In order to get these geometries on a computer screen map we must be able to somehow retreive the single points building these geometries and transform them from coordinates to pixels, and then use some graphics functions to draw lines and polygons. The following steps were used to achieve this task.

1. Define a minimal bounding rectangles (MBR) representing the boundaries of the map we will display on the screen and compare this geometry with the lines and polygons in the database and see if they should be selected. If this box is expressed in coordinate space we will be able to take advantage of functions comparing the spatial relationship between two geometries in MySQL. The MBR of the following Linestring reaching from one corner to the opposite, represents the map given its width, height and scale.

`SET @g = GeomFromText('LineString($longitude1 $latitude1,$longitude2 $latitude2)')`

2. The geometries holding the coordinates for rivers or territories has a spatial column of type LINESTRING or POLYGON and has been given the name SHAPE2 on all database-tables of this kind (layers). We will Select values from this column that

*Intersects*the MBR defining the boundaries of the map (@g). We will then retreive the data as a binary string (BLOB). The Well-Known Binary (WKB) representation for geometric values is defined by the OpenGIS specification. WKB uses one-byte unsigned integers, four-byte unsigned integers, and eight-byte double-precision numbers. For fast access of the data, we use the Unbuffered Query in MySQL.

`$query="SELECT AsBinary(SHAPE2), color FROM $layer WHERE MBRIntersects(SHAPE2,@g)";`

...

$result=mysql_unbuffered_query($query);

3. In the code snippet above, we are retreiving two columns of each row in the database table. Unpack is a PHP-function that unpacks data from a binary string into an associative array according to a given format, in this case defined by the WKB. The first parameter of the geometry is the byte order (1 byte), the second, geometry type (4 bytes), and the third, number of elements (4 bytes). This sums to 9 bytes, and at this position (offset) in the binary string, we will find the number of elements of the given geometry type. The class-function

`drawLine`

deals with Linestrings and the function `drawRings`

deals with Polygons. We will not deal any further with Multipolygon and Point geometries.

while($row = mysql_fetch_row($result)) {

$color=$row[1];

$g = unpack("Corder/Ltype/Lnum",$row[0]);

$type = $g['type'];

$num = $g['num'];

$offset = 9;

switch($type) {

case 1: // POINT

break;

case 2: // LINESTRING

$this->drawLine($offset,$num,$color,$row[0]);

break;

case 3: // POLYGON

$this->drawRings($offset,$num,$color,$row[0]);

break;

case 6: // MULTIPOLYGON

break;

}

}

mysql_free_result ($result);

4. Now we are ready to transform the coordinates to pixels on the map. We will begin with the Linestring-geometry. Offset is the position in the binary string ($row) where we will start to read. $numpts is the number of points in the Linestring. From the offset position in the buffer we start to unpack into the points-array (

`pts`

) eight-byte double-precision numbers (d) until we reach the end of the buffer (*). Next we assign an array `geom`

, that will hold the resulting pixel-values of the points defining the Linestring. When reading the points-array, we know that the first number is the longitude and the second the latitude, on so forth. When reaching every second number we have a longitude/latitude pair that will be transformed using the gRef-function. The resulting pixel-values are assigned to the `geom`

-array. Then we use the PHP graphics-function `imageline`

to draw a line-segment between x1,y1 and x2, y2, in the given color.

function drawLine($offset,$numpts,$color,$row) {

$pts = unpack("@$offset/d*", $row);

$geom = array();

$lon=0;$lat=0;

$odd=true;

foreach ($pts as $value) {

$odd ? $lon=$value : $lat=$value;

if (!$odd) {

$this->gRef($lon, $lat);

$geom[]= $this->Xp();

$geom[]= $this->Yp();

}

$odd=!$odd;

} // end foreach

$max=($numpts-1)*2;

for ($i=0;$i < $max; $i+=2) {

imageline ( $this->img, $geom[$i], $geom[$i+1],

$geom[$i+2], $geom[$i+3], $color);

}

} // end function

5. Now we deal with the Polygons. In it's core this is the same function as the one for Linestrings, because polygons are Linestrings with the same start- and end-point.

function drawRings($offset,$numrings,$color,$row){

$off=$offset;

$x=$numrings;

while($x > 0) {

$h=unpack("@$off/Lnumpts",$row);

$numpts=$h['numpts'];

$off+=4;

$nump=$numpts*2;

$pts = unpack("@$off/d$nump", $row);

$geom = array();

$lon=0;$lat=0;

$odd=true;

foreach ($pts as $value) {

$odd ? $lon=$value : $lat=$value;

if (!$odd) {

$this->gRef($lon, $lat);

$geom[]= $this->Xp();

$geom[]= $this->Yp();

}

$odd=!$odd;

} // end foreach

imagefilledpolygon ( $this->img, $geom, $numpts, $color);

// If we wish to draw a border

imagepolygon ( $this->img, $geom, $numpts,

$this->bordercolor);

// Increase byte pointer

$off+=($nump*8);

$x--;

} // end while x

return($off);

} // end function

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ReplyDeletethanks a lot.

ReplyDeleteBut how can we do this using raster maps