Thursday, October 4, 2012


For purposes of printing, we need to fetch a giant image from a WMS. As in, 9 megapixels. Unfortunately, the WMS server isn't configured to allow requests of that size, and since we don't have an "in" with them we're not able to have them correct this.
Using tiles isn't an option, for reasons I won't get into here. Point is, we need a giant image from a WMS service that just won't give out images that large.


My solution: I wrote a proxy server in PHP, which takes our large request, splits it into 4 logical requests, fetches the 4 images, and then composites them and returns that finished image. Ta-da, an interface that accepts any arbitrary parameters which the source WMS supports, but which also provides for very-large requests.
function ohioimagery() {
    $BASE_URL = "";

    // parse the bbox into 4 numbers: NOTE THIS IS NOT THE USUAL SEQUENCE
    // this is S W N E, and I have no idea how they do that. Maybe that's WMS 1.3.0 ?
    list($miny,$minx,$maxy,$maxx) = explode(",",$_GET['BBOX']);
    //list($minx,$miny,$maxx,$maxy) = explode(",",$_GET['BBOX']); // the normal way

    // calculate the W/S/E/N for the 4 quadrants, e.g. a logical "nw" box bounded by $nw_w $nw_s $nw_e $nw_n
    $halfx = ($maxx + $minx) / 2.0;
    $halfy = ($maxy + $miny) / 2.0;
    $nw_w = $minx; $nw_s = $halfy; $nw_e = $halfx; $nw_n = $maxy;
    $ne_w = $halfx; $ne_s = $halfy; $ne_e = $maxx; $ne_n = $maxy;
    $sw_w = $minx; $sw_s = $miny; $sw_e = $halfx; $sw_n = $halfy;
    $se_w = $halfx; $se_s = $miny; $se_e = $maxx; $se_n = $halfy;

    // split the width and height in half, but use subtraction so we don't fall victim to rounding
    $nw_width = $sw_width = round($_GET['WIDTH'] / 2.0);
    $ne_width = $se_width = $_GET['WIDTH'] - $nw_width;
    $nw_height = $ne_height = round($_GET['HEIGHT'] / 2.0);
    $sw_height = $se_height = $_GET['HEIGHT'] - $nw_height;

    // now generate a new set of WMS params and then URLs for each of the 4 boxes
    $nw = $_GET; $nw['WIDTH'] = $nw_width; $nw['HEIGHT'] = $nw_height; $nw['BBOX'] = "$nw_s,$nw_w,$nw_n,$nw_e";
    $ne = $_GET; $ne['WIDTH'] = $ne_width; $ne['HEIGHT'] = $ne_height; $ne['BBOX'] = "$ne_s,$ne_w,$ne_n,$ne_e";
    $sw = $_GET; $sw['WIDTH'] = $sw_width; $sw['HEIGHT'] = $sw_height; $sw['BBOX'] = "$sw_s,$sw_w,$sw_n,$sw_e";
    $se = $_GET; $se['WIDTH'] = $se_width; $se['HEIGHT'] = $se_height; $se['BBOX'] = "$se_s,$se_w,$se_n,$se_e";
    $nw_url = $BASE_URL . '?' . http_build_query($nw);
    $ne_url = $BASE_URL . '?' . http_build_query($ne);
    $sw_url = $BASE_URL . '?' . http_build_query($sw);
    $se_url = $BASE_URL . '?' . http_build_query($se);
    header("Content-type: text/plain");
    print "ORIGINAL\n"; print_r($_GET); print "\n\n";
    print "NW\n"; print_r($nw); print "\n\n";
    print "NE\n"; print_r($ne); print "\n\n";
    print "SW\n"; print_r($sw); print "\n\n";
    print "SE\n"; print_r($se); print "\n\n";
    print "NW\n$nw_url\n";
    print "NE\n$ne_url\n";
    print "SW\n$sw_url\n";
    print "SE\n$se_url\n";

    // fetch the four images and composite them into a single canvas
    $canvas = imagecreatetruecolor($_GET['WIDTH'],$_GET['HEIGHT']);
    $nw_img = imagecreatefrompng($nw_url);
    $ne_img = imagecreatefrompng($ne_url);
    $sw_img = imagecreatefrompng($sw_url);
    $se_img = imagecreatefrompng($se_url);
    imagecopy($canvas, $nw_img, $sw_width-1, $ne_height-1, 0, 0, imagesx($nw_img), imagesy($nw_img) );
    imagecopy($canvas, $ne_img, $nw_width-1, 0, 0, 0, imagesx($ne_img), imagesy($ne_img) );
    imagecopy($canvas, $sw_img, 0, $nw_height-1, 0, 0, imagesx($sw_img), imagesy($sw_img) );
    imagecopy($canvas, $se_img, 0, 0, 0, 0, imagesx($se_img), imagesy($se_img) );

    // spit it back out
    header("Content-type: image/png");
    imagepng($canvas, null, 0);

The result, is that I can now make exactly the same WMS request, along with whatever changes in layers, styles, and SRS, and get back oversized images.



For reasons I do not know, the remote WMS takes their BBOX parameter in an unusual order. Instead of the usual W,S,E,N they do S,W,N,E The code above has been modified to fit their bizarre case, and I have noted it.

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