Pixel-density augmentation and adjustment with minimum data,...

Details Pixel-density augmentation and adjustment with minimum data,... Pixel-density augmentation and adjustment with minimum data,...

1999-02-18

2004-02-10

Rogers, Scott (Department: 2624)

Facsimile and static presentation processing

Static presentation processing

Attribute control

C358S003270, C358S502000, C358S520000

Reexamination Certificate

active

06690485

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, or other glossy media; and more particularly to a scanning machine and method that construct text or images from individual colorant spots created on a printing medium, in a two-dimensional pixel array. Thermal-inkjet printers and processes are of greatest interest; however the invention is applicable in other types of units such as, for example only, piezodriven inkjet printers and hot-wax transfer printers.
BACKGROUND OF THE INVENTION
(a) Mechanisms of color-intensity gradation—An incremental printer forms an image on a printing medium by placing colorant thereon in the form of tiny dots. An inkjet printer, for instance, ejects ink droplets that fly across a narrow gap to the print medium and so form the dots on the medium.
A principal way to reproduce color tones is by varying the areal density, i. e. number per unit area, of such dots on the medium. This variation ranges from none (white, if the print medium is white) to full density.
The phrase “full density” means that every location in the pixel array contains at least one dot. Making a color as intense as possible requires full density, to cover the printing medium with colorant.
Another way to produce various color tones is to vary the density of the colorant itself, i. e. the concentration of the colored substance that makes up each dot. This can be done for instance by applying plural quanta of colorant at a common pixel location.
Here the phrase “quanta of colorant” means the discrete units in which a printing system transmits colorant toward a printing medium. In inkjet and other liquid-ink systems, colorant quanta thus are drops of ink.
Variation in color tones can also be accomplished by storing colorants at various concentrations within the printing mechanism, and using those colorants in printing. This approach is relatively more costly and is not taken up further in this document.
(b) Texture and resolution vs. dynamic-range stability—The smaller the dots, the less visible are individual dots, thus producing an image that is less grainy and more realistic. These effects are desirable for ideal image quality at the low end of the dynamic range of color intensity—namely for very light colors such as highlight regions in photo-quality images, pastel washes and the like.
Certain factors, however, may cause the dots to be different in diameter than the nominal. Holding a nominal dot size is more difficult for smaller dots, and failure in this regard can introduce major problems, particularly at the opposite end of the dynamic range—in other words, in densely packed regions where color should be very intense.
Among the sources of divergence from nominal dot size are use of different media types, printing under differing environmental conditions, and variations in parameters of colorant quanta as defined earlier. Inkdrops for example vary in weight, volume, viscosity, cohesion, adhesion (including electronegativity and chemical affinities), pH, temperature, physical integrity (some drops break into pieces having various relationships), shape or shapes, dimensions, speed, and direction—as well as molecular weight, shape and dimensions—and all of these can affect resulting dot shape or size.
If dots are smaller than nominal, white spaces may appear in the areas where maximum color intensities should be, thus reducing such intensity through poor coverage. On the other hand, if dots are larger than nominal, the associated excess colorant leads to oversaturation of the printing medium with colorant or vehicle (particularly for colorant that is a liquid ink), which commonly creates undesirable image-quality defects.
Therefore dots cannot simply be made too small (or too large) so as to be on the safe side. It is important that dot size be controlled as well as practical or cost-effective.
Pursuing that objective, system designers often select the nominal dot size in such a way that variations about that nominal value minimize loss of image quality. Usually neighboring dots slightly overlap, to provide some allowance for error in case of slight misplacement or the chance of producing smaller dots than expected due to the variables that affect dot size.
In the end, dot-size selection is limited by the fact that positive variations produce oversaturation of colorant, again particularly for liquid ink. With that limitation, the native resolution of the system determines the dot size and in consequence the visibility of the individual dots—permitting only relatively little control over texture quality in highlight regions, as noted above.
(c) Nonlinearity—The foregoing discussion relates to the difficulties of obtaining nominal dot size and therefore stable dynamic range. Even with correct dot size, however, in single-bit incremental printing several factors make linear response
211
,
212
(
FIG. 1
a
) of color intensity very difficult to obtain.
From the
FIG. 1
a
graph, some of these factors can be appreciated based on geometrical relationships between a pixel grid
201
(
FIG. 2
a
) and a correctly sized dot
202
. For tutorial purposes, two simplifying assumptions have been adopted in the graphs.
The first is that the distribution of dots in some analyzed uniform-intensity region of the image—i. e., for some moderately large portion of the pixel grid—at fifty-percent input intensity (0.5 on the abscissa) is a simple alternating-pixels checkerboard pattern. With such a dot distribution, if the dots are all correctly positioned there is no overlap of dots anywhere in the image region.
The second assumption is that, at all lower input intensities (zero through 0.5 on the abscissa), too, the dots are distributed with no overlap in the image region. As a result the perceptual response is simply the fraction of that image region which is covered with colorant.
The lower quadrant of the graph shows an idealized but nearly realizable response (i. e. perceived intensity) to an idealized progression of input-data intensities. The curve corresponds to placing drops of diameter d in a square pixel grid of pixel-size s (
FIG. 2
a
), such that the diameter d just fits across the diagonal of the pixel.
Because the pixel square is exactly inscribed within the dot circle, d=s·′2, and the dots do not overlap along diagonals. If there are i such drops within a grid region of n squares, then the fractional area covered by the drops is:
fractional
⁢
 
⁢
coverage
=
π
⁢
 
⁢
r
2
s
2
·
(
i
n
)
=
π
4
·
(
d
s
)
2
·
(
i
n
)
=
k
⁢
(
i
n
)
,
k being a constant. Thus the printer response is linear. Inserting the value d
2
=2s, the coverage fraction becomes
π
4
·
(
2
)
·
(
i
n
)
=
π
⁢
 
⁢
i
2
⁢
n
.
This applies from i=0 (zero input intensity) up to the point where i=n/2 (fifty-percent input intensity). There the expression reduces to &pgr;/4≈0.785, thus establishing the rectilinear portion
211
of the curve.
If there were no sources of nonlinearity for higher values, the curve would continue along the dashed, angled straight line
212
to &pgr;/2≈1.57. In that upper half of the input-intensity range, however, a progressively greater fraction of the space in the image region is occupied by overlapping chordal areas of the circular dots.
The dashed straight line
212
represents a fictitious case in which the colorant deposited in an overlapping area has just as much coloring power as if it were printed in two separate nonoverlapping spaces. In actuality the first-deposited colorant in those overlapping areas is muted by the overprinted additional colorant, so that its effect is substantially lessened.
On the other hand, if it had no effect at all—in other words, if the incremental effect of overprinted additional colorant were zero—then the only impact of adding the second half of the input intensity would be to fill in the previously unprinted, p

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