the 4096-size microword area of the micro-PROM 71 can be carried out. A LIFO stack provides return-coupling and loop-in possibilities in a microsubroutine. The stack is nine steps deep. For each microinstruction the address sequencer 70 provides a 12-bit address which is initiated from one of the four following sources: The microprogram address register (PC), which usually indicates an address by an address increase of 1 with respect to the address being handled. However, if a "wait" mode is generated by wait selector 77, the PC is not increased. An external input connected to lines 92 which obtains its data from the branch address control unit 79. A nine-step deep LIFO stack which is loaded with the content of the microprogram address register (PC) during a preceding microinstruction. A register/counter which retains the data which have been loaded from an external input during a preceding microinstruction. The processor 74 is a microprogrammable 16-bit microprocessor, type Am 29116, with an instruction set which is optimised for graphical applications. The instruction set for the processor 74 comprises, in particular, single and double operand, rotate n bits and rotate and merge. The processor 74 receives its instructions for performing an operation from the microinstruction register 72 via bus 83 and an instruction-modification circuit 73. The instruction input is also used as data-input for "immediate" instructions. If the "instruction-enable" (IEN) input of the processor 74 is activated via line 94, the results of the instruction performed are retained in the accumulator and the status register in the processor 74. If an "output enable" (OE) is activated via line 95, the data lines of the CPU 43 are switched as outputs and they contain the content of the ALU of processor 74. Conversely, if the "output enable" is rendered inactive via line 95, the databus of the CPU 43 functions as a 16-bit input, and data which are present on the RIP bus can be supplied via data lines 47 to the processor 74. Said data can then be retained in an internal register. The data bus of the processor 74 is connected directly to the data lines 47 of the RIP bus. The status of the ALU (for example, carry, negative, zero, overflow) is available on the "status bus" 87 of the processor 74 during each cycle. The instruction-modification circuit 73 makes it possible to adapt the instructions recorded in the micro-PROM 71 to indicate in the case of instructions such as, for example "rotate n bits", the number of bits with which rotation has to be performed. This number of bits is then specified via a number of lines (91) from the processor data lines 47. If an IEN signal on the line 94 renders the instruction input of the processor 74 inactive, the same processor instruction bits which are supplied to the processor 74 on bus 83 can be supplied via bus 84 also to the branch address control unit 79 and be used to cause the address sequencer 70 to branch to a randomly different address. Normally the unit 79 receives its branch address from the content of a register which is filled with data from data lines 47 via bus 90. The condition selector 75 incorporates a one-out-of eight multiplexer, and the output thereof is connected via line 89 to the address sequencer 70. One of the eight possible conditions which are applied from the condition lines 49 of the RIP bus or from the processor status lines 87 via status buffer 76 and lines 88 to the input of the condition selector 75 can be selected. The selected condition is used by the address sequencer 70 to perform the desired conditional instruction. Any new conditions can be loaded into the status buffer 76 by supplying a selection-enable signal (SLE) via lines 85 to the status buffer 76. The "wait selector" 77 likewise incorporates a one-out-of-eight multiplexer which in the active mode connects one of the eigh "wait" lines 97 via line 93 to the address sequencer 70. A zero level on a wait line stops the program counter of the program address register. The wait lines are connected to the condition lines of the RIP bus. The enable block 78 has various functions and generates, in addition, all the signals which are necessary for the control lines on the RIP bus. It performs three different funtions: a. The generation of "enable signals". The enable signals determine which of the data registers which are connected by their outputs to the RIP bus have to be activated. For each register there is one enable line. b. The generation of register clocks. The clock lines determine which data registers which are connected by their inputs to the RIP bus have to clock in data. There is one clock line present for each register. c. The generation of other signals. The signals on the signal lines are used as flags and function selectors in the interface modules which are connected to the RIP bus. Raster image bus interface The connection between the RIP bus 46 and the RI bus 17 is formed by the RI bus interface 45 (Fig. 7). Said interface buffers the bidirectional data, the addresses to be called up and the modification code. The buffering is performed by means of registers. The "data-out" register 120, "address-low" register 122, "address-high" register 123 and the modification register 124 can be loaded from the RIP bus 46. The loading takes place under the control of address and control lines 48 of the enable block 78 of the CPU 43. The "data-in" register 121 can be read out under the guidance of control by means of an enable line of the enable block 78. The "address-high" register 123 contains the most significant bits of the address. The "address-low" register 122 contains the least significant bits. After the loading of the "address-high" register 123 the RI-bus read/write cycle is automatically started. This means that the following processes are carried out by