|Publication number||USRE43149 E1|
|Application number||US 12/031,151|
|Publication date||31 Jan 2012|
|Filing date||14 Feb 2008|
|Priority date||27 Mar 2001|
|Also published as||CA2442100A1, CA2442100C, EP1374557A2, US6999202, US20020159094, WO2002078320A2, WO2002078320A3|
|Publication number||031151, 12031151, US RE43149 E1, US RE43149E1, US-E1-RE43149, USRE43149 E1, USRE43149E1|
|Inventors||Daniel P. Bybell, Jay E. Thornton, Dana F. Schuh|
|Original Assignee||Senshin Capital, Llc|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (203), Non-Patent Citations (51), Referenced by (1), Classifications (7), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
1. Field of the Invention
The present invention relates to digital halftoning and, more particularly, to retaining high-frequency details and reducing visual artifacts in digitally-halftoned images.
2. Related Art
It is often desirable to reproduce a continuous-tone image, such as a black-and-white or color photograph, using an output device that is only capable of producing a discrete set of tones. For example, it has become increasingly desirable to reproduce continuous-tone images using bilevel output devices, such as laser printers and some types of thermal-transfer printers, that are only capable of printing black ink or toner on a page.
Discrete-tone output devices typically simulate continuous-tone images by producing a halftone of the continuous-tone image. Halftones use patterns of dots to simulate various tones. Such dot patterns tend to approximate the appearance of continuous tones when viewed by the human eye from a suitable distance.
With conventional halftoning techniques, there is typically a tradeoff between the spatial resolution (the number of pixels within a given area) and the intensity resolution (the number of tones that may be simulated) of the halftone. The higher the spatial resolution, the lower the intensity resolution, and vice versa. Furthermore, halftones may exhibit visual artifacts, such as Moiré patterns, that reduce the perceived quality of the halftone.
What is needed, therefore, are improved techniques for performing digital halftoning that preserve high-frequency details with less of an impact on intensity resolution than conventional halftoning techniques. What is also needed are techniques for reducing the appearance of visual artifacts in digital halftones.
In one aspect of the present invention, a method is provided for producing a halftone of a source image. The halftone includes halftone pixels. The halftone pixels are suitable for containing halftone dots. The method selects glyphs corresponding to intensities of regions (e.g., pixels) in the source image. The glyphs contain one or more halftone dots. The method locates halftone dots within the halftone pixels such that for at least one pair of halftone dots contained within a pair of halftone pixels sharing a common boundary, the halftone dots in the pair of halftone pixels extend in opposite directions from the common boundary.
Halftones produced by this method contain pairs of adjacent halftone dots that share a common pixel boundary. Such adjacent pairs of dots are referred to herein as dot pairs. In one embodiment of the present invention, each dot pair may be rendered by an output device as a single contiguous mark. For example, when rendered using a thermal printer, a dot pair may be rendered on an output medium as a single contiguous mark by printing one or more connected spots to form the dot pair. This technique results in fewer, larger dots being printed, thereby increasing the average dot perimeter-to-area ratio, which results in higher dot quality.
In a further embodiment of the present invention, the size of each dot in a dot pair is independently determined by the intensity of a unique region (e.g., a pixel) in the source image. This technique may result in greater retention of high-frequency detail than techniques that use a combination (e.g., average or sum) of multiple source image region intensities to determine the size of halftone dots.
As described above, halftone dots generated by the method described above abut halftone pixel boundaries. Such a method may advantageously be used to produce higher quality halftones than those produced by methods that use dots that are centered at pixel centers. By rendering each pair of adjacent dots as a single contiguous mark, larger dots are produced which are more robust and less susceptible to process variation that manifests itself as grain. This allows a greater number of tones to be simulated by the halftone, thereby resulting in higher-quality halftones.
The method described above may be used to produce halftones in which halftone dots are arranged according to various angles that reduce the presence of certain visual artifacts. For example, in one embodiment of the present invention, dots are arranged according to a 45-degree pattern, and according to a further embodiment of the present invention, spots are arranged according to a 38-degree pattern.
In one embodiment of the present invention, each halftone dot in the halftone corresponds to the intensity of a region (e.g., a single pixel) in the source image. In another embodiment of the present invention, a family of glyphs is used to generate dots in the halftone. Each of the glyphs corresponds to a range of intensities. For each region in the source image, the glyph corresponding to the intensity of the region is selected, and the dots in the glyph are generated in the halftone at coordinates corresponding to those of the source image region.
