A simple perl::GD script to convert .hgt to a .png for visualization purposes.

The explicite draw each pixel method is inefficient, but used anyway to make
it utterly clear what the data order is in the original .hgt file.

src/Lib/HGT/draw_picture.pl

@@ -0,0 +1,62 @@
+#!/usr/bin/perl
+
+use GD;
+
+$output = "picture.png";
+
+# load data
+$width = <>; chomp($width);
+$height = <>; chomp($height);
+$min = 9999;
+$max = -9999;
+
+for ( $i = 0; $i < $height; $i++ ) {
+    for ( $j = 0; $j < $width; $j++ ) {
+        $d = <>; chomp($d);
+        if ( $d < -9999 ) {
+            $d = 0;
+        }
+        if ( $d < $min ) {
+            $min = $d;
+        }
+        if ( $d > $max ) {
+            $max = $d;
+        }
+        $data{ "$i:$j" } = $d;
+    }
+}
+print "min = $min  max = $max\n";
+
+$im = new GD::Image( $width, $height );
+($w, $h) = $im->getBounds();
+print "create $w x $h\n";
+
+for ( $i = 0; $i < 255; $i++ ) {
+    $color{$i} = $im->colorAllocate( $i, $i, $i );
+}
+$red = $im->colorAllocate( 255, 10, 10 );
+
+# draw image
+for ( $i = 0; $i < $height; $i++ ) {
+    for ( $j = 0; $j < $width; $j++ ) {
+        if ( $data{ "$i:$j" } > 0 ) {
+            $level = 55 + int(200 * ( $data{ "$i:$j" } - $min ) / ( $max - $min ));
+        } else {
+            $level = 0;
+        }
+        
+        $im->setPixel( $j, $i, $color{$level} );
+    }
+}
+
+$im->line( 0, 0, $width, 0, $red );
+$im->line( 0, 1, $width, 1, $red );
+$im->line( 0, 2, $width, 2, $red );
+
+# write out gif
+$data = $im->png;
+open( OUTPUT, ">$output" ) ||
+    die "cannot open $output\n";
+binmode OUTPUT;
+print OUTPUT $data;
+close OUTPUT;