Converting world informations (PGW file - pixels) to geographical coordinates?

Question

I'm trying to load an image on a Leaflet map and to display it at the right place using the world information exported by QGIS.

For image overlays, Leaflet needs the coordinates of the upper left point and of the lower right point (boundaries):

var imageUrl = 'http://www.lib.utexas.edu/maps/historical/newark_nj_1922.jpg',
imageBounds = [[40.712216, -74.22655], [40.773941, -74.12544]]; L.imageOverlay(imageUrl, imageBounds).addTo(map);

In QGIS, when I export my image, I always use the 1:25000 scale and 72 DPI, which is a constant in my workflow.

It outputs a world file (PGW) that contains the information:

8.86711729729699094 //xscale
0 //yskew
0 //xskew
-8.86711729729699094 //yscale
491355.98836270271567628 //xpos
6600748.93414135184139013 //ypos

Knowing all that information, is there a way to convert the world file information into geographical coordinates I could use in Leaflet?

Answer 1

I found out ! No need to check DPIs or map scale, all required informations are contained in the world file. Here, I'm parsing a PGW file sent through a form. I'm also using the proj4php library to handle projections.

public function get_overlay_bounds_from_pgw($pgwfile,$imagewidth,$imageheight){

    $filename = isset($pgwfile['name']) ? $pgwfile['name'] : null;
    if ( !$filename ) return new WP_Error('tdp_empty_pgw_file','Missing PGW file');

    $arr_file_type = wp_check_filetype( basename( $filename ) );
    if ( !$arr_file_type || ($arr_file_type['ext'] != 'pgw') ) return new WP_Error('tdp_invalid_pgw_file','Invalid PGW file');

    $content = file_get_contents($pgwfile['tmp_name']);
    $lines = explode(PHP_EOL, $content);
    if ( count($lines) < 6 ) return new WP_Error('tdp_invalid_pgw_file','Invalid PGW file');

    //https://en.wikipedia.org/wiki/World_file
    $world = array(
        'xscale'=>  $lines[0],
        'yskew'=>   $lines[1],
        'xskew'=>   $lines[2],
        'yscale'=>  $lines[3],
        'xpos'=>    $lines[4],
        'ypos'=>    $lines[5],
    );

    // Initialise Proj4
    $proj4 = new Proj4php();
    // Create two different projections.
    $projQgis    = new Proj('EPSG:3857', $proj4);
    $projLeaflet  = new Proj('EPSG:4326', $proj4);

    //
    $topleft = new Point($world['xpos'],$world['ypos'], $projQgis);
    $topleft = $proj4->transform($projLeaflet, $topleft);

    $xmeters = $imagewidth * $world['xscale'];
    $ymeters = $imageheight * $world['yscale'];

    $bottomright = new Point($world['xpos'] + $xmeters,$world['ypos'] + $ymeters, $projQgis);
    $bottomright = $proj4->transform($projLeaflet, $bottomright);

    $bounds = array(
        'north'=>   $topleft->x,
        'west'=>    $topleft->y,
        'south'=>   $bottomright->x,
        'east'=>    $bottomright->y,
    );

    return $bounds;
}