* File->New (select desired size and Advanced->Fill->Transparent)
* Select foreground color desired (black is the default)
* Select blend tool (“L”)
* Now, in the options at the bottom of the tool window:
* Click on “Gradient” widget to get popup
* Select “FG to transparent”
* Clock the arrow next to Gradient to reverse direction
* Select “Radial” under “Shape”.
* Now, in the image window:
* Click and hold mouse where you want the alpha overlay to be centered (transparent)
* Drag to farthest (from where you clicked) corner, and just a little past it.
* Release to draw.

Done.

Not sure if it works for you, but I’ve done something similar to pre-process big texture files before loading them into OpenGL applications. And yes, this is in Perl.

I couldn’t test it right now, because my Win32 Imager doesn’t support PNG, but it should work fairly well. Let me know.

http://github.com/cosimo/imager-alpha-mask/tree/master

#!perl
use strict;
use warnings;
use Image::Imlib2;

my $source = Image::Imlib2->load(‘square-gradient-mask.png’);
my $destination = Image::Imlib2->new( $source->width, $source->height );

foreach my $x ( 0 .. $source->width ) {
foreach my $y ( 0 .. $source->height ) {
my ( $r, $g, $b, $a ) = $source->query_pixel( $x, $y );
my $average = int( ( $r + $g + $b ) / 3 );
$destination->set_colour( 255, 255, 255, $average );
$destination->draw_point( $x, $y );
}
}

$destination->save(‘square-alpha.png’);