In yet a further embodiment of the present invention, the intensity of each source image region is used to select the glyph corresponding to the intensity. A dot within the glyph is selected based on the coordinates of the source image region, and the dot is generated in the halftone at the coordinates of the source image region.
Additional aspects and embodiments of the present invention will be described in more detail below.
Before describing various embodiments of the present invention, certain terms will be defined.
Source image. As used herein, the term “source image” refers to any continuous-tone or discrete-tone image for which a halftone is to be generated. For example, referring to
Halftone. As used herein, the term “halftone image”(or simply “halftone”) refers to a discrete-tone image that simulates the appearance of a continuous-tone or discrete-tone source image using fewer tones than the source image. In one embodiment, a halftone consists of a two-dimensional array of halftone cells, each of which consists of a two-dimensional array of halftone pixels. As described in more detail below, a halftone pixel may contain one or more halftone dots, and a halftone dot may consist of one or more spots.
Halftone cell. As used herein, the term “halftone cell” refers to a collection of halftone pixels, such as a two-dimensional array of halftone pixels. A halftone cell may, for example, contain a glyph or halftone pixels containing any other collection of halftone dots. A halftone cell is the smallest unit of a halftone that is capable of containing any glyph in the family of glyphs used in the halftone.
Glyph. As used herein, the term “glyph” refers to a pattern consisting of one or more halftone dots. A particular halftone typically uses a collection of glyphs, referred to herein as a “family” of glyphs, to simulate various tones. A glyph in a digital halftone is typically a two-dimensional array of halftone pixels containing halftone dots, and as used herein, the term glyph may refer to a single halftone dot.
Pixel. An abbreviation for “picture element,” a pixel is the smallest spatial unit of a digital image. A digital image is composed of a collection of pixels typically arranged in a rectangular array. Each pixel has a location, typically expressed in terms of x (column) and y (row) coordinates, and an intensity, which may represent any tone such as a color or a shade of gray. Pixels typically adjoin each other when rendered on various output media, although they may overlap or be spaced apart to various degrees when rendered. Various well-known techniques have been developed for representing the locations and tones of pixels.
As described herein, the source image 800 may be a digital image represented and stored as an array of pixels, referred to herein as “source image pixels” for ease of identification and explanation. Similarly, the halftone 804 may be a digital image consisting of an array of pixels, referred to herein as halftone pixels.
Halftone dot. A halftone dot, also referred to herein simply as a “dot,” consists of one or more spots. For example, a thermal printer typically renders a dot as a collection of vertically-adjacent spots, while other devices may render a halftone dot as a two-dimensional array of spots. The resulting halftone dot may be any shape, such as a rectangle, rounded rectangle, or circle. Halftone dots in bilevel halftones are typically rendered as solid shapes of a uniform tone. A single halftone may contain halftone dots of various shapes and sizes. Each halftone pixel in a digital halftone may contain one or more halftone dots.
Spot. A “physical spot,” as used herein, is a small shape, such as a rectangle or disk, that an output device has rendered at a particular point or within a particular area on an output medium. A physical spot is the smallest unit of output that an output device can generate. For example, a physical spot may be a spot of ink printed by a printer or a pixel displayed by a monitor. A physical spot may be any shape, such as a rectangle, rounded rectangle, or circle. Different output devices may render physical spots of different shapes and sizes, and a single output device may be capable of printing physical spots of varying sizes. For example, thermal-transfer printers typically pulse their heating elements to create physical spots. Each pulse of a heating element transfers a small amount of wax or ink to the output medium creating a small physical spot. A single heating element may be pulsed many times in succession to create many physical spots that together form a larger physical dot.
A “logical spot,” as used herein, is a digital representation of a physical spot. A logical spot may be represented as, for example, a single bit in a bitmap. A logical spot may be stored in, for example, a computer-readable memory such as a RAM or in a file on a disk. As used herein, the term “spot” refers to both physical spots and to logical spots.
Point. A “point” is a mathematical construct that specifies a location that may be addressed by an output device such as a printer, plotter, or monitor. With respect to images, a point is defined by two-dimensional coordinates. The spatial resolution of an output device indicates how many points the output device is capable of addressing in a given area. For example, a printer that has a resolution of 300 points per inch (usually expressed as 300 dots per inch or 300 dpi), is capable of addressing (drawing spots) at 300 discrete points within an inch. Although this measure of resolution is relevant to the perceived quality of the output device's output, the perceived quality is also a function of the size of the spots that the output device prints at each point.
