Adaptive control of second page printing to reduce smear in an inkjet printer

A high density graphics image can be printed without smearing by contact with a second page, without any unnecessary reduction of throughput. Throughput enhancement logic is inhibited during the printing of the second page for a variable delay related to the image density of the first page. The variable delay is calculated as a linear function of both the density (relative to a predetermined grid size) and the location if the densest portion of the first page, using coefficients which are different for different print modes. In one preferred embodiment, a maximum density is calculated by counting drops of ink in each of several overlapping grids, and the magnitude and location of the maximum density grid on a prior page is used to limit the throughput of a next page until a sufficient delay has elapsed to ensure that ink on the prior page will not be smeared when it comes into contact with the next page.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates in general to inkjet printers and in 
particular to printing high quality images having densely inked areas 
without smearing the print media. 
CROSS-REFERENCE TO RELATED APPLICATIONS 
The following commonly assigned U.S. patent application filed concurrently 
herewith claims an invention which, although believed to be patentably 
distinguishable, has objectives and which is based on principles that are 
closely related to those of the present invention: 
J.R. Arbeiter et al, "Densitometer for Adaptive Control of Ink Drying Time 
for Inkier Printer" Ser. No. 08/056,330 filed Apr. 30, 1993. 
BACKGROUND OF THE INVENTION 
Inkjet printers operate by sweeping a pen with one or more inkjet nozzles 
above a print medium and applying a precision quantity of liquid ink from 
specified nozzles as they pass over specified pixel locations on the print 
medium. 
The print medium becomes damper and remains damp for a longer time as more 
ink is applied on the same area of the print medium. As a first 
approximation, the drying time, before which the ink is not subject to 
smearing by contact with an adjacent sheet is a linear function of amount 
of ink applied. In certain prior art inkjet printers, a fixed delay is 
introduced between any physical contact between successively printed 
sheets, which is greater than the maximum time required to dry the densest 
possible image to the point that it is not susceptible to smearing. 
However, this unnecessarily restricts throughput when the printed images 
on some pages do not contain any densely inked portions and/or when large 
unprinted areas appear on succeeding pages which can be completely 
bypassed by the print head. 
Thus, the prior art has failed to provide a satisfactory solution for 
printing a high quality graphics image at a high throughput rate, which is 
further exacerbated if additional dots of ink are selectively applied 
between adjacent pixels, thereby effectively doubling the number of dots 
of ink, in order to increase image density and/or to provide smoother 
boundaries for any curved or diagonal images ("Resolution Enhancement 
Technology"). 
SUMMARY OF THE INVENTION 
Therefore, an overall objective of the present invention is to provide an 
improved inkjet printer whereby a page of high density graphics images can 
be printed without smearing by contact with a second page, without any 
unnecessary reduction of throughput. 
In accordance with one aspect of the present invention, throughput 
enhancement logic is inhibited during the printing of the second page for 
a variable delay related to the image density of the first page. In 
accordance with specific aspects of the invention, the variable delay is 
calculated as a linear function of both the density (relative to a 
predetermined grid size) and the location if the densest portion of the 
first page, using coefficients which are different for different print 
modes.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a diagram of an inkjet printer 100 wherein the present invention 
is embodied. The printer 100 performs printing on sheets of paper 101 or 
other print media which are supplied from an input tray 102. The print 
media are printed by a plurality of inkjet nozzles 103 in the printer 100. 
After a print medium is printed, it is output and stacked onto an output 
tray 104. 
FIG. 2 is a side view which shows the path along which a sheet of paper 
travels within the printer 100. When a sheet of paper is picked from tray 
102, it is pushed by a feeder mechanism (not shown) into a paper path at 
the lower part of a forward paper guide 105. Before the paper passes 
inside the paper path defined by guide 105, it is preheated by heat 
generated from a preheater (not shown). 
The paper path directs the paper to an interface between a pinch wheel 106 
and a main drive roller 107 which is rotated by a motor (not shown). The 
main drive roller 107 and the pinch wheel 106 operate together to advance 
the paper over a platen 109 which is heated by a heater 108. A swath of 
ink (typically 96 nozzles high, or about 8 mm) is applied to the paper 
lying over the heated platen and the heater 108 accelerates the 
evaporation of solvent absorbed by the paper. 
