How It Works - Page 10
December 29, 2000
After including the include file common_db.inc, we
start by creating our blank image canvas and allocating 2 colors
to the image:
Header("Content-type: image/png");
$image = ImageCreate(501,501);
$white = ImageColorAllocate($image,255,255,255);
$black = ImageColorAllocate($image,0,0,0);
We then connect to the MySQL server on the local machine using
the db_connect() function that we defined back in
Chapter 11. We set up and execute a SQL query to return all of
the m_area fields:
$link_id = db_connect('mapping');
$query = "SELECT m_area FROM mall";
$mallResult = mysql_query($query, $link_id);
We then loop around through the rows that our query returned. The
important bits to do with graphics start now.
The first thing we have to do is to grab that comma-separated
list of x-y coordinates and turn it into something that we can
easily use. The simplest way to do that is to use the
explode() function to break up the list (or in this
case $mallRow[0]) on commas. We therefore define
$area as an array of all of our x and y coordinates:
while ($mallRow = mysql_fetch_array($mallResult)) {
$area = explode(",", $mallRow[0]);
for ($i=0; $i < count($area)-2; $i=$i+2) {
ImageLine($image,$area[$i],$area[$i+1],
$area[$i+2],$area[$i+3],$black);
}
ImageLine($image,$area[count($area)-2],
$area[count($area)-1], $area[0], $area[1], $black);
}
[Lines 4 and 5 above are one line. They were split for
formatting purposes.]
The for() statement that we now use may look a
little bit strange:
for ($i=0; $i < count($area)-2; $i=$i+2) {
ImageLine($image,$area[$i], $area[$i+1],
$area[$i+2], $area[$i+3],
$black);
}
[Lines 2 and 3 above are one line. They were split for
formatting purposes.]
What we're doing here is looping around from 0 while
$i is less than the count of $area
minus 2, each time incrementing $i by 2. The trick
to understanding this is that we have an array of alternating x
and y coordinates, so we have to use the coordinates in pairs in
order to make any sense of them. If $mallRow[0] was:
150,0,150,100,200,100,200,0
then $area would be an array of elements, so the
first element $area[0] would be 150, while the last
one $area[7] would be 0.
|
Each pair of elements represents a coordinate set, and our first
line should be drawn from the first set to the second. Likewise,
the second line will be from the second set to the third and the
third line from the third set to the fourth set. The last line is
drawn between the last set of coordinates and the first; this
completes the shape.
|
The first time we get into the loop, $i equals 0, so
looking at the code that draws the lines:
ImageLine($image, $area[$i], $area[$i+1],
$area[$i+2], $area[$i+3],
$black);
[Lines 1 and 2 above are one line. They were split for
formatting purposes.]
Our first coordinate set corresponds to the first two elements of
the array [$i] and [$i+1]. The second
set corresponds to [$i+2] and [$i+3].
The variable <$i then has 2 added to its value, and
the next pair of coordinates are linked.
The diagram below shows how the loop progresses in steps of 2.
The square bracketing on the right (around groups of four
elements from $area) show the coordinates that will
be used to draw the line in each loop. Remember that our loop is
set to continue while $i is less than
count($area) –2. Since there are 8 values in
the array, it will stop on reaching $i = 6:
|
This has only drawn 3 lines for us and the shape will need 4, so
once we have dropped out of the bottom of the loop we draw a line
from the last pair of elements to the first pair:
|
|
ImageLine($image, $area[count($area)-2],
$area[count($area)-1],
$area[0], $area[1], $black);
The last thing we need to do then is output the graphic and clean
up memory:
ImagePNG($image);
ImageDestroy($image);
mysql_free_result($mallResult);
?>
Adding Data - Page 9
Beginning PHP4
|