the controller 125: Cycle 1 - sets an address on the RI bus and activates RAV (RI-bus address valid), Cycle 2 - sets data-out on the RI bus and inactivates RAV, Cycle 3 - reads the data on the RI bus into the "data-in" register 121. Before the starting of an RI-bus cycle, the CPU 43 has to test whether the RI-bus busy line 57 is inactive. Said busy line 57 is connected to one of the wait lines of the CPU 43. The raster image bus (RI bus) The RI bus 17 connects the RIP 15 to the bit-map memory 16 (RIM) and is made up of 64 lines. It comprises a 32-bit wide multiplexed address/data bus. The RIP 15 acts on the RI bus 17 as a master. The RIM 16, which comprises one or more RIM boards, does not itself take any initiative on the bus. In addition, there may further be connected to the RI bus 17 RI-bus DMA devices which can submit a request to the RIP 15 to obtain control over the bus. The RI bus 17 is a synchronous bus. A clock signal (BCLK) is presented to the RI bus by the RIP 15. All the actions on the bus are performed at the edges of the biphase clock. Thus, all the actions of the RIP take place on the rising edge, and all the actions of the RIM on the falling edge of the clock signal (BCLK). The other modes on the RI bus 17 can be described by means of three signal levels, high, low and high-impedance (tristate). All the alterations in signal levels take place after they have been initiated by an active edge of the biphase clock. There are thus three groups of signals: the clock signals, the address/data signals and the remaining signals. The signals which occur on the bus are defined below: BCLK: this is a symmetrical clock pulse which is presented to the RI bus by the RIP. RAD 00 .. RAD 31 (RIP address/data lines): this is a multiplexed address/data way which is driven by all the bus devices with tristate drivers. All the lines are active when "high". RMC 0 .. RMC 3 (RIM modify code): on these lines a code is presented to the RIM board by the RIP or a DMA device. This code specifies the "modify" function which takes place during the logic operation which is performed on the RIM board on the content of the addressed memory word. These signals are also of the tristate type. RROFF (RI-bus refresh-off): this signal indicates that the RIM boards can switch off the refresh to achieve a minimum cycle time. To prevent loss of data, a special addressing sequence is maintained between the RIM boards and the RIP. RBR 0, RBR 1 (RI-bus bus request): with these open-collector signals two DMA devices can request the bus arbiter for access to the bus. The devices are of a different priority. RBG (RI-bus bus grant): with this line the bus arbiter indicates that the bus is available to the requesting device with the highest priority. RBUSY (RI-bus bus busy): with this open-collector signal an addressed RIM board can indicate that the board is unable to process an new bus cycle for a certain time. RAV (RI-bus address valid): this tristate signal which is active when low indicates that there is a valid address on the RI bus. The RI bus is constructed with a 32-bit wide data and address way which is multiplexed for RAD 00 .. RAD 31. The allocation of these lines is as follows: A 24 - D 16: in this situation the address lines RA00 .. RA23 are used. The lines RAD 24 .. RAD 31 are then in the "don't care" mode. For the data lines RAD00 .. RAD15 are used. The lines RAD16 .. RAD31 are at that instant in the "don't care" mode. Data transfer thus takes place on the basis of 16-bit words and the addresses are 24 bits wide. Another option for using the 32 data and address lines is: A24 - D16 - D16: This situation is the same as the A24 - D16 situation as regards the address lines. By adding a second board in the same address space, in which the data flows through the bus via the lines RAD16 .. RAD31, it is possible for a 32-bit wide databus to be created with two boards which are internally 16 bits wide. On a RIM board it is possible for the section of the address/databus over which the data is transferred to be selected. By means of the RBR0, RBR1 and RBG signals access to the RI bus is regulated between the RIP and optional DMA devices. This arbitration takes place completely outside the RIM 16. Each cycle on the bus consists of a WRITE/READ cycle. If the bus is free (RBUSY not active), the RIP is able to set an address (ADR[n]) on the bus on lines (RAD00 .. RAD23). This takes place together with the presentation of a RAV-signal and a RI-bus modify code (RM code) via lines RMC00 .. RMC03. After the address the RIP presents its data (DATA0[n]) to the bus on lines (RAD00 .. RAD15). The RIM board addressed by ADR[n] renders the RBUSY signal active. The RIP then leaves the RI bus in order to give the RIM board addressed by the ADR[n-1] the opportunity to set DATAI[n-1] on the bus so that the RIP is able to read these data in. Two consecutive WRITE/READ cycles are, as it were, pushed into each other as a result of this. This is still further optimised by making the time which the RIP requires to decide whether RBUSY has become inactive coincide with the last processing phase of the RIM board addressed by ADR[n] in the current cycle. This is achieved by the RIM board making RBUSY already inactive before the RIM board is already completely ready but when it is already quite certain that it will be ready when the RIP has been able to confirm this. The first cycle thus contains invalid data and an additional cycle is also necessary to fetch up the final data from the RIM. In the manner described above a minimum cycle time is achieved on the bus. By "minimum cycle time" is meant the timing sequence of bus modes in which a maximum transfer rate is achieved on the bus. As a result of the refresh of the RIM it may happen that a RIM board is unable to achieve the minimum cycle time. The RIM boards indicate