Addressability. As used herein, the term “addressability” refers to the maximum number of individual (not necessarily distinguishable) spots per inch that an output device is capable of printing. The addressability of a particular output device may differ in the horizontal and vertical directions. Addressability in the x (horizontal) direction is equal to the reciprocal of the distance between the centers of spots at addresses (x, y) and (x+1, y); similarly, addressability in the y (vertical) direction is equal to the reciprocal of the distance between the centers of spots at addresses (x, y) and (x, y+1). The image quality that is achievable with a particular output device depends on both the device's addressability and the spot size of the device.
Resolution. As used herein, the term “resolution” refers to the number of distinguishable lines per inch that a device can create. Resolution is defined as the closest spacing at which adjacent black and white lines can be distinguished. For example, if 40 black lines interleaved with 40 white lines can be distinguished across one inch, the resolution is 80 lines per inch.
Render. As used herein, the term “rendering” refers to the process of producing output on an output medium using an output device. For example, “rendering” includes printing ink or toner on a printed page, displaying pixels on a computer monitor, and storing a bitmap in RAM or other storage.
Region. As used herein, a “region” of an image may refer to any area within the image. For example, a region in a digital source image may include an area containing a single pixel or a collection of pixels, such as a two-dimensional array of pixels.
A continuous-tone image may contain any colors (tones) within a continuous range of colors. A grayscale image is a special kind of continuous-tone image that may contain any shades of gray within a continuous range of gray tones ranging from black to white. Conventional color and black-and-white photographs are examples of continuous-tone images.
In contrast, discrete-tone images may contain only a limited number of tones selected from a discrete set of tones. Computer output devices, such as monitors, printers, and plotters, are only capable of rendering discrete-tone images. Such devices are therefore referred to as discrete-tone output devices or digital output devices. For example, an 8-bit grayscale image printed on an inkjet printer may contain at most 256 (28) different shades of gray. Similarly, a 16-bit color image displayed on a computer monitor may contain at most 65,535 (216) different colors.
The increased use of computers for capturing, storing, transmitting, and rendering images has increased the need for faithful reproduction of continuous-tone images using discrete-tone output devices. Although such devices are not capable of directly producing the full range of tones that may be present in continuous-tone images, various techniques have been developed for using discrete-tone images to simulate continuous-tone images. Such discrete-tone images approximate the appearance of continuous-tone images when viewed from a suitable distance.
For example, some discrete-tone output devices, such as line printers, pen plotters, electrostatic plotters, some thermal printers, and some laser printers, produce output by depositing black ink or toner on an output medium such as plain paper. Such devices are referred to as bilevel devices because they are only capable of producing images containing two tones: a first tone (such as black) produced by depositing the ink or toner and a second tone (such as white or gray) produced by the output medium. Various techniques have been developed to enable bilevel devices to simulate the appearance of additional tones and thereby to produce discrete-tone images that appear similar to continuous-tone images when viewed from a suitable distance.
Various techniques for rendering continuous-tone images on discrete-tone output devices, such as bilevel devices, take advantage of the spatial integration that the human eye performs. We will term the amount of light reflected by a particular region of an image as the region's intensity. Perceived intensity is referred to as lightness. For example, in a continuous-tone grayscale image, regions with higher intensities appear closer to white and regions with lower intensities appear closer to black. The opposite of lightness is blackness. When the human eye views a small region of an image from a sufficiently large viewing distance, the eye averages fine (high spatial frequency) detail within the small region and records only the overall intensity of the region. This process is referred to as spatial integration.
Spatial integration is exploited, for example, when black-and-white continuous-tone photographs are printed in newspapers, magazines, and books using a technique referred to as halftoning. The image of the photograph that is printed on the page is referred to as a halftoned image or simply as a halftone. The photograph is considered to be composed of a two-dimensional array of small rectangular regions. To print the halftone, black spots of ink are typically printed at regularly-spaced locations in the halftone corresponding to each rectangular region in the original photograph. The area of each ink spot in the halftone is proportional to the amount of blackness in the corresponding rectangular region in the original photograph. When viewed at a suitable distance, the individual ink spots produce the appearance of appropriate shades of gray, thereby simulating the appearance of the original photograph.