The inkjet nozzles 103 are carried by a carriage which is driven along the 
support shaft by a mechanism which comprises, for example, a motor and a 
belt. Each trip along the support shaft is conventionally called a sweep. 
The inkjet nozzles 103, when activated, apply droplets of ink onto the 
paper. Typically, the inkier nozzles are mounted on the carriage in a 
direction perpendicular to the direction of the sweep, so that columns of 
dots are printed in one sweep. The columns of dots made by inkjet nozzles 
across a horizontal portion of the paper is sometimes called a swath. A 
swath may be printed by one or more passes of the inkier nozzles across 
the same horizontal portion, depending upon the required print mode. In 
order to reduce undesirable "banding", some of the known printing modes 
advance the print medium relative to the carriage in the vertical 
direction by only a fraction of the height of a single swath; in order to 
reduce "bleeding", multipass printing modes may be used in which the dots 
applied in successive passes are interleaved vertically and horizontally. 
Moreover, both single pass and multiple pass print modes may employ 
"Resolution Enhancement Technology" in which additional dots of ink are 
selectively applied between adjacent pixels to increase image density 
and/or to provide smoother boundaries for curved or diagonal images. 
When a swath is completely printed, the paper is advanced and ejected into 
the output tray 104, with the assistance of starwheel 110 and an output 
roller 111 which cooperate to produce a pulling force on the paper. A 
starwheel is used so that its pointed edges can pull the paper at the 
printed surface without smearing. 
FIG. 3 is a logic diagram showing the main hardware components of the 
printer 100 and the related software. The hardware components include a 
controller 120 which operates to control the main operations of the 
printer 100. For example, the controller controls the sheet 
feeding/stacking mechanism 121, including the pinch wheel 106, the main 
drive roller 107, the starwheel 110 and the output roller 111, to feed and 
position a sheet of paper during a printing process. The controller 120 
also controls the carriage drive mechanism 122 to move the carriage across 
the paper, The controller 120 also controls the inkjet nozzles 123 to 
activate them at appropriate times so that ink can be applied at the 
proper pixels of the paper. 
The controller 120 performs the control functions by executing instructions 
and data accessed from a memory 125. For example, data to be printed are 
received by the controller 120 under the control of a software driver. The 
data received are stored in a "plot file" within a data area 126 in the 
memory 125. 
The instructions can be classified logically into different procedures. 
These procedures include different driver routines 127 such as a routine 
for controlling the motor which drives the main drive roller, a routine 
for controlling the motor which drives the output roller/star wheel, a 
routine for controlling the motor which drives the carriage and a routine 
for controlling activation of the inkjet nozzles. 
One or more timers 1 are available to controller 120. A timer may be simply 
a starting clock value stored at a predetermined location in the memory. 
To obtain an elapsed time value, the stored starting value is then 
subtracted from an instantaneous clock value from a realtime clock (not 
shown). 
The memory 125 also stores a throughput procedure 129. The throughput 
procedure operates to control the throughput of the printer 100. 
Throughput may be thought of as the sum of a first duration T1 and a 
second duration T2, where T1 is the time duration between the time 
immediately before a first swath is printed on a sheet of paper and the 
time immediately after the last swath is printed, and T2 is the time 
duration between the final position of one sheet and the initial position 
of the next sheet. T2 represents the sheet feeding delay of the printer, 
which is typically constrained only by the drive mechanism and is therefor 
a constant; however T1 is also constrained by various factors related to 
the complexity and density of the image and the desired print quality, 
which in turn determine how much time is required for each of the 
sequential process steps of the selected print mode. Throughput procedure 
129 uses horizontal and vertical logic seeking to identify blank lines 
between adjacent swaths (vertical logic seeking) and blank portions at 
either end of (or possibly within) a swath, altogether avoiding any 
unnecessary carriage movements and slewing the carriage at maximum slew 
rate over any unprinted areas over which the carriage must be slewed. 
