The present invention is a method for rendering two-dimensional continuous-tone images on bi-level digital output devices. Increasingly sophisticated and powerful computer resources are able to manipulate a variety of generic data types. Text data and organized databases were the earliest of data types. Currently, graphics and image data are being created, transferred and manipulated in general purpose computer systems. These new forms of data pose new problems for the computer systems designer. To the user, displaying an image on any of a wide variety of devices must be as transparent as displaying ASCII text documents. Office video displays with differing gray level capacity, laser printers, and home dot-matrix printers, all of various resolutions and aspect ratios, must render a given image in a similar way. To achieve this transparency of display, each output device should have a dedicated pre-processor that transforms generic digital image data to a form tailored to the characteristics peculiar to that device.
The binary nature of a given output device, for example a laser printer or video display, creates particular problems in rendering continuous-tone images. Outside of photographic film and some thermal sensitive materials, there does not exist a practical method of producing true continuous-tone hard copy. Computer hard copy devices are almost exclusively binary in nature. An output device is binary if the lowest resolvable image portion (often called a pixel or bit) is either on or off, not some intermediate value in between. While the video displays associated with workstations and terminals are certainly capable of true continuous-tone representations, they are often implemented with frame buffers that provide high spatial resolution rather than full gray-scale capability.
Digital half-toning techniques, implemented in an output pre-processor system, comprise any algorithmic process which creates the illusion of continuous-tone image from careful arrangement of binary picture elements. Since most output devices are designed for display of binary dot-matrix text or graphics, digital half-toning provides the mechanism to display images on them as well.
An adequate digital half-toning technique must contend with two particular problems. Both problems stem from the interactions of local groups of displayed pixels in the final perception of the rendered image. First, peculiarities in the output device may allow neighboring pixel intensities to affect other pixels. For example, a printer may allow the bleeding of dye or toner from one localized pixel to another, altering the printed image. Second, the human eye itself tends to read groups of pixels together. This effect is often used to advantage to create the illusion of continuous-tone color in video displays. However, the effects of low-level processing by the human eye and brain can alter the perception of a theoretically accurate display of an image, leading to further undesired results.
A satisfactory method for rendering continuous-tone images onto any available binary display must take into account both display interactions and perceptual interactions in the processing of images. Such a method should allow a variety of different display system characteristics to be used interchangeably, without great difficulty, in order to process an image for a given display. In addition, the method should explicitly take into account the interactions of pixels on each other, both in the physical output of the display and in the perception by the human eye and brain. The method should also permit the processing of images for a given display system in a minimum amount of time, and without requiring inordinate amounts of computational power.