Printing the halftone may be accomplished by covering the source image (e.g., a photograph) with a fine cross-hatched screen during exposure at a suitable stage in the photographic process leading to the printing plate. The result is to divide the halftone into very small, regularly spaced spots whose size varies with the image density being reproduced. Full-color printing is achieved by using three or four printing plates, one for each of the primary colors being used. Because undesirable Moiré patterns can be produced by the interactions among the spots produced by these different screens, the screens are usually arranged at various angles to each other (referred to as screen angles). In black-and-white bilevel halftoning, the single screen is oriented at a 45-degree angle to reduce the visibility of the halftone pattern.
Digital output devices, such as bilevel printers and plotters, can also be used to render approximations of continuous-tone images using a process referred to as digital halftoning (also referred to as spatial dithering). Rather than approximating continuous tones using a continuum of variable-area dots used in traditional halftoning, conventional digital halftoning typically makes use of rectangular arrays of dots (each of which may be a shape such as a rectangle or a circle) referred to as glyphs. When viewed from a distance, each repeated glyph pattern appears to be a different color or shade of gray. Glyphs may therefore be combined to simulate continuous-tone images.
For example, referring to
In general, an n×n group of bilevel pixels can be used to simulate n2+1 intensity levels. Furthermore, there is typically an inverse relationship between the spatial resolution and intensity resolution of a halftone. For example, referring to
Various glyphs of various dimensions have been designed for use in digital halftoning, as is well known to those of ordinary skill in the art. See, for example, Ulichney, Robert (1987) Digital Halftoning, Cambridge, Mass.: The MIT Press. Note that the family of glyphs 102a–e shown in
A family of glyphs, such as the family of glyphs 102a–e shown in
As described earlier, the blackness of a pixel refers to the opposite of its intensity. For example, in an image with five gray levels, where each pixel may have an intensity, I, ranging from 0 to 4, the blackness of a pixel may also range from 0 to 4 and is equal to 4−I. When generating a glyph corresponding to a pixel in the source image, the blackness of the source image pixel is compared to the value of each element in the dither matrix D(2). If the blackness of the source image pixel is greater than the element value, a dot is generated in the glyph pixel corresponding to the element. Otherwise, no dot is generated. For example, consider a pixel having an intensity of 1. This is equal to a blackness of 3 (4−1). The blackness of 3 is greater than or equal to 0 (the value of the upper-left element in the dither matrix), so a dot is rendered in the upper-left pixel of the glyph. Making similar comparisons of the blackness to the remaining elements of the dither matrix results in the glyph 102d shown in
Digital halftoning may be performed by devices other than bilevel devices. For example, consider an output device that has two bits per pixel and is therefore capable of outputting pixels of four different intensity levels. If 2×2 glyphs are used, there is a total of four pixels per glyph, and each pixel may display three intensities other than black. This allows 13 (4×3+1) intensities to be simulated using the glyphs. Various techniques for performing halftoning using such output devices are described in Computer Graphics: Principles and Practice (2nd Ed.), James D. Foley et al, Addison-Wesley (1997), pp. 568–574.
In the examples described above, the source image has fewer pixels than the output medium, so that multiple halftone pixels may be used to simulate a single pixel from the source image. Various techniques have also been developed for simulating a continuous-tone source image on an output medium having the same number of pixels as the source image, as will be discussed below.
Various kinds of conventional printers exist for printing discrete-tone images on physical output media, such as paper. Such printers include, but are not limited to, dot-matrix printers, plotters (such as pen plotters, flatbed plotters, drum plotters, desktop plotters, and electrostatic plotters), laser printers, inkjet printers, thermal-transfer printers, and thermal sublimation dye transfer printers.
For example, thermal-transfer printers contain a linear array of heating elements spaced very close together (e.g., 84.7 microns) which typically transfer colored pigments in wax from a donor sheet to plain paper. The wax-coated donor and plain paper are drawn together over the strip of heating elements, which are selectively heated to cause the pigment transfer. For color printing, the wax on the donor roll may be pigmented into alternating cyan, magenta, yellow, and black strips, each of a length equal to the paper size.
Dye sublimation printers are similar to thermal-transfer printers, except that the heating and dye transfer process permits varying intensities each of cyan, magenta, and yellow to be transferred, creating high-quality full-color images with a typical resolution of 300 dots per inch (dpi) Although this process is slower than wax transfer, the quality of the resulting output is higher. Thermal-transfer printers, dye sublimation printers, and other printers that use thermal energy to deposit ink or wax on an output medium are referred to herein as thermal printers.