this by means of the RBUSY signal. By extending this RBUSY signal by a certain number of clock cycles (BCLK), the RIP postpones its subsequent access to the memory by an integral number of clock cycles. A same situation may occur if the RIP has not yet finished a certain task. The RIP indicates this to the bus by postponing the RAV signal by an integral number of clock cycles. Raster image memory The RIM 16 (Fig. 11) comprises a 24 Mbit dynamic memory 220 organised in 16-bit words and is used as page-size bit-map memory. Each memory location in the memory 220 corresponds to one precise position on the final printed page. The RIM 16 is connected via the RI bus 17 to the RIP 15 and is filled by the RIP 15 with expanded font data and graphical data. An important process which takes place in the RIM 16 is the modify process which is performed on an addressed word. The modify process comprises 16 different logic operations which can be applied to the incoming data and the data already present at a certain address. One particular modify function is selected by presenting a RIM modify code to the lines RMC0 .. RMC3 221 of the RI bus 17. Said modify code is set in the RMC register 222 and supplied to the logic processing unit 223 (ALU) which is made up of programmable logic. The new data (ND) is supplied via the DATA0 register 227 over data lines 225 to the ALU 223 while the old data (OD) already present in the memory 220 is supplied via data output lines 226 to the ALU 223. The result of the processing (MD) in the ALU 223 is written into the memory 220 via lines 224. In the table below some of the modify functions are represented along with the RM codes associated therewith and the corresponding logic functions. Because the RI bus 17 is a multiplexed bus, the separate addresses and the data have to be clocked into the registers. For this purpose the RI bus 17 is connected to an address/data bus buffer 228 and when an address is presented to the RI bus 17 it is supplied to the address register 229 via buffer 228. When the data is presented (one clock cycle later), said data are stored in the DATO register 227 via buffers 228. The data-in register 230 ("in" for the RIP but "out" for the RIM) has been added in order to be able to set the data originating from the memory 220, which belong to the previously presented address, on the RI bus 17. The control of the RIM 16 is provided by the memory control circuit 231. The memory control circuit 231 contains a bus state sequencer for starting a number of actions in response to a RAV signal, which actions consist of the clocking in of an address, the clocking in of data associated therewith, the clocking in of the modify code and the setting of data belonging to the preceding address on the RI bus 17. In addition the memory control circuit 231 incorporates a memory state sequencer which is synchronised with the bus state sequencer. The memory state sequencer can be initiated by a refresh request or by a bus cycle for a memory access. If a refresh cycle is performed, the next bus cycle has to be suspended. The circuit 231 employs programmable logic. The memory 220 is built up from 256 K dynamic memory chips and is organised in six "banks" of 256 K words of 16 bits. Bank selection takes place through decoding the address lines A18, A19 and A20 in the address multiplexer 232. The addressing of a memory location in one bank takes place by means of address lines A0 - A7 and A16 and the generation of a row address strobe (RAS) from the control circuit 231, and address lines A8, A15 and A17 are then supplied to the memory address lines via the address multiplexer 232, and a column address strobe (CAS) is also generated by the control circuit 231. Because dynamic memories are used all the memory locations of the memory 220 have to undergo a refresh at least once every 4 msec. This is done by periodically adding a "RAS-only" cycle. During this cycle a row address is presented to all the banks. The refresh address in a row is derived by means of a 9-bit counter which is increased by 1 after each refresh cycle. By activating the RROFF line of the RI bus 17 the normal refresh cycle is interrupted and the RIP 15 ensures that the minimum cycle time of the next cycle is met. Non-addressed banks of the memory then use the address on the RI bus 17 to perform a refresh. On the addressed bank refresh takes place by an access to the selected address. If only one copy of a page has to be made the "CLEAR" modify code will be set on the RI bus 17 during the reading out of the RIM 16 because after reading out the RIM 16 must be completely filled with zeros. If the page has to be kept in order to be printed yet once more, the "NOP" modify code will be set on the RI bus 17. Graphical instructions The bit-map filler 28 (Fig. 2) is designed to perform various textual and graphical instructions such as CHAR, MCHAR, LINE and CIRCLE. All these instructions, which are stored in the micro-PROM 71 (Fig. 6), are performed as microinstructions for the CPU 43. The algorithms for these instructions are implemented in a manner such that a bit-map filling speed which is as large possible is obtained. CHAR: is an instruction for the placing of a character at the correct location in the bit-map memory 16. Since the word boundaries of a character generally do not correspond to the word boundaries of the bit-map memory, a displacement is necessary (see Fig. 9). In the font memory 13 the bit-map representation 200 of a character 201 is stored in 16-bit words. A character usually comprises a number of 16-bit words, some of which are indicated by 203, 204 and 205. The corner point 202 of the character 201 is taken here as the reference point as an example, and the first 16-bit word 203 contains 16 bits of which the first bit is denoted by "0" and the last bit by "F". The bit