The memory 125 also stores a densitometer procedure 128 which determines a 
maximum density of dots of ink to be printed in the current swath, and a 
second page anti-smear procedure 130 which operates in response to the 
results from the densitometer procedure 128 to ensure that the ink of a 
preceding sheet of paper is not smeared when the current sheet of paper is 
output. 
Typically, a sheet of paper is printed by applying ink at the specified dot 
positions (pixels). The dots may be printed in single (e.g., black) or 
multiple colors. To print a multiple color image, the carriage may have to 
make more than one sweep across the print medium and make two or more 
drops of ink with different primary colors at the same dot locations 
("pixels"), as disclosed in U.S. Pat. No. 4,855,752 which is assigned to 
the assignee of the present invention. 
The printer 100 has several different modes of printing. Each of the 
different modes is used to produce a different type or quality of an 
image. For example, one or more "high quality" modes can be specified 
whereby density of the print dots is increased to enhance the quality of 
the printed images. In some printers, a "high quality" mode of printing 
may require the printer 100 to make multiple passes across substantially 
the same horizontal portion of the page. 
For example, in its high quality three pass mode, printer 100 make three 
sweeps across the page to print a single swath. In each of the three 
sweeps, the printer would print one of every three consecutive dots so as 
to allow more time for one dot to dry before the neighboring dot is 
printed, and thereby preventing the possibility that the ink of the two 
neighboring dots would combine to produce an unwanted shape or color. Such 
a three pass printing mode may also be used to reduce banding by dividing 
the swath into three reduced-height bands, printed in successive but 
overlapping printing cycles each providing for three passes across an 
associated reduced-height band. 
In known manner, the image to be printed is defined by the "plot file" 
which specified which pixels are and which pixels are not to be coated 
with dots of ink. For color images, the color of the ink is also specified 
in the plot file. 
FIG. 8 is a flow chart showing the general steps performed by the printer 
in printing an image. 
To print a page, a plot file is first sent to the printer 100 (step 201). 
As the plot file is being received by the printer 100, it is scanned by 
the controller 120. The controller 120 scans the plot file to divide it 
into one or more printed swaths and at the same time produces a density 
profile for the entire page (step 201). 
More particularly, when the controller 120 scans the plot file, it also 
divides it into a plurality of grids each with a predetermined shape and 
size, each identified by an x-coordinate and a y-coordinate. For each 
grid, the controller 120 determines the number of dots that need to be 
printed with each type of ink. 
According to one method, each swath to be printed in a single sweep of the 
carriage is subdivided into a plurality of rows and each row is subdivided 
into a plurality of non-overlapping grids; each dot on the page may belong 
to only one grid. The density of each grid is then determined by counting 
the number of pixels to be printed in a representative randomly selected 
sample of the pixels in the grid. A maximum row density is then obtained 
from the individual grid densities in each row, and a maximum sweep 
density is then obtained from the individual row densities in the sweep. 
Although such non-overlap scanning using only a representative sample is 
faster, it may, however, produce inaccurate results. To illustrate, assume 
an image to be printed by the printer has the shape 160 as shown in FIG. 4 
and assume that the scanning is performed by square grids 161, 162, . . . 
, 169. Depending upon the position of the image 160 with respect to the 
grids, different density profiles may result. For example, if the image 
160 falls by chance in the middle of a grid 165 as shown in FIG. 4 the 
density profile would show a high density, D1, in grid 165. On the other 
hand, if same image 160' per chance falls in the intersection of grids 
161', 162', 164' and 165' as shown in FIG. 5, then the highest density of 
the image 160' would be about a fourth of the density D1 obtain from the 
scanning performed as shown in FIG. 4. 
Moreover, accuracy of the local density profile is also a function of the 
size of the grid. For example, a density profile which is made with a 
non-overlapping grid size of 150.times.150 dots will more accurately 
reflect a dense image having a size of only 300.times.300 dots than a 
density profile which is made with a non-overlapping grid size of 
300.times.300 dots. However, if grid size were so small that a single grid 
could have a density of 100% but the solvent could nevertheless rapidly 
diffuse into adjacent unprinted areas, such a small grid size would not 
provide a useful measure of the probability of an image being sufficiently 
dense to adversely affect print quality. 
