Abstract:
A method for digitally constructing variable width cartographic lines, the invention provides an algorithm for generating symbolized lines using multiply-stroked centerline data. The algorithm allows generation of software which utilizes trigonometric functions and which can be implemented in applications using vector data with vector or raster plotters. The algorithm further accounts for basic cartographic principles, existing algorithm characteristics and hardware constraints imposed by specific output devices.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to construction of lines in cartographic applications by software simulation and particularly to a method for digitally constructing minimum width and variable width lines via vector-to-raster conversion. 
     2. Description of the Prior Art 
     Generation of map images by automated equipment often requires that lines of differing widths be produced. Equipment commonly referred to as vector equipment provides this capability in an essentially mechanical fashion through the hardware employed and includes such options as pen selection on pen plotters or aperture selection on photoplotters. When utilizing graphic devices operable by binary raster output, these hardware options are not available, such equipment requiring the generation of line widths by software simulation. A primary difficulty encountered with such equipment in the specific field of cartographic symbolization is that lines generated by software simulation typically do not meet necessary standards. The deficiencies of the prior art become of even greater moment in view of recent developments in the use of output devices for vector data which involves the generation of lines in raster form using vector-to-raster conversion. Given such advances in the state of the art relative to the generation of cartographic lines in automated image generation, it has become imperative that the ability to control the quality of line widths generated by software simulation be improved. 
     The prior art as represented by issued United States patents does not provide such improvement with even those devices closest in function to the present problem being of little assistance in meeting cartographic requirements. In particular, Kemplin et al, in U.S. Pat. No. 4,135,245, describe an X-Y plotter having a programmable pen changer. However, the Kemplin et al device is not useful in the generation of variable width cartographic lines by software simulation. Rieger, in U.S. Pat. No. 4,283,765, teaches the use of a matrix multiplier for rotation, scaling, shearing, and stretching of three-dimensional caligraphic images with particular application to incremental vector generators. Andrews et al, in U.S. Pat. No. 4,259,725, disclose a system for displaying a rasters-scanned image on a screen wherein signals are also fed for causing a cursor having distinctive light intensity to be overlayed on the image. Andrews et al further disclose programs for defining multiple cursors which are stored in a readable memory. As a further example of the prior art, Hennessee, in U.S. Pat. No. 4,062,648, discloses a plotter system having an adaptive velocity device for improving plotted line quality by selectively varying the velocity of the plot head. However, the systems of the prior art do not provide the ability to construct variable-width cartographic lines through software simulation and which is of a quality adequate to the requirements of the generation of map images. 
     The present invention provides a technique for generating lines via vector-to-raster conversion wherein the lines have a minimum distinguishable width associated with cartographic production, the invention further allowing the generation of variable-width lines in addition to the generation of lines of minimum width. 
     SUMMARY OF THE INVENTION 
     The invention provides a digital technique for constructing variable-width cartographic lines and includes an algorithm developed to generate symbolized lines using multiply-stroked centerlines data. The invention particularly improves the quality of lines generated by software simulation to the level necessary to maintain the standards of cartographic symbolization. In the automated map image generation environment such as exists for example in the United States Army Engineer Topographic Laboratory, the requirement exists for simulating line widths on binary raster devices, this requirement being related to the development of devices such as the Large-Format Laser Platemaker. Use of devices such as the Large-Format Laser Platemaker as output devices for vector data involve the generation of lines in raster form using vector-to-raster conversion. Software used in this conversion process commonly creates a series of single pixels for each vector. Present vector data output devices are excessively fine when generating single pixel vectors, a line created from single pixels typically having a width of only 1.5 mils. Since lines of such widths are not distinguishable, the present technique particularly intends the generation of lines via vector-to-raster conversion which have a minimum distinguishable width associated with cartographic production, this width being on the order of 4 to 5 mils. The present invention particularly provides for the generation of lines of variable width in addition to the generation of lines of minimum width. 
     The present digital technique particularly improves upon prior binary raster devices such as electrostatic plotters and the software packages provided therewith which cause such binary raster devices to be compatible with basic pen plotter software. In such situations, the vector-to-raster conversion involved is commonly used by the applications programmer. Such software packages often contain additional routines which take advantage of the capablilities afforded by raster plotting including the generation of variable-width lines. Typically, the variable-width lines generated are derived from multiply-stroked centerline data. Subsequent vector-to-raster conversion utilizes the additional &#34;strokes&#34;  and generates a single pixel line for each stroke. Algorithms thus employed generate variable width lines by repeatedly plotting the centerline data parallel to itself at a distance equal to the step size between two adjacent pixels. Each stroke is thus the same length as the centerline which causes gaps to form at vertices of rectangular line segments intersecting at angles to each other. The algorithm utilized according to the present invention generates lines which more closely resemble those lines produced by traditional drafting techniques by variation of the length of the strokes based from the centerline. 
     Accordingly, it is a primary object of the present invention to provide a digital technique for constructing variable-width cartographic lines and which includes an algorithm developed for generating symbolized lines using multiply-stroked centerline data. 
     It is another object of the invention to provide a method for generating variable-width lines from multiply-stroked centerline data and wherein the lines more closely resemble lines produced by traditional drafting techniques such that intersecting lines are not gapped at the vertices. 
     It is a further object of the present invention to provide a digital technique for generating lines via vector-to-raster conversion which have a minimum distinguishable width and which permits generation of variable-width lines of a quality associated with cartographic production and particularly automated map image generation. 
     Further objects and advantages of the present invention will become more readily apparent in light of the following detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatical representation of a single line such as is generated according to the prior art; 
     FIG. 2 is a diagrammatical representation of the connection of the lines such as that shown in FIG. 1; 
     FIG. 3 is a diagram illustrating the connection of lines generated according to the present invention; and 
     FIG. 4 is a diagrammatical view illustrating the particular technique of the present invention for generating cartographic lines. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and particularly to FIGS. 1-3, a line is seen at 10 to have been generated by conventional equipment such as binary raster devices controlled by software. The line 10 is substantially rectangular in geometrical conformation and is typical of lines generated by the capabilities afforded by raster plotting such as occurs through vector-to-raster conversion processes and wherein lines are generated from multiply-stroked centerline data. The vector-to-raster conversion process utilizes what is known as additional &#34;strokes&#34; to generate a single pixel line for each stroke. Algorithms used in such situations thus repeatedly plot the centerline data parallel to itself at a distance equal to the step size between two adjacent pixels. Since each stroke is the same length as the centerline, the resultant symbolized line 10 has a rectangular appearance as noted in the drawing. Due to proximity, the multiple single-pixel raster lines give the appearance of one multiple-pixel raster line. However, angular connection of individual rectangular lines such as the lines 10 as seen in FIG. 2 results in gaps 12 formed at the vertices of the intersection between such lines. The present invention particularly provides an algorithm which allows the generation of lines which more closely resemble lines produced by traditional drafting techniques. Such traditionally-produced lines such as are formed by drafting tools including reservoir pens and scribers have rounded ends. The present multi-stroked algorithm generates variable width lines which have rounded ends by varying the length of each stroke. Intersecting lines formed of variable-width lines 14 as seen in FIG. 3 and which have rounded ends do not have gaps at the vertices and are therefore much preferred in graphic situations including cartographic usage. 
     Referring now to FIG. 4, the method by which the lines 14 are generated is shown. The length of each stroke 16 forming each line 14 is varied by calculating plot coordinates Px1, Py1 and Px2, Py2 which lie upon the circumference of semicircles 18 and 20 centered about centerline end points 22 and 24. To calculate the plot of the end point Px1, Py1, and Px2, Py2, it is necessary to know the location of the centerline end points 22 and 24 which are represented by x1, y1 and x2, y2 respectively, the width of a single stroke W and the number of strokes necessary to generate as represented by NTIM. Working at one end of the centerline, the plot coordinates Px1, Py1 are found by: 
     