It is important to preserve both spatial resolution and intensity resolution to faithfully simulate the source image, and to do so without introducing any visually objectionable artifacts. As described above, however, with conventional digital halftoning techniques there is typically a tradeoff between spatial resolution and intensity resolution. Halftones having a higher spatial resolution are more capable of accurately displaying finer details from the source image, while halftones having higher intensity resolution are capable of more accurately reproducing the full range of tones in the source image.
In one aspect of the present invention, a method is provided for producing a halftone of a source image. The halftone includes halftone pixels. The halftone pixels are suitable for containing halftone dots. The method generates glyphs corresponding to intensities of regions (e.g., pixels) in the source image. The glyphs contain one or more halftone dots. The method positions halftone dots within the halftone pixels such that for at least one pair of halftone dots contained within two halftone pixels that are adjacent along a predetermined axis of the halftone (e.g., a horizontal or vertical axis), the halftone dots in the pair of halftone pixels extend in opposite directions from a boundary shared by the two halftone pixels. These pairs of adjacent halftone dots, referred to as dot pairs, may be rendered by an output device, such as a thermal printer, as a single contiguous mark. The method may be used to produce halftones containing dots at various angles, such as 45 or 38 degrees, to reduce the visibility of the pattern. In one embodiment, each halftone dot corresponds to one source image pixel. Additional features and advantages of various embodiments of the present invention will be described in more detail below.
As shown in
The smallest spot that may be printed by a thermal printer is approximately equal to the area of the face of each of the heating elements 302a–d. For example, referring to
It is possible to create larger spots by activating a particular heating element for a longer period of time. The activated heating element will continue to transfer black pigment to the output medium 304 as long as the heating element is activated, thereby creating a larger spot. For example, referring to
As described above, thermal printers may be used to print digital images. For example, referring to
One technique for printing a halftone of a digital image, such as the digital image 400 shown in
As shown in
Different-sized dots, such as the dots 414, 416, and 418 shown in
It should be appreciated that thermal printers are typically limited in the sizes of spots that they are capable of printing, and as a result such printers may not be capable of printing a unique-sized spot for each possible source image pixel blackness. For example, a particular thermal printer may only be capable of printing 64 different sizes of spot within a halftone pixel even though the source image to be printed may contain 256 different shades of gray. Therefore, the same size spot may be printed for source image pixels within a range of blackness.
The use of a rectangular pattern of dots, such as that shown in
For example, referring to
Further improvement may be made by combining dots together to produce a pattern of dots at 45-degree angles as shown in
In one embodiment of the present invention, a halftone of a source image is produced in which halftone dots extend either upward or downward from halftone pixel boundaries, rather than outward from halftone pixel centers. The direction of dot extension (either upward or downward) depends on the coordinates of the halftone pixel in which the dot is printed. For example, according to one embodiment of the present invention, dots in vertically adjacent halftone pixels extend in opposite directions from a pixel boundary shared by the adjacent halftone pixels. Pairs of dots that extend in opposite directions from a common halftone pixel boundary are referred to herein as dot pairs. In a further embodiment, each pair of dots vertically- or horizontally-adjacent halftone pixels extend in opposite directions. For example, if a halftone dot extends upward from the bottom boundary of a halftone pixel, then a halftone dot in a bordering halftone pixel extends downward from the top boundary of the bordering halftone pixel. As used herein, the term “bordering” refers to adjacent pixels that are either within the same row or the same column.
In one embodiment of the present invention, a dot pair is rendered as a single contiguous mark. For example, a dot pair may be rendered as a single contiguous mark by a single thermal print head element. The mark may consist of one or more connected spots. This process for generating a halftone of a source image is described in more detail below.
Each of the halftone pixels 612a–i contains a dot whose size is proportional to the blackness B (255−I) of the corresponding source image region. The dot extends either upward from the bottom boundary of the halftone pixel or downward from the top boundary of the halftone pixel, depending on the coordinates of the halftone pixel. For example, in
The result of this pattern of dot extension is to create pairs of vertically-adjacent dots that extend from (abut) a common halftone pixel boundary. For example, vertically-adjacent dots 614a and 614d extend from a common halftone pixel boundary 616 between halftone pixels 612a and 612d. Similarly, vertically-adjacent dots 614c and 614f extend from a common halftone pixel boundary 618 between halftone pixels 612c and 612f. These pairs of vertically-adjacent dots that are joined by a common halftone pixel boundary (i.e., dots 614a and 614d, 614e and 614h, and 614c and 614f) are referred to herein as dot pairs. In one embodiment of the present invention, a dot pair is rendered (e.g., printed) as a single contiguous mark using, for example, a single thermal print head element.
