Digital Equipment Corporation

Todd R Comins

Willem A Engelse

A computer graphics technique for rendering shaded areas by using pseudo-random noise localized to image boundaries is disclosed. Calculated pixel values are initially generated using an arithmetic accuracy larger than the word size of an associated bitmap memory. The calculated pixel values thus have a most significant portion, corresponding to the portion which is to be stored in bitmap memory, and a least significant portion, corresponding to the additional accuracy. Pseudo-random noise, preferably generated by a linear feedback shift register, is then added to some or all of the least significant portion, and the carry output from this operation is added to the most significant portion. The most significant portion is then written into bitmap memory. Changes in the value of the integer portion thus occur with increased frequency as the distance to an intensity boundary decreases, since the random noise causes the dithering to occur with increased frequency. The result is a more natural display than would otherwise be available with a given bitmap memory word size.

Rendering shaded areas with boundary-localized pseudo-random noise

Method and apparatus for rendering a smooth-shaded graphics primitive vertical series of horizontal pixel spans which extend between beginning and trailing edges of the primitive, the pixel locations being spaced from each other by selected distances dX and dY. The apparatus produces X and Y addresses of the first and last pixels in the beginning span of the primitive and the number of spans in the primitive. Also produced are step values (dX/dY) indicating the slopes of the beginning and trailing edges, and a beginning intensity value for the first pixel in the beginning span. DX is added to the X address of each pixel in each span to determine the X address of the next pixel in that span and one of the step values is added to the X address of the first pixel in each span to calculate the X address of the first pixel in the next span of the primitive. In addition, the other of the step values is added to the X address of the last pixel in each span of the primitive to calculate the X address of the last pixel in the next span of the primitive. The distance dY is added to the Y address of each span to calculate the Y address of the next span until the number of spans has been rendered. One or more intensity values is cumulatively added to the beginning intensity value to calculate the intensity value for the next pixel.

Method and apparatus for displaying smooth-shaded objects

A graphics subsystem (10) employs an image memory (12) that includes an on-screen part (44) and an off-screen part (46). The locations of the on-screen memory (44) are continually scanned to provide data to be displayed on a display device (16). The off-screen part (46) stores data that represent image segments that have been occluded by windowing. To transfer data between the on-screen part (44) and the off-screen part (46), an address generator (24) generates a sequence of two-dimensional addresses representing the two-dimensional positions of the pixels whose data are to be transferred. These addresses are converted to one-dimensional addresses by a simple circuit consisting of multiplexers (26 and 28), adders (30 and 32) and a multiplier (34).

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North Chelmsford, MA, United States of America

Todd R Comins