However, more accurate measurement of the dot density may be obtained by 
overlapping the larger grids vertically and/or horizontally, to thereby 
obtain the advantages of both the larger and the smaller grid sizes. FIG. 
6 shows how horizontal overlapping is performed with respect to three 
exemplary grids G(1,1), G(1,2) and G(1,3). As shown, the left half of grid 
G(1,2) overlaps right half of grid G(1,1). On the other hand, the right 
half of grid G(1,2) is overlapped by the left half of grid G(1,3). 
FIG. 7 shows how both vertical and horizontal overlapping may be combined. 
A first row of grids G(1 ,x), comprising grids G(1,1 ), G(1,2) and G(1,3) 
of FIG. 6 and a second row G(2,x) of grids which overlap with the first 
row G(1,x). For example, the upper 5/6 of grid G(2,1) in the second row 
overlaps the lower 5/6 of grid G(1,1) of the first row, and the upper 5/6 
of grid G(2,2) overlaps the lower 5/6 of grid G(1,2). 
FIG. 9 is a flow chart illustrating the basic steps required to generate a 
density profile. The steps are performed by the densitometer procedure 
when it is executed by the controller 120. 
In step 301, a grid of the image to be printed is scanned. In scanning the 
grid, each dot position of the grid is examined (step 302). Within the 
grid, the number of dot positions which will be printed with black dot and 
the number of dot positions which will be printed with colored dots are 
counted (step 303). Separate counts are made of black and colored dots 
because they are typically produced by inks having different formulations 
and concentrations. Because all the grids have the same size, the count 
can therefore be used directly to represent the density of the grip. After 
all the dot positions are examined, the count and the coordinates of the 
grid are stored into the memory 125 (step 304). The controller 120 then 
examines the plot file to determine whether the current grid is the last 
grid of the page (step 305). If the current grid is not the last grid, 
then the process is repeated on the next grid (step 306). Otherwise, the 
procedure terminates. 
In practice, rather than maintaining a density history for each grid, only 
a maximum density for one or more rows of grids is stored, with the size 
of the individual grids preferably being preferably decreased. As a row of 
grids is being scanned, the grid with the maximum density in the row is 
located, along with its density value. This is accomplished by providing a 
variable, GRID-ROW-MAX, and the additional steps shown in FIG. 10 which 
are performed between steps 303 and 305. In step 307, the count obtained 
from step 303 is compared with the value stored in GRID-ROW-MAX. If the 
count of the current grid is greater than GRID-ROW-MAX, its value is 
stored into GRID-ROW-MAX (step 308); otherwise, step 308 is bypassed. It 
will be understood that GRID-ROW-MAX is initialized (by setting it to "0") 
at the beginning of the procedure shown in FIG. 9. If it is necessary to 
determine a maximum density for an area covering more than one grid row, 
this can be done by using a similar procedure to determine the maximum of 
the previously stored GRID-ROW-MAX values for each grid row involved. 
Alternatively, GRID-ROW-MAX is not re-initialized at the beginning of each 
row, but is re-initialized only once at the beginning of the area and is 
used until all the rows in that area have been processed. Similarly, if it 
is desired to determine a local density based on a grid size larger than 
that used to process the individual rows, this may be approximated by 
assuming that the maximum density locations in adjacent rows relate to 
adjacent portions of the image, and thus may be approximated by averaging 
the maximum densities of the adjoining rows; in any event, such an 
assumption would provide a calculated maximum density that is no less than 
the actual density. 
Referring back to FIG. 8, after the plot file is scanned and the required 
density information has been stored as a function of grid or row location, 
the page is printed (step 204). In practice, because only one swath is 
printed at a time, it is preferable to perform the printing operation 
(step 204) concurrently with the scanning operation (step 202), in which 
case as soon as all the pixels in one swath have been scanned, that swath 
can be printed, thereby increasing throughput and reducing the size of the 
buffer necessary to store the plot file. 
