Rendering shaded areas with boundary-localized pseudo-random noise

An apparatus for rendering a visual representation of an image, the apparatus comprising:

• means for rendering a representation of the image as an array of calculated pixel values, wherein each calculated pixel value represents an intensity of a corresponding pixel of the image;
• random data value generating means, for generating a plurality of random data values;
• adding means, connected to the means for rendering and the random data value generating means, the adding means for adding the random data values to the calculated pixel values, and for thereby generating an array of dithered pixel values;
• pixel value truncation means, connected to receive the array of dithered pixel values, for truncating the dithered pixel values, and thereby generating an array of truncated pixel values;
• a bitmap memory, having address terminals, data terminals, and a plurality of memory locations for storing data values, the data terminals connected to receive the array of truncated pixel values, and to store the truncated pixel values in the plurality of memory locations as memory data signals;
• a two-dimensional raster scan display having horizontal and vertical refresh timing cycle timing, and an intensity input terminal;
• address generator means for generating address signals, the address signals connected to the address terminals of the bitmap memory, and such that the memory data signals are sequentially and repetitively read out of the memory locations in the bitmap memory, and such that the memory data signals are read in synchronizm with the horizontal and vertical refresh cycle timing of the raster scan display;
• and
• signal conversion means, connected to receive the memory data signals from the bitmap memory, for converting the memory data signals into an intensity signal, and the intensity signal being connected to the intensity input terminal of the display, thereby generated the visual representation of the image.

An apparatus for rendering a visual representation of an image, the apparatus comprising:

• a data processor, connected to provide a sequence of pixel data words at an output bus, the pixel data words each comprising M bits, the M bits of each pixel data word representing an intensity of a corresponding pixel of the image to be displayed;
• a linear feedback shift register having a plurality of stages, with a predetermined number, N, of the stages being connected to a shift register output bus, to provide a sequence of pseudo-random data words each having N bits, where N is less than M;
• an adder, connected to receive the M bits of the pixel data words from the processor bus, and to receive the N bits of the pseudo-random data words from the linear feedback shift register, the adder connected to add the N bits of each received pseudo-random data word to the N least significant bits of each correspondingly received pixel data word, the adder connected to provide a sequence of adder output data words;
• a truncator, connected to receive the sequence of adder output data words, and connected to select the M-N most significant bits of each adder output data word, and connected to provide a series of truncated data words;
• a bitmap memory, having address terminals, data terminals, and a plurality of memory locations for storing data values, the data terminals connected to receive the sequence of truncated data words, and to store the truncated data words in the plurality of memory locations as memory data signals;
• a two-dimensional raster scan display having horizontal and vertical refresh timing cycle timing, and an intensity input terminal;
• an address signal generator which generated sequential and repetitive address signals which are connected to the address terminals of the bitmap memory, such that the memory data signals are read in synchronism with the horizontal and vertical refresh cycle timing of the raster scan display;
• and
• a signal converter, connected to receive the memory data signals from the bitmap memory, and connected to provide converted memory data signals as an intensity signal, which is connected to the intensity input terminal of the display.

A method for displaying a visual representation of an image on a computer graphics workstation, the method comprising the steps of:

• using a host data processor to generate a graphic primitive instruction representing the image to be displayed;
• rendering a representation of the image as an array of pixel values, wherein each pixel value represents an intensity of a corresponding pixel of the image to be displayed;
• dithering the pixel values such that the probability of dithering a pixel value increases as the distance between the corresponding pixel in the image and an intensity boundary in the image decreases, thereby providing dithered pixels;
• storing the dithered pixel values in a bitmap memory;
• periodically scanning the bitmap memory to retrieve the dithered pixel values, thereby generating intensity input signals;
• and
• producing a visual representation of the image by connecting the intensity input signals to a display.