Documentation Source Text

<p>
To rotate a polygon by R radians around the point 0, 0:

<codeblock>
geopoly_xform($polygon, cos($R), sin($R), -sin($R), cos($R), 0, 0)
</codeblock>


<tcl>hd_fragment regpoly geopoly_regular</tcl>
<h2>The geopoly_regular(X,Y,R,N) Function</h2>

<p>
The geopoly_regular(X,Y,R,N) function returns a convex, simple, regular,
equilateral, equiangular polygon with N sides, centered at X,Y, and with
a circumradius of R.  Or, if R is negative or if N is less than 3, the
function returns NULL.  The N value is capped at 1000 so that the routine
will never render a polygon with more than 1000 sides even if the N value
is larger than 1000.

<p>
As an example, the following graphic:

<blockquote>
<svg width="600" height="300" style="border:1px solid black">
<polyline points="140.003,100 80.0019,134.644 80.0019,65.3565 140.003,100" style="fill:none;stroke:red;stroke-width:2"></polyline> <text x="100" y="106" alignment-baseline="central" text-anchor="middle">3</text>
<polyline points="240.003,100 200,140.003 159.997,100 200,59.9973 240.003,100" style="fill:none;stroke:orange;stroke-width:2"></polyline> <text x="200" y="106" alignment-baseline="central" text-anchor="middle">4</text>
<polyline points="340.003,100 312.358,138.042 267.637,123.511 267.637,76.4893 312.358,61.9583 340.003,100" style="fill:none;stroke:green;stroke-width:2"></polyline> <text x="300" y="106" alignment-baseline="central" text-anchor="middle">5</text>
<polyline points="440.003,100 419.998,134.644 380.002,134.644 359.997,100 380.002,65.3565 419.998,65.3565 440.003,100" style="fill:none;stroke:blue;stroke-width:2"></polyline> <text x="400" y="106" alignment-baseline="central" text-anchor="middle">6</text>
<polyline points="540.003,100 524.94,131.276 491.101,138.995 463.959,117.353 463.959,82.6471 491.101,61.005 524.94,68.7243 540.003,100" style="fill:none;stroke:purple;stroke-width:2"></polyline> <text x="500" y="106" alignment-baseline="central" text-anchor="middle">7</text>
<polyline points="140.003,200 128.286,228.286 100,240.003 71.7143,228.286 59.9973,200 71.7143,171.714 100,159.997 128.286,171.714 140.003,200" style="fill:none;stroke:red;stroke-width:2"></polyline> <text x="100" y="206" alignment-baseline="central" text-anchor="middle">8</text>
<polyline points="240.003,200 232.363,223.511 212.358,238.042 187.642,238.042 167.637,223.511 159.997,200 167.637,176.489 187.642,161.958 212.358,161.958 232.363,176.489 240.003,200" style="fill:none;stroke:orange;stroke-width:2"></polyline> <text x="200" y="206" alignment-baseline="central" text-anchor="middle">10</text>
<polyline points="340.003,200 334.644,219.998 319.998,234.644 300,240.003 280.002,234.644 265.356,219.998 259.997,200 265.356,180.002 280.002,165.356 300,159.997 319.998,165.356 334.644,180.002 340.003,200" style="fill:none;stroke:green;stroke-width:2"></polyline> <text x="300" y="206" alignment-baseline="central" text-anchor="middle">12</text>
<polyline points="440.003,200 436.956,215.305 428.286,228.286 415.305,236.956 400,240.003 384.695,236.956 371.714,228.286 363.044,215.305 359.997,200 363.044,184.695 371.714,171.714 384.695,163.044 400,159.997 415.305,163.044 428.286,171.714 436.956,184.695 440.003,200" style="fill:none;stroke:blue;stroke-width:2"></polyline> <text x="400" y="206" alignment-baseline="central" text-anchor="middle">16</text>
<polyline points="540.003,200 538.042,212.358 532.363,223.511 523.511,232.363 512.358,238.042 500,240.003 487.642,238.042 476.489,232.363 467.637,223.511 461.958,212.358 459.997,200 461.958,187.642 467.637,176.489 476.489,167.637 487.642,161.958 500,159.997 512.358,161.958 523.511,167.637 532.363,176.489 538.042,187.642 540.003,200" style="fill:none;stroke:purple;stroke-width:2"></polyline> <text x="500" y="206" alignment-baseline="central" text-anchor="middle">20</text>
</svg>
</blockquote>

<p>Was generated by this script:

<codeblock>
SELECT '&lt;svg width="600" height="300">';
WITH t1(x,y,n,color) AS (VALUES
   (100,100,3,'red'),
   (200,100,4,'orange'),
   (300,100,5,'green'),
   (400,100,6,'blue'),
   (500,100,7,'purple'),
   (100,200,8,'red'),
   (200,200,10,'orange'),
   (300,200,12,'green'),
   (400,200,16,'blue'),
   (500,200,20,'purple')
)
SELECT
   geopoly_svg(geopoly_regular(x,y,40,n),
        printf('style="fill:none;stroke:%s;stroke-width:2"',color))
   || printf(' &lt;text x="%d" y="%d" alignment-baseline="central" text-anchor="middle">%d&lt;/text>',x,y+6,n)
  FROM t1;
SELECT '&lt;/svg>';
</codeblock>


<h1>Implementation Details</h1>

<p>The geopoly module is an extension to the [R-Tree extension].  Geopoly
uses the same underlying logic and shadow tables as the [R-Tree extension].
Geopoly merely presents a different interface, and provides some extra logic
to compute polygon decoding, overlap, and containment.