FIG. 11 is a block diagram showing the procedure performed by the 
controller 120 for printing a page N among a series of pages. 
In step 401 of the procedure, the controller 120 performs an initialization 
of the printer 100 to print the page N. The initialization includes 
executing the appropriate driver routines to position the inkier nozzles 
in a known position relative to a top corner of the page. When 
initialization is complete, the controller 120 causes the first swath of 
the page to be printed (step 402). 
Before each swath is printed or skipped over in whole or in part by the 
throughput enhancement logic, the controller 120 checks a page timer to 
see if the time elapsed since the printing of the last page, page N-1, has 
exceeded the throughput enhancement delay needed to avoid any possibility 
of smearing the previous page N-1 when page N is output (step 403). This 
delay is based upon the maximum density of page N-1. 
As a first approximation, there is a linear relationship between the local 
density of a particular portion of the image and the required drying time 
before the ink in that portion is sufficiently dry that it will not be 
smeared when it comes into contact with another sheet. Accordingly, it is 
necessary to delay any contact of the particular portion of the first 
sheet with any part of the next sheet by a time: 
EQU Tdry=Kdry.multidot.Den 
where Tdry is the total drying time required, Kdry is an experimentally 
derived constant and Den is the density of the selected portion. 
Although a separate Tdry could be calculated for each swath of the first 
page which would be used to start a timer as soon as that swath was 
printed, the required computations are simplified by determining only a 
single maximum density for the entire first page, and using that maximum 
density to calculate a worst case Tdry for that page. Since for equal ink 
density, the last portion to be printed will be the wettest, the 
implementation is further simplified by using only one timer and not 
starting the timer until the entire page has been printed. 
Consideration should also be given to the fact that in the preferred 
embodiment illustrated in FIG. 1, as the next page is being printed, its 
leading edge (typically the top of the page) is propelled by the paper 
advance mechanism (starwheel 110 and output roller 111) away from the 
platen 109 and into the output tray 104 in which the previously printed 
sheets are stacked, with the last printed sheet on the top of the stack 
with its printed side facing up. Thus, the leading edge of the page 
currently being printed is free to curve downward under the influence of 
gravity in the direction of output tray 104 and first contacts the printed 
area of the previous sheet at a predetermined distance of about 91/2" 
(about 240 mm) from the top. The leading edge of the next sheet then 
glides over the upper portion of the previous sheet until the current page 
has been printed and the two sheets are more or less aligned one on top of 
the other. Accordingly, the vertical location of the densely inked portion 
on the first page determines when it will first contacted by the next 
page. 
It will also be appreciated that, in the absence of throughput enhancement 
strategies such as vertical and horizontal logic seeking, there is a fixed 
delay between the time page N is output into tray 24 and the time page N+1 
will come into contact with page N. As a practical matter, it is 
advantageous to use that fixed delay to specify process variables such as 
ink drying time, in order to guarantee a minimum throughput rate for an 
entire page of graphics having at least some densely inked areas. 
Accordingly, the calculation of the required delay can be further 
simplified by realizing that rather than determine how much delay is 
required, it is sufficient to inhibit such throughput enhancement under 
certain degenerate conditions wherein a page having inked portions of 
higher than normal density is immediately followed by a page having 
relatively large printed areas. 
In an exemplary embodiment, these considerations are reflected in the 
following equation: 
EQU 0sec&lt;Inhibit=K1+K2*(Den)+K3*(Loc)&lt;Inhibit.sub.Max 
where 
Inhibit is the elapsed time during which any throughput enhancement should 
be inhibited 
K1 is an empirical offset constant 
K2 is an empirical density coefficient 
K3 is an empirical location coefficient and 
Inhibit.sub.Max is predetermined maximum. 
In the exemplary embodiment, Inhibit.sub.Max is 48 seconds, (Den) ranges 
from 0 to 1 (1 being solid black) and (Loc) ranges linearly from 1 (at the 
top of the page) to 4 (at 240 mm from the top); for all modes except high 
quality three pass mode, K1, K2 and K3 are zero (ie, there is no need to 
inhibit throughput enhancement). In the case of a high quality three pass 
mode (which prints a large black image with two drops of ink at every 
pixel), K1 is -15, K2 is 48 and K3 is 1. 
