Graphic arts applications frequently require the accurate reproduction of a high resolution image, i.e., artwork, such as a black and white or a color photograph. A typical application might involve printing a high resolution color image on a page of a periodical such as a magazine or on a postcard.
Color images can be generated either photographically, i.e., on film, or electronically, i.e., on some electronic media. When generated, these images are recorded on a continuous tone ("contone") basis. A continuous tone is defined as the variation in the ability of a material to absorb light within a photographic or printed image which corresponds to the graduated range of lightness or darkness in the original copy or scene. To represent a continuous tone, the color existing at any point in the image is recorded by an amplitude value, either exposure in the case of film or a voltage level in the case of electronic media.
Color reproduction equipment takes advantage of the principle that any color can be separated into a specific linear combination of primary subtractive colors (yellow, magenta, cyan and black, hereinafter referred to collectively as "the primary colors") in which the amount of each primary color is set to a predetermined amount. In the case of printed reproductions of an image, the use of primary color printing obviates the need to use differently colored ink for each different color in the image. However, in some cases specialty inks are used. As such, each image is converted into a succession of color separations wherein each separation is either a negative or positive transparency with an altered tone reproducing characteristic that carries the color information for one of the primary colors and also any other color that is required to reproduce an image. The term "printing color" will be used to refer to the primary colors and any other colors that may be utilized to reproduce an image.
A separation is frequently made by photographing or electronically scanning an artwork through an appropriately colored filter. If a digital scanner is used, then each resulting continuous tone value is represented by a value, e.g., binary number, within a range of values that represents the relative intensity of a corresponding printing color. This relative intensity is commonly referred to as the "density" of the printing color. The density is actually a measurement of the light stopping ability of a tone area--less light will be stopped by a brighter tone whereas more light will be stopped by a darker tone. A separate set or file of corresponding continuous tone values are created for each separation for each of the printing colors.
Since printing presses are only capable of either applying or not applying a single amount of ink for a given location and cannot apply differential amounts of ink at a given location, the full range of continuous tone gradations are achieved by utilizing halftone dots. A halftone image is essentially a pulse width modulation representation of the amplitude modulation form of the continuous tone which is converted by the human eye into a desired color. The size of the dots which comprise the halftone image are varied to yield variations in the printing color densities. Thus, smooth tonal variations are generated in a reproduced image by smoothly changing dot sizes. A full color image is formed by overlaying single halftone reproductions, i.e., layers, for all of the printing colors where each layer is formed from a halftone dot separation that contains dots of appropriate sizes and in one of the printing colors. In the case where specialty inks are used to reproduce colors in addition to the primary colors, additional layers are formed for each additional color.
An image created using halftones is created where the range of tones consists of dots of varying area but of uniform density. Thus, with halftone techniques, it is possible to break the continuous tone into tiny dots of different sizes to create the illusion of continuous tone when seen at a distance. This illusion is achieved by confusing the human eye to perceive a continuous pattern. The human eye is receptive to light radiation throughout what has become known as the visible spectrum. Receptors within the human eye allow the brain to differentiate between wavelengths of radiated light to produce the sensation of color. The graphics arts industry has used this basic color theory in the development of color printing. Color is divided into three basic additive colors--red, green and blue--and the subtractive primary colors--yellow, magenta, cyan and black. Subtractive colors are those frequencies reflected from a surface or object.
The process of digitally transforming continuous tone values into halftone dots is called "screening". This process creates a pattern which confuses the human eye so that patterns of a reproduced image appear continuous when in reality they comprise small, varying sized dots.
As the size of the dots decrease, an increasing amount of detail can be encoded in a dot pattern which, in turn, increases the detail in the reproduced image. The higher the number of dots utilized for a reproduction, however, the more sensitive the image quality of the reproduction is to printing press variations. For these reasons, in graphics arts applications, resolutions ranging from 65 to 200 dots per inch are considered suitable to balance these conflicting image requirements. The number of halftone dots per inch in a halftone screen is referred to as the "screen ruling".
To generate a color halftone image, separate runs of yellow, magenta, cyan and black are performed on a printing press. When each color separation is screened, an undesirable low frequency beat pattern may appear as a repeating pattern called a Moire pattern when the layers are superimposed on a printed sheet. This pattern is discernable by the human eye because the human eye is capable of perceiving images quickly in the horizontal and the vertical direction, but has more difficulty with images that are angular. Thus, to minimize the effect of Moire patterns, each color separation for each printing color is screened using a halftone dot pattern which is physically oriented along a different "screen angle" on the printed sheet. The "screen angle" is an angle measured from a line vertical to the base of the screen to a line formed through the diagonally opposite corners of the dot pattern. The orientation of the halftone dot pattern at preselected screen angles shifts the low frequency beat patterns to a higher frequency which is more difficult for the human eye to detect thereby improving the image quality. Commonly used screen angles in graphic arts applications are 45 degrees for black, 75 degrees for magenta, 90 degrees for yellow and 105 degrees for cyan.
The angular displacement of screens to overcome the effects of Moire patterns finds its origins in the pre-electronics days. Screens made of glass were used to create the halftone dots that create halftone screens. The glass was ruled, i.e., straight lines were drawn, in the vertical direction on the glass sheet. An identical set of lines were then superimposed upon the glass sheet perpendicular to the vertical lines thereby creating what has become known as a "cross-lined screen". The halftones were created by laying the screen over an image and vignetting light through the screen thereby creating dots. Thus, the angular displacement of screens was developed because the concept of the cross-lined screen was carried over as the electronics were adapted to create halftone images.
The screening of halftone dot patterns for each of the primary colors is done at the foregoing screen angles. The color separations created through this screening process are then superimposed one upon the other to create a pattern for a color image. The screening of the halftone dot patterns for each one of the primary colors at the specified angles requires the dot patterns to be arranged with reference to a specific point to avoid Moire patterns. Therefore, the physical placement, i.e., registration, of the dot patterns is critical. If the screen angles are not accurate, Moire patterns will develop. Thus, angular registration must be precise.
Furthermore, high resolution screening, i.e., 133 or 150 dots/inch, is generally used for high image quality reproductions in graphic arts applications. The cost of a device which prints the actual reproduced image increases as the desired resolution increases. Moreover, the storage requirements placed upon an imaging system that produces high resolution images is greater than one that produces lower resolution images. This is because more samples of the image to be reproduced are required which, in turn, requires the use of a greater number of smaller halftone dots to increase the amount of detail that is encoded in a halftone dot pattern. The greater the number of dots utilized for a reproduction, however, the more storage space that is required to store the data utilized to create the halftone dots.
It is apparent from the foregoing that there are problems in the prior art in accurately reproducing high quality halftone color images. In particular, the sensitivity of the angular registration of the halftone dot patterns renders the associated electronics and mechanical features highly complex. In addition, more expensive, higher quality typesetters or printers capable of producing high resolution images are required when high quality halftone color images are desired.