Each of the dots 614a–i shown in
Dots in halftone pixels 632a, 632c, 632e, 632g, and 632i extend upward from the bottom boundary of the halftone pixels, while dots in halftone pixels 632b, 632d, 632f, and 632h extend downward from the top boundary of the halftone pixels. Referring to
As a result, in this embodiment each dot in the halftone 630 extends in a direction that is opposite to the direction of extension of dots in horizontally- and vertically-adjacent halftone pixels. This results in vertically-adjacent pairs of dots which extend outward from a common halftone pixel boundary. For example, dot 634c and 634f extend outward from the common boundary 618 between halftone pixels 632c and 632f. Similarly, dots 634e and 634h extend outward from the common boundary 617 between halftone pixels 632e and 632h. Furthermore, the dots 634a–i in the halftone 630 are arranged in a 45-degree pattern, which has various advantages that are described in more detail below.
It should be appreciated that the vertically-adjacent dot pairs (such as the pair of dots 634c and 634f) may be rendered as individual marks using, e.g., a single heating element in a thermal-transfer printer. For example, because the dots 634c and 634f form an adjoining vertical dot pair, they may be printed as a single contiguous mark using, e.g., the heating element that is over the halftone pixels 632c and 632f while the output medium 304 passes under the heating element to form the shape made up of dots 634c and 634f.
It should be appreciated that although dot pairs (such as the dot pair consisting of dots 634c and 634f) may be printed as single marks, the size of each component dot of a dot pair is independently determined by the intensity of a corresponding region in the source image, such as a source image pixel. For example, the size of each of the dots 634c and 634f in the dot pair that they form is independently determined by the intensities of source image pixels 622c and 622f.
It should be appreciated that dot pairs need not be arranged in a 45-degree pattern, such as that shown in
A variable DirectionStart is used to keep track of the direction in which the first dot in the current row is to extend. The value of DirectionStart may either be UP, indicating that the current dot is to extend upward from the bottom of a halftone pixel boundary, or DOWN, indicating that the current dot is to extend downward from the top of the halftone pixel boundary. The process 700 initially assigns a value of UP to the variable DirectionStart (step 702), indicating that the first dot in the halftone is to extend upward from the bottom of the first halftone pixel boundary. This initial value is chosen arbitrarily, and may alternatively be a value of DOWN. It should be appreciated that the values UP and DOWN may correspond to any opposing directions and do not specify a particular orientation.
The process 700 enters into a loop over each row R in the source image (step 704). A variable D is used to keep track of the direction in which the current dot is to extend (either UP or DOWN). The variable D is assigned the current value of DirectionStart (step 706). The process 700 enters into an inner loop over each column C in the source image (step 708).
The process 700 is now ready to process the source image pixel at coordinates (C, R), where C is the current column and R is the current row. A variable BND is used to store an identifier of the boundary (either BOTTOM or TOP) of the current halftone pixel from which the current spot is to extend. If the value of D is equal to UP (step 710), a value of BOTTOM is assigned to BND (step 712). Otherwise, a value of TOP is assigned to BND (step 714).
The blackness of the source image pixel at coordinates (C, R) is stored in a variable B (step 716). The process 700 generates a dot: (1) in the halftone pixel H at coordinates (C, R), (2) having a size proportional to B, and (3) extending in direction D from boundary BND of halftone pixel H (step 718). For example, if the source image pixel at coordinates (0, 0) has a blackness of 128, then the process 700 generates a dot extending upward from the bottom of the halftone pixel at coordinates (0, 0) having a size that is proportional to 128.
To prepare for generating the next dot, the process 700 toggles the value of Direction (step 720). More specifically, if the value of Direction is UP, then Direction is assigned a value of DOWN. Similarly, if the value of direction is DOWN, then Direction is assigned a value of UP. This ensures that horizontally-adjacent dots extend in different directions. Steps 710–720 are repeated for the remaining columns in row R (step 722).
To prepare for generating the next row of dots, the process 700 toggles the value of DirectionStart (step 724). This ensures that vertically-adjacent dots extend in opposite directions from halftone pixel boundaries.
Steps 706–724 are repeated for the remaining rows in the source image (step 726).