representation of the first word is thus: 0000 0000 0001 1111. When this character 201 is placed at the desired y location in the bit-map memory 16, the word boundary 207 of the bit-map memory will on the whole only rarely coincide with the word boundary 0′ of the character 201. The operation to be carried out will thus correspond to the displacement of the bit-map representation of the character 201 over a number of (n) bits denoted by Δ y in the Figure. At the microinstruction level the following steps have to be carried out for this purpose: Rotate: from bit 0 to bit F over Δ y (n: =Δ y) Merge: mask = 1 rotate mask = 0 non-rotate MCHAR: is an instruction for placing a character in the bit-map memory 16 in mirrored form. The reading out of the bit-map representation of the character by the VME-bus interface takes place in reverse sequence. The displacement of the word boundaries of the character in the bit-map memory 16 takes place in an identical manner to that described for CHAR. The circuit for the mirroring is also accommodated in the VME-bus interface. For writing characters, lines, circles and so on into the bit-map memory 16 the front end has a number of overlay options (Fig. 10). These options are represented diagrammatically in Fig. 10 by reference to a letter V (210). A hatching 211 means that the content of the RIM 16 is unaltered. It is assumed that a "0" in the RIM produces "white" and a "1" in the RIM produces "black". WRITE: the existing content of the RIM 16 is made "0" and the bit-map representation of a character is written with ones (212). INVERT: the existing content of the RIM is made "1" and the bit-map representation of a character is written with zeros (216). PAINT: the content of the RIM is not deleted and the ones of the character undergo an "0R" function with the content of the RIM (213). MASK: the content of the RIM is made "0" at the locations where the character contains zeros, and where the character contains ones the content of the RIM is maintained (214). ERASE: the content of the RIM is made "0" at the locations where the character contains ones, and where the character contains zeros the content of the RIM is maintained (215). INVERTING PAINT: the content of the RIM is maintained where the character contains a "0" and where the character contains a "1" the content of the RIM is inverted. The invention is not limited to the method or embodiment described but can be performed by the skilled addressee in many corresponding ways all of which will fall within the scope of the following claims.

Claims: Printing system for printing information by means of a raster output scanner adapted to form an image on an image-forming medium (405) having fixed imaging sections and to transfer the image to a receiving sheet at a repetition frequency corresponding to one machine cycle time (T), the raster output scanner also comprising a control device (460) which in response to a set-up-page signal starts the formation of a print when an imaging section is situated at a predetermined position and which is connected to a front-end system (461), being part of the printing system, which can deliver the set-up-page signal, comprising a front-end processor (10), a memory (12, 13), a raster-image processor (15) and a page-size bit-map memory (16), the front-end system (461) being adapted to receive coded information about a complex page and the raster-image processor (15) being adapted to decode the information about the coded complex page and place it in the bit-map memory (16), the raster-output scanner comprising an exposure device (462) which line by line exposes a fixed imaging section image-wise a fixed time after the set-up-page signal and in response to the electrical signals originating from the bit-map memory (16), characterised in that the front-end processor (10) can postpone the set-up-page signal one or more whole multiples of a machine cycle time (T) on the basis of the coded information by means of a prediction algorithm stored in the memory (12), the prediction algorithm determining the individual fine-format times (IFFT) of each character occurring on the complex page and summing all the individual fine-format times occurring. A method of operating a printing system according to claim 1, comprising starting forming a print on a set-up-page signal, storing coded information about a complex page in the front-end system (461), expanding this coded information, and storing the expanded information in the bit-map memory (16), reading-out the bit-map memory (16) a fixed time after the set-up-page signal is delivered and line by line exposing a fixed imaging section image-wise by means of the read-out information, characterised by determining on the basis of the coded information by means of a prediction algorithm the decoding time required to fine-format this information and place it in the bit-map memory (16), the prediction algorithm determining the individual fine-format times (IFFT) of each character occurring on the complex page and summing all the individual fine-format times occurring, and postponing the set-up-page signal, on the basis of this determined decoding time, for a time corresponding to one or more whole multiples of a machine cycle time (T). A method according to claim 2, characterised by determining the decoding time by determining the bit-map filling time by determining the character time or average process time (ACPT) as the average time required to place a character of a given size in the bit-map memory, determining the number of characters of each size occurring on the page, multiplying these character times by the associated numbers and summing the obtained products.

Inventors: NOTERMANS, RONALD MARIA WILLEM
Applicants: OCE-NEDERLAND B.V.
Application year: 1987
Main classification: G06K15
IPC codes: H04N1, B41J2, G06K15, G06F3