1993-01-26
1997-01-21
Jankus, Almis R.
G06T 1100
Patent
active
055966926
DESCRIPTION:
BRIEF SUMMARY
This invention relates to computer graphics and is particularly concerned with computer graphic systems in which lines may be drawn as in painting or sketching.
Conventional computer graphic systems comprise drafting means, such as an electronic tablet and stylus with which an artist may carry out drawing and painting movements, a computer system which receives signals from the drafting means representing the movements made by the artist, one or more frame stores for storing data representing the lines drawn or painted by the artist and output means for reading out the contents of the frame stores, for example for display on a VDU. Each frame store comprises a matrix of pixels with different pixels in the matrix corresponding to respective different positions on the tablet. Thus, placing the stylus at a particular position on the tablet designates a corresponding pixel in the frame store. In order to simulate a line produced by an artist's implement such as a particular form of paint brush or a pencil, it is known to process the patch of pixels surrounding the designated pixel by combining the data representing the colour and/or intensity of the line to be drawn with the data already in the frame store representing the background. This processing utilises a set of coefficients, of which there is a respective one for each different pixel in the patch, chosen so that the new data is combined with the old in a manner which simulates the chosen implement. Thus, as the stylus moves across the tablet, the "patch" moves across the frame store and each frame store pixel which falls beneath the patch is processed a number of times dependent upon the size of the patch and the distance moved per iteration. A problem with the above described process is that since pixel which falls beneath the patch is processed a number of times, substantial computing power is needed and the process is necessarily somewhat slow.
In one aspect, the invention alleviates this problem 20 by providing means to perform a first step in which data representing the line drawn is stored separately from data representing the background and means to perform a second step in which the stored data representing the line drawn in the background are combined.
Another problem with the prior art system described above arises due to the fact that the stylus position relative to the table is sensed at fixed time intervals. The distance between successive sensed positions of the stylus will therefore vary dependent upon the speed of movement of the stylus. The patch of pixels which is processed is, as explained, that surrounding the designated pixel in the frame store corresponding to the sensed brush position. The line which is drawn, therefore, is essentially made up of a series of overlapping patches of pixels or blobs the degree of overlap reduces as the speed of stylus movement is increased. Thus, for high stylus speeds, the line produced will be percieved as having wavey edges or appears to consist of discrete blobs of paint. To avoid this, the stylus must be moved relatively slowly.
With a view to solving this problem, a further aspect of the invention provides means to compute data representing a substantially continuous line drawn between spaced apart detected position of the drafting means and to store said computed data.
In the prior art system described above, the coefficients utilised for processing the pixels in the patch, have a constant relationship to each other after the artist has selected a particular implement. Inevitably, therefore, the line drawn does not truely represent strokes made with a real artists implement since variation arises along the stroke with a real implement, for example, due to the amount of paint remaining on the brush reducing as the stroke proceeds. One proposal for dealing with this problem has been made in the article entitled "Hairey Brushes" by Strassmann at pages 225 to 232 in the proceedings of SIGGRAPH '86, Vol. 20, No. 4, 1986. In this paper, Strassman proposes that the individual bristles of a brush shou
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patent: 5208904 (1993-05-01), Sasaki
Martin Philip D.
Shaw Simon G.
Woodhouse Kenneth A.
Accom, Incorporated
Jankus Almis R.
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