It should be appreciated that the process shown in
The halftone generated by the process 700 may be rendered on an output medium either after the completion of the process 700 or as part of the process 700. As described in more detail below with respect to
In the examples described above with respect to
For example, in one embodiment of the present invention, glyphs are used to simulate a larger number of grayscale tones than can be simulated using individual dots. Referring to
It should be appreciated that the glyphs 900a–m are provided merely for purposes of illustration and do not constitute a limitation of the present invention. Furthermore, any family of glyphs may be used with various embodiments of the present invention.
To generate a halftone of a source image using the glyphs 900a–m, each region in the source image may be used to select one of the glyphs 900a–m. For example, consider a source image in which the blackness of each source image region ranges from 0–255. In one embodiment of the present invention, this range of blackness is subdivided into thirteen sub-ranges, and each of the glyphs 900a–m is assigned to one of the sub-ranges. To simulate a source image region, the glyph corresponding to the source image pixel's blackness is generated at corresponding coordinates in the halftone. Various other schemes may be used to simulate tones using these glyphs.
If the resolution of the output device is not sufficient to produce four dots for each source image pixel, various techniques may be used to generate halftones corresponding to the source image. For example, the source image may be downsampled, and the blackness of pixels in the downsampled source image may be used to select one of the glyphs 900a–m for use in the halftone, as described above. Although this technique allows source images to be printed on output devices that do not have a sufficiently high spatial resolution to print one glyph for each source image pixel, high-frequency detail is lost by downsampling the source image.
In another embodiment of the present invention, a method that avoids downsampling is used to produce a halftone from a source image using the glyphs 900a–m. For example, referring to
The variables Xs and Ys are assigned the coordinates of the pixel P in the source image (step 1106). For example, the coordinates of the pixel in the upper-left corner of the source image 1000 are (0, 0), while the coordinates of the pixel in the lower-right corner of the source image 1000 are (3, 3). The variables XA and YA are assigned the coordinates of the pixel P relative to the upper-left corner of the array AS (step 1108). For example, the pixel in the lower-right corner of the source image 1000 has (XA, YA) coordinates (1, 1) relative to the upper-left corner of the array 1002d, and the pixel in the lower-left corner of the source image 1000 has coordinates (0, 1) relative to the upper-left corner of the array 1002c.
The glyph G corresponding to the blackness of pixel P is selected (step 1110). The glyph G may be selected in any of a variety of ways. For example, a family of glyphs (such as the family of glyphs 900a–m) may be pre-generated and stored in a lookup table. The blackness of pixel P may be used as an index in the lookup table to select the glyph G that corresponds to pixel P's blackness. The lookup table may, for example, represent each glyph as either: (1) a bitmap suitable for output on the output device 810, or (2) a pattern of halftone dots that is rasterized (converted into a bitmap) at a later stage. Alternatively, the glyph G may be generated on the fly by step 1110. For example, if the patterns in the family of glyphs are defined by a dither matrix, step 1110 may use the dither matrix to generate the appropriate glyph G based on the blackness of pixel P. These and other techniques for selecting the glyph G will be apparent to those of ordinary skill in the art.
The dot (or dot pattern) D at coordinates (XA, YA) within glyph G is selected (step 1112). For example, if pixel P is in the lower-left hand corner of array As, then coordinates (XA, YA) are (0, 1). In such a case, the dot at coordinates (0, 1) of the glyph G (i.e., the dot in its lower-left hand corner) would be selected as dot D in step 1112.
The dot D is generated (e.g., stored or rendered) at coordinates (Xs, Ys) in the halftone (step 1114). It should be appreciated that various techniques described above with respect to the process 700 (
Steps 1106–1114 are repeated for the remaining pixels in the array As (step 1116). Steps 1104–1118 are repeated for the remaining pixel arrays in the source image 1000 (step 1118).
One advantage of using the process 1100 described above with respect to
Various embodiments of the present invention may be used in a variety of contexts. For example, referring to
The computer 816 optionally stores the source image 814 in a computer-readable memory, such as a Random Access Memory (RAM) or a hard disk. The computer 816 may also perform additional processing on the source image 814, such as reducing or enlarging the size of the source image 814, applying filters to the source image 814, or modifying the spatial or intensity resolution of the source image 814. After performing any such processing, the computer 816 transmits the results as a logical halftone 818 to an output device 820 such as a thermal-transfer printer 820 using a digital transport mechanism. As used herein, the term “logical halftone” refers to a halftone that is stored on a computer-readable medium.