         Px1=x1+dX and ps 
    
     
         Py1=y1+dY 
    
     The coordinate off-sets dX, dY are calculated by 
     
         dX=R* COSINE c and 
    
     
         dY=R* SINE c 
    
     where R is the radius of the &#34;thickening&#34; semicircle and c is the angle of the plot coordinates with respect to the centerline end coordinates. The radius R is found by 
     
         R=0.5*NTIM*W 
    
     where NTIM is the number of strokes and W is the stroke width. 
     The angle c is found by 
     
         c=(a-π)+b 
    
     where a is the absolute angle of the centerline and b is the relative angle of the plot coordinates Px1, Py1 measured from the centerline, extended. The angle a is found by 
     
         a=ARCTANGENT(dY&#39;/dX&#39;) 
    
     where 
     
         dX&#39;=x2-x1 and 
    
     
         dY&#39;=y2-y1 
    
     The relative angle b is found by 
     
         b=ARCSINE(S/R) 
    
     where S is the distance of the generated plot coordinate from the centerline and R is the radius. The value of S or the length of the side opposite the relative angle b is calculated as 
     
         S=0.5*((2*T-NTIM-1)*W) 
    
     where I is the incremented stroke count, NTIM is the number of strokes, and W is the stroke width. It should be noted that the calculation of S compensates for the difference between odd and even stroke counts, the generated strokes always exhibiting symmetry with respect to the vector centerline. 
     The above calculations take place at both ends of the centerline x1, y1, and x2, y2, and generate the plot coordinates Px1, Py1 and Px2, Py2 for each stroke 16. The total width of the multiply-stroked line 14 equals the width W multiplied by the number of strokes generated NTIM. The calculation can be changed such that the number of strokes NTIM equals the total width divided by the stroke width W, thereby permitting the generation of constant line widths regardless of available pen width. For aplications using raster output devices, the stroke width W is set to the step size between pixels. On devices such as the Laser Plate-maker, the step size is approximately 0.001 inch. For applications using vector output devices, the stroke width W is set to the actual &#34;pen&#34; width. The method is also useful in installations that have a limited selection of &#34;pen&#34; widths. 
     This algorithm thus developed is thus defined as follows: ##SPC1## 
     The invention thus provides the ability to produce lines of differing width as is commonly required in automated map image generation. In those particular situations wherein graphic devices include hardware unable to allow selection of line width, line widths must be generated by software simulation such as according to the present invention. High resolution devices particularly require the generation of lines through software simulation which have minimum associated with cartographic symbolization standards. On those devices where hardware line width selection is available but limited, a variable-width routine provides additional capability through the use of the present digital technique for constructing variable-width lines based on multiply-stroked centerline data. It is to be understood that the invention can be practiced other than as is explicitly described hereinabove, the scope of the invention being defined by the appended claims.