COMPUTER METHOD AND AN APPARATUS FOR
GENERATING A DISPLAY PICTURE REPRESENTING A SET OF OB JECTS INCLUDING
A BRUSH ELEMENT 5
This is a continuation of application Ser. No. 384,226, filed Jul. 21, 1989, now abandoned, which was a continuation-in-part of application Ser. No. 197,077, filed May 20, 1988, now abandoned. 10
BACKGROUND OF THE INVENTION
The invention relates to a computer method for generating a display picture representing a set of object elements including a brush object element. U.S. Pat. 15 No. 4,631,690 to the same assignee, herein incorporated by reference, discloses a multiprocessor system for forming a color picture from object elements defined in a hierarchical data structure. The present invention is not restricted to a multilevel hierarchy. The present 20 invention may be applied to a two-color system (e.g. black and white) and to a one-processor implementation. The state of the art shows that even for a complicated pictorial situation the number of calculation steps necessary for color-assignment to a particular pixel 25 remains limited to a low average value. For the specific case of Bezier curves and shapes the calculation generally boils down to elementary shift and add operations that straightforwardly lead to a silicon implementation of the processing. The nature of the present approach is 30 point-driven versus the conventional object-driven approach now generally in use. The present invention envisages the use of brushes. For conceptualization thereof, a Bezier curve is defined as a primitive element that is expressed by a Bezier formula. A path is a concat- 35 enation of curves; a path may be closed or non-closed. A path may consist of a single curve. A shape is described by one or more closed paths. A certain inside versus outside definition specifies the points of the shape. 40
For better explanation of the description to follow, the following three frames of references are specified:
first there are continuous or analog application coordinates in which the objects to be displayed are conceptually modelled; these will not be considered hereinaf- 45 ter;
second there are discrete application coordinates, which are used for computer models of these objects and for digital calculations to generate results that are to be displayed; 50
finally, discrete device coordinates specify pixels in the picture field to be displayed according to a particular color specification.
shape. The brush has an origin or reference point that moves along the trajectory. The area to be colored need not be defined in the form of a Bezier shape. The present invention allows for easy modelling of the trajectory, for a wide variety of applicable brush shapes and yet for easy implementation.
b. according to a second field of use, the trajectory is formed by the outline of characters and the like, that may be though of as relatively small trajectories with much tiny detail. A problem then occurs in that notwithstanding changes in magnification, such as dezooming, the characters must remain readable as long as possible. One attractive solution is using the character perimeter as a trajectory for the brush. All pixels being covered by the running brush get the same color as the rest of the character. In itself, the shape of the brush may be arbitrary. Thus, the character is dilated somewhat; in this way problems are avoided that could occur because of undersampling of narrow colored shapes during a high degree of dezooming. In this case, the trajectory must be a closed shape. If the trajectory is not a closed shape, the system of field a. prevails, that also mitigates extensive dezooming. For example, connecting lines between two blocks in a block diagram would retain connectivity. For correct solution of the dezooming problem, the dimension of the brush in digital application coordinates would be chosen so that it would correspond to be equal or nearly equal to the pixel periodicity. A particular advantage of this application is the easy implementability of elementary and locally applicable algorithms for anti-aliasing; anti-aliasing by itself is known in picture processing.
c. a third field of use is the testing of spatial coherence, that is testing a window for color uniformity. This coherence information allows for various strategic measures, such as deciding on whether an icon, subpicture or other pictorial element when placed in this window would interfere with pre-existing information. In this case a brush is used that has a shape that corresponds to the window shape, but is inverted with respect to a predetermined window reference point. The new brush travels with its reference point along any relevant color edge in the picture and it is detected whether the brush ever touches the original reference point. If so, the window color is nonuniform. The course of action described is faster to execute than the reverse, wherein all window pixels would have to be interrogated in succession with respect to all Bezier paths.
Note that in all cases part of the trajectory may extend outside the viewing window. The display may be crt, liquid crystal, hard copy or otherwise. According to one aspect of the invention, the object is realized by a computer method for pixel-wise generating a display picture representing an object set of object elements, including a brush object element that is a Bezier shape to be moved with its guiding centre along a guiding
outside predicate for any display pixel and thereby, a 60 trajectory that is a non-rational Bezier path included in
color. The usefulness of such superposition may be one of the following.
a. according to a first field of use, the trajectory is the guiding line of the brush motion, whereas any pixel covered by the brush at any time gets an -insideBoolean as distinct from any pixel never covered by the brush. Here, the trajectory is a non-rational Bezier curve, and the inside of the brush is a Bezier
said object set, said method comprising the steps of: a. for each trajectory Bezier curve providing a trajectory set of trajectory control points in an associated coordinate-wise oriented trajectory curve bounding box and for each brush Bezier curve providing a brush set of brush control points in an associated brush curve bounding box, and providing a brush bounding box containing all points of the brush shape;