It should be appreciated that the output device 820 may be any output device, such as a thermal printer, laser printer, inkjet printer, or multi-function device. The output device 820 renders a physical halftone 822 based on the logical halftone 818 using any of the various techniques described herein on an output medium such as paper or film. As used herein, the term “physical halftone” refers to a halftone that has been rendered on an output medium.
It should be appreciated that the system 810 shown in
It should be appreciated that the various features of embodiments of the present invention described above and described in more detail below provide numerous advantages.
For example, in various embodiments of the present invention, individual pixels in the source image 800 contribute to (i.e., control the size of) individual dots in the halftone. This correspondence between individual source image pixels and halftone dots enables the halftone 804 to retain high-frequency details from the source image 800 while simulating a large number of gray levels, thereby resulting in more faithful reproduction of the source image 800.
Another advantage of various embodiments of the present invention is that by combining dots together to form dot pairs, each such dot pair may be rendered in the halftone 804 using a single contiguous mark. This can reduce the number of dots that are output in the halftone, thereby increasing the perimeter-to-area ratio of the dots. Reduction of this parameter can result in higher-quality dots because dots produced by various output devices tend to exhibit poorer quality at the perimeter than within the perimeter.
Various embodiments of the present invention also reduce the presence of certain visual artifacts that may be introduced by digital halftoning. For example, arranging dots at 45-degree angles (as shown in
The present invention has been described above in terms of various embodiments. Various other embodiments, including but not limited to the following, are also within the scope of the claims.
As described above, the term intensity typically refers to the amount of light reflected from within a particular region of an image. As used herein, the term intensity also refers to a value that may be used to represent a tone for a region. For example, an intensity may be a value representing a shade of gray in a grayscale image or a color in a color image. As described above, an intensity may be represented as a scalar value. For example, an intensity may be represented as an 8-bit scalar value having a range of 0–255. An intensity may also be represented, for example, as a floating point value having a range of 0–1.
The descriptions of various embodiments refer to halftone dots that “extend” from a halftone pixel boundary. As used herein, a halftone dot “extends” from a halftone pixel boundary if an edge of the dot abuts the halftone pixel boundary. For example, referring again to
Portions of the description above refer to dots that are “proportional” in size to the blackness of source image regions. It should be appreciated that such proportionality includes, but is not limited to, strict numerical proportionality. For example, in one embodiment of the present invention, there are n dot sizes D1–Dn, and there are m levels of blackness B1–Bm. For any two dots having dot sizes DA and DB, corresponding to blacknesses BA and BB, respectively, if DB>DA, then BB>BA. Various other mappings between levels of blackness and dot size are also within the scope of the present invention. Furthermore, as used herein, the term “dot size” includes, but is not limited to, measures of size such as area and length.
In general, the techniques described above may be implemented, for example, in hardware, software, firmware, or any combination thereof. The techniques described above may be implemented in one or more computer programs executing on a programmable computer and/or printer including a processor, a storage medium readable by the processor (including, for example, volatile and non-volatile memory and/or storage elements), at least one input device, and at least one output device. Program code may be applied to data entered using the input device to perform the functions described herein and to generate output information. The output information may be applied to one or more output devices.
Printers suitable for use with various embodiments of the present invention typically include a print engine and a printer controller. The printer controller receives print data from a host computer and generates page information, such as a logical halftone to be printed based on the print data. The printer controller transmits the page information to the print engine to be printed. The print engine performs the physical printing of the image specified by the page information on the output medium.
Elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions.
Each computer program within the scope of the claims below may be implemented in any programming language, such as assembly language, machine language, a high-level procedural programming language, or an object-oriented programming language. The programming language may be a compiled or interpreted programming language.
Each computer program may be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a computer processor. Method steps of the invention may be performed by a computer processor executing a program tangibly embodied on a computer-readable medium to perform functions of the invention by operating on input and generating output.
It is to be understood that although the invention has been described above in terms of particular embodiments, the foregoing embodiments are provided as illustrative only, and do not limit or define the scope of the invention. Other embodiments are also within the scope of the present invention, which is defined by the scope of the claims below. Other embodiments that fall within the scope of the following claims includes include, but are not limited to, the following.
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|Citing Patent||Filing date||Publication date||Applicant||Title|
|US20110122213 *||1 Feb 2011||26 May 2011||Alain Bouchard||Technique for printing a color image|
|U.S. Classification||358/3.06, 358/3.13|
|International Classification||B41J2/52, H04N1/405, G06K15/00|
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