MySQL implementation of ray-casting Algorithm? - mysql

Question

We need to figure out a quick and fairly accurate method for point-in-polygon for lat/long values and polygons over google maps. After some research - came across some posts about mysql geometric extensions, and did implement that too -
SELECT id, Contains( PolyFromText( 'POLYGON(".$polygonpath.")' ) , PointFromText( concat( \"POINT(\", latitude, \" \", longitude, \")\" ) ) ) AS
CONTAINS
FROM tbl_points
That did not however work with polygons made up of a large number of points :(
After some more research - came across a standard algorithm called the Ray-casting algorithm but before trying developing a query for that in MySQL, wanted to take my chances if someone had already been through that or came across a useful link which shows how to implement the algorithm in MySQL / SQL-server.
So, cutting it short - question is:
Can anyone please provide the MySQL/SQL-server implementation of Ray casting algorithm?
Additional detail:
Polygons are either of concave, convex or complex.
Targeting quick execution over 100% accuracy.

Answer 1

The following function (MYSQL version of Raycasting algorithm) rocked my world :
CREATE FUNCTION myWithin(p POINT, poly POLYGON) RETURNS INT(1) DETERMINISTIC
BEGIN
DECLARE n INT DEFAULT 0;
DECLARE pX DECIMAL(9,6);
DECLARE pY DECIMAL(9,6);
DECLARE ls LINESTRING;
DECLARE poly1 POINT;
DECLARE poly1X DECIMAL(9,6);
DECLARE poly1Y DECIMAL(9,6);
DECLARE poly2 POINT;
DECLARE poly2X DECIMAL(9,6);
DECLARE poly2Y DECIMAL(9,6);
DECLARE i INT DEFAULT 0;
DECLARE result INT(1) DEFAULT 0;
SET pX = X(p);
SET pY = Y(p);
SET ls = ExteriorRing(poly);
SET poly2 = EndPoint(ls);
SET poly2X = X(poly2);
SET poly2Y = Y(poly2);
SET n = NumPoints(ls);
WHILE i<n DO
SET poly1 = PointN(ls, (i+1));
SET poly1X = X(poly1);
SET poly1Y = Y(poly1);
IF ( ( ( ( poly1X <= pX ) && ( pX < poly2X ) ) || ( ( poly2X <= pX ) && ( pX < poly1X ) ) ) && ( pY > ( poly2Y - poly1Y ) * ( pX - poly1X ) / ( poly2X - poly1X ) + poly1Y ) ) THEN
SET result = !result;
END IF;
SET poly2X = poly1X;
SET poly2Y = poly1Y;
SET i = i + 1;
END WHILE;
RETURN result;
End;
Add
DELIMITER ;;
before the function as required.
The usage for the function is:
SELECT myWithin(point, polygon) as result;
where
point = Point(lat,lng)
polygon = Polygon(lat1 lng1, lat2 lng2, lat3 lng3, .... latn lngn, lat1 lng1)
Please note that the polygon ought to be closed (normally it is closed if you're retrieving a standard kml or googlemap data but just make sure it is - note lat1 lng1 set is repeated at the end)
I did not have points and polygons in my database as geometric fields, so I had to do something like:
Select myWithin(PointFromText( concat( "POINT(", latitude, " ", longitude, ")" ) ),PolyFromText( 'POLYGON((lat1 lng1, ..... latn lngn, lat1 lng1))' ) ) as result
I hope this might help someone.

Answer 2

I would write a custom UDF that implements the ray-casting algorithm in C or Delphi or whatever high level tool you use:
Links for writing a UDF
Here's sourcecode for a MySQL gis implementation that looks up point on a sphere (use it as a template to see how to interact with the spatial datatypes in MySQL).
http://www.lenzg.net/archives/220-New-UDF-for-MySQL-5.1-provides-GIS-functions-distance_sphere-and-distance_spheroid.html
From the MySQL manual:
http://dev.mysql.com/doc/refman/5.0/en/adding-functions.html
UDF tutorial for MS Visual C++
http://rpbouman.blogspot.com/2007/09/creating-mysql-udfs-with-microsoft.html
UDF tutorial in Delphi:
Creating a UDF for MySQL in Delphi
Source-code regarding the ray-casting algorithm
Pseudo-code: http://rosettacode.org/wiki/Ray-casting_algorithm
Article in drDobbs (note the link to code at the top of the article): http://drdobbs.com/cpp/184409586
Delphi (actually FreePascal): http://www.cabiatl.com/mricro/raycast/