Thus, in the exemplary embodiment, throughput enhancement in high quality 
three pass mode is inhibited for a maximum of 34 seconds for a 100% dense 
square at the top of the preceding page, for 33 seconds for the same 
square at the bottom of the page, or for 37 seconds for the same square at 
the more critical location 240 mm from the top. If the density of the 
densest square is only 50%, the corresponding throughput enhancement 
delays are 11, 10 and 13 seconds, and for a 25% density are 0, 0 and 1 
second. 
In steps 404a and 404b, the controller performs a procedure for printing 
the next swath. 
If the time elapsed since the printing of page N-1 has not exceeded the 
delay required to prevent smearing of page N-1 when page N is output, then 
a throughput reduction procedure (step 405) is executed. On the other 
hand, if the elapsed time has exceeded the required delay, then the 
throughput reduction procedure is not executed. 
Referring back to FIG. 11, in step 406, the controller 120 checks whether 
the last swath of page N has been processed. If not, steps 403-406 are 
repeated. 
If the last swath of page N has already been printed, then the elapsed time 
clock is restarted (step 407). The elapsed time clock is restarted so that 
it can be used in step 403 when page N+1 is being printed. 
FIG. 12 is a flow chart showing the procedure which the controller 120 
performs to print a swath. 
Before printing or skipping over the next swath, the controller 120 first 
determines the upper and lower boundaries of the previous swath (step 411 
). The upper boundary can be defined as the y-coordinate of the highest 
row of pixels in the swath and the lower boundary can be defined as the 
y-coordinate of the lowest row of pixels in the swath. 
In step 412, the controller 120 scans the density profile for all the grids 
(or the density profiles for all the rows, if only GRID-ROW-MAX was 
stored), whose y-coordinates are within the values of upper and lower 
boundaries of the previous swath and retrieves the maximum density 
associated with those grids (or rows), and stores its density in the 
memory 125 (step 413). To facilitate the concurrent scanning of the plot 
file and the printing of the individual swaths, a respective location can 
be reserved in the memory 125 for storing the value of the maximum density 
of each swath. The controller 120 also checks to see if the maximum 
density of the previous swath is the highest density of the page (step 
414). If so, the highest density of the page is then updated with the 
maximum density of the sweep (step 415). The value of the highest density 
of the page is used in step 403 of the procedure shown in FIG. 11 for 
determining when the current page can be output without smearing the 
previous page. 
The controller 120 then determines whether a delay is required for the 
previous swath to dry so that it will not be smeared by the upcoming 
sweep. 
The delay for preventing smearing of the previous swath can be determined 
by several methods. 
One such method is to perform a table look-up based upon the maximum 
density of the swath to find a minimum time delay for which the previous 
swath should remain over the heated platen 109 before the paper is 
advanced or the carriage is moved over any portion of the previously 
printed swath, to thereby prevent any possibility of smearing. In order to 
speed up and simplify the required computations, separate tables are 
preferably maintained for different paper sizes and print modes; the table 
look-up is preferably performed using only the maximum density of the 
swath as determined in the densitometer procedure and preferably assumes a 
worst case condition that the maximum density is representative of average 
density over an area larger than a single grid. The controller 120 
performs the table look-up to determine the minimum time required for the 
swath. 