Answer 3

Just in case, one MySQL function which accepts MULTIPOLYGON as an input: http://forums.mysql.com/read.php?23,286574,286574
DELIMITER $$
CREATE DEFINER=`root`#`localhost` FUNCTION `GISWithin`(pt POINT, mp MULTIPOLYGON) RETURNS int(1)
DETERMINISTIC
BEGIN
DECLARE str, xy TEXT;
DECLARE x, y, p1x, p1y, p2x, p2y, m, xinters DECIMAL(16, 13) DEFAULT 0;
DECLARE counter INT DEFAULT 0;
DECLARE p, pb, pe INT DEFAULT 0;
SELECT MBRWithin(pt, mp) INTO p;
IF p != 1 OR ISNULL(p) THEN
RETURN p;
END IF;
SELECT X(pt), Y(pt), ASTEXT(mp) INTO x, y, str;
SET str = REPLACE(str, 'POLYGON((','');
SET str = REPLACE(str, '))', '');
SET str = CONCAT(str, ',');
SET pb = 1;
SET pe = LOCATE(',', str);
SET xy = SUBSTRING(str, pb, pe - pb);
SET p = INSTR(xy, ' ');
SET p1x = SUBSTRING(xy, 1, p - 1);
SET p1y = SUBSTRING(xy, p + 1);
SET str = CONCAT(str, xy, ',');
WHILE pe > 0 DO
SET xy = SUBSTRING(str, pb, pe - pb);
SET p = INSTR(xy, ' ');
SET p2x = SUBSTRING(xy, 1, p - 1);
SET p2y = SUBSTRING(xy, p + 1);
IF p1y < p2y THEN SET m = p1y; ELSE SET m = p2y; END IF;
IF y > m THEN
IF p1y > p2y THEN SET m = p1y; ELSE SET m = p2y; END IF;
IF y <= m THEN
IF p1x > p2x THEN SET m = p1x; ELSE SET m = p2x; END IF;
IF x <= m THEN
IF p1y != p2y THEN
SET xinters = (y - p1y) * (p2x - p1x) / (p2y - p1y) + p1x;
END IF;
IF p1x = p2x OR x <= xinters THEN
SET counter = counter + 1;
END IF;
END IF;
END IF;
END IF;
SET p1x = p2x;
SET p1y = p2y;
SET pb = pe + 1;
SET pe = LOCATE(',', str, pb);
END WHILE;
RETURN counter % 2;
END

Answer 4

In reply to zarun function for finding lat/long within polygon.
I had a property table having lat/long information. So I had to get the records whose lat/long lies within polygon lats/longs (which I got from Google API). At first I was dumb how to use the Zarun function. So here is the solution query for it.
Table: properties
Fields: id, latitude, longitude, beds etc...
Query:
SELECT id
FROM properties
WHERE myWithin(
PointFromText(concat( "POINT(", latitude, " ", longitude, ")")),
PolyFromText('POLYGON((37.628134 -77.458334,37.629867 -77.449021,37.62324 -77.445416,37.622424 -77.457819,37.628134 -77.458334))' )
) = 1 limit 0,50;
Hope it will save time for dumbs like me ;)

Answer 5

I wanted to use the above mywithin stored procedure on a table of polygons so here are the commands to do just that.
After importing a shapefile containing polygons into mysql using ogr2ogr as follows
ogr2ogr -f "mysql" MYSQL:"mydbname,host=localhost,user=root,password=mypassword,port=3306" -nln "mytablename" -a_srs "EPSG:4326" /path/to/shapefile.shp
you can then use MBRwithin to prefilter your table and mywithin to finish as follows
DROP TEMPORARY TABLE IF EXISTS POSSIBLE_POLYS;
CREATE TEMPORARY TABLE POSSIBLE_POLYS(OGR_FID INT,SHAPE POLYGON);
INSERT INTO POSSIBLE_POLYS (OGR_FID, SHAPE) SELECT mytablename.OGR_FID,mytablename.SHAPE FROM mytablename WHERE MBRwithin(#testpoint,mytablename.SHAPE);
DROP TEMPORARY TABLE IF EXISTS DEFINITE_POLY;
CREATE TEMPORARY TABLE DEFINITE_POLY(OGR_FID INT,SHAPE POLYGON);
INSERT INTO DEFINITE_POLY (OGR_FID, SHAPE) SELECT POSSIBLE_POLYS.OGR_FID,POSSIBLE_POLYS.SHAPE FROM POSSIBLE_POLYS WHERE mywithin(#testpoint,POSSIBLE_POLYS.SHAPE);
where #testpoint is created, for example, from
SET #longitude=120;
SET #latitude=-30;
SET #testpoint =(PointFromText( concat( "POINT(", #longitude, " ", #latitude, ")" ) ));

Answer 6

It is now a Spatial Extension as of MySQL5.6.1 and above. See function_st-contains at Docs.

Answer 7

Here is a version that works with MULTIPOLYGONS (an adaptation of Zarun's one which only works for POLYGONS).
CREATE FUNCTION GISWithin(p POINT, multipoly MULTIPOLYGON) RETURNS INT(1) DETERMINISTIC
BEGIN
DECLARE n,i,m,x INT DEFAULT 0;
DECLARE pX,pY,poly1X,poly1Y,poly2X,poly2Y DECIMAL(13,10);
DECLARE ls LINESTRING;
DECLARE poly MULTIPOLYGON;
DECLARE poly1,poly2 POINT;
DECLARE result INT(1) DEFAULT 0;
SET pX = X(p);
SET pY = Y(p);
SET m = NumGeometries(multipoly);
WHILE x<m DO
SET poly = GeometryN(multipoly,x);
SET ls = ExteriorRing(poly);
SET poly2 = EndPoint(ls);
SET poly2X = X(poly2);
SET poly2Y = Y(poly2);
SET n = NumPoints(ls);
WHILE i<n DO
SET poly1 = PointN(ls, (i+1));
SET poly1X = X(poly1);
SET poly1Y = Y(poly1);
IF ( ( ( ( poly1X <= pX ) && ( pX < poly2X ) ) || ( ( poly2X <= pX ) && ( pX < poly1X ) ) ) && ( pY > ( poly2Y - poly1Y ) * ( pX - poly1X ) / ( poly2X - poly1X ) + poly1Y ) ) THEN
SET result = !result;
END IF;
SET poly2X = poly1X;
SET poly2Y = poly1Y;
SET i = i + 1;
END WHILE;
SET x = x + 1;
END WHILE;
RETURN result;
End;