The values of the table can be obtained empirically. Several sets of 
exemplary values are listed in the following tables: 
______________________________________ 
density Minimum Time (seconds) 
______________________________________ 
A-size, Plain 
&gt;150 1.50 
&gt;75 1.20 
&gt;25 0.80 
&gt;0 0.45 
A-size, Color Transparency 
&gt;150 1.35 
&gt;75 1.10 
&gt;25 0.80 
&gt;0 0.45 
B-size, Plain, or Color Transparency 
&gt;150 1.70 
&gt;75 1.40 
&gt;25 0.90 
&gt;0 0.45 
______________________________________ 
Another method for determining the delay, which is preferred for its 
greater accuracy, but which is computationally more complex, is 
illustrated in the flow chart of FIG. 14. In step 431, the controller 120 
determines a delay factor (Sp) used to adjust the nominal advance delay 
(for each pass, if a multiple pass mode) of the current print mode based 
upon the swath's maximum density. This delay allows the solvent to 
evaporate sufficiently to prevent scraping of a previously printed swath 
while printing of the next swath. The swath density may include a value 
(Bden) which is the density of single color dots and a value (Cden) which 
is the density of multi-color dots obtained by the densitometer procedure. 
In general, the delay factor (Sp) is determined by the formula: 
Sp=f(Mode, Bden, Cden) 
where f(Mode, Bden, Cden) is a mode-dependent function of the density 
(Bden) of black dots and the density (Cden) of color dots on the swath. 
In the preferred embodiment, the delay factor Sp is determined by the 
formula 
EQU 100%.gtoreq.Sc-[K1*Bden+K2*Cden].gtoreq.Smin 
where Sc, K1, K2 are empirically established coefficients, with only Sc and 
Smin dependent on print mode. Exemplary values for K1 and K2 are 2.5 and 
0.75 respectively. Exemplary values for Sc and Smin are set forth in the 
following Table: 
TABLE 
______________________________________ 
Print Mode Sc Smin 
______________________________________ 
Normal 300 75 
Performance 300 75 
High-quality 1-pass 200 30 
High-quality 3-pass 237 50 
______________________________________ 
To illustrate the application of the equation, assume that a page is 
printed in normal mode (i.e., the value of Sc is 300) and that the densest 
grid has 80% of its pixels printed with black dots. From the above, the 
preferred delay factor Sp is 
EQU 300%-2.5*80%=300%-200%=100% 
Thus, in normal and performance modes, a maximum black density of 80% or 
less will not cause any reduction of throughput. Similarly, a black 
density of 90% will cause a maximum reduction of throughput by reducing 
the nominal advance delay by the minimum delay factor of 75%; for density 
values between 80% and 90%, the advance delay will vary linearly between 
100% and 75% of its nominal value. 
For high quality 1 pass mode, the maximum slowdown (50%) is utilized for 
black densities greater than 68%, which increases linearly to 100% at a 
density of 40%. For the high quality 3 pass mode, the corresponding 
figures are 74.8% density (50% slowdown) and 54.8% density (no slowdown). 
The controller 120 then uses the delay factor Sp to determine the required 
advance delay (tp) for printing the swath upon the specified print mode of 
the swath (step 432). The time tp is determined in the preferred 
embodiment by dividing a nominal advance time tn by the delay factor Sp. 
The nominal advance time tn is dependent on the print mode and may be 
stored in a look-up table; in an exemplary embodiment, it is 0.527 seconds 
for a high quality three pass mode and 0.512 seconds for all other modes. 
The result of the above identified division is then used to set a swatch 
delay timer. After the required advance delay time has elapsed (step 433), 
the controller 120 activates the appropriate drivers to advance the print 
medium in preparation for the next sweep (step 416). When the delay has 
elapsed, the controller 120 then activates the appropriate drivers to 
cause the inkjet to make a sweep (step 417). After the sweep is made, the 
controller 120 checks to see if the sweep just made is the last sweep of 
the page (step 406). If the sweep is not the last one for the page, steps 
411 to 418 are then repeated. 
To summarize, in a preferred embodiment, a variable delay for preventing 
smearing of the swath just printed by contact with the nozzle plate or 
other parts of the printer mechanism is a function of the density profile 
of the swath, and a variable delay for preventing smearing of a previous 
page by contact with a next page is a function of the density profile of 
the previous page. These related concepts enable the printing of 
densely-inked images without smearing and without sacrificing throughput 
and print quality. 
It is understood that the above-described embodiment is merely provided to 
illustrate the principles of the present invention, and that other 
embodiments may readily be devised using these principles by those skilled 
in the art without departing from the scope and spirit of the invention.