I again seek your wisdom fellow Monks...

As part of the website for my partner's artwork, I am producing two copies of each image that she will upload. One is a small thumbnail with quite high JPEG compression so that it is deliberately pixelated if it is enlarged. The other image is high resolution and large, but with a watermark copyright notice on it. I am using GD for the image processing and Image::Resize to make the sizing easier.

GD is being used because I know my way around it better than the alternatives like Image::Magick and I know GD better because it is available to me by default.

All the resizing works fine and I am able to add a watermark in two parts to get two different font sizes. However, regardless of what I add in the first parameter of ->stringFT() the text is always black. This is fine for some images...

this watermark looks fine

But the black watermark doesn't work very well on artwork where a significant amount of the background is black or close to black.

this watermark doesn't look right

My first thought was to try to have semi-transparent text for the watermark. However, searching suggests that this isn't possible although I haven't found anything which categorically rules it out. So instead I thought of trying to read the general "darkness" of areas of the background where the watermark is going to be and programmatically choose a colour that will contrast against this. Detecting the "darkness" seems far from trivial...

Here is the code... 

sub XHRupload {
    no strict 'subs';
    my $full;
    my $image = Image::Resize->new(GD::Image->new($file{'joolzimage', 
+'file'}));
    if ($image->width() > 1800) {
        $full = $image->resize(1800, 99999);
    } else {
        $full = $image->gd();
    }
    
    my $year        = (localtime)[5] + 1900;
    my $watermark    = 'Artwork by Joolz';
    my $copyright    = "copyright $year";
    
    # Centre text components and centre on image
    my @bounds        = new GD::Image->stringFT('silver', "$ENV{'DOCUM
+ENT_ROOT'}/cgi-bin/Image/watermark.ttf", 140, 0.18, 0, 0, $watermark)
+; 
    my $left           = ($full->width() / 2) - (($bounds[2] - $bounds
+[0]) / 2) + 5;
    my $top            = ($full->height() / 2);
    $full->stringFT('white', "$ENV{'DOCUMENT_ROOT'}/cgi-bin/Image/wate
+rmark.ttf", 140, 0.18, $left, $top, $watermark); 

    @bounds        = new GD::Image->stringFT('silver', "$ENV{'DOCUMENT
+_ROOT'}/cgi-bin/Image/watermark.ttf", 80, 0.18, 0, 0, $copyright); 
    $left           = ($full->width() / 2) - (($bounds[2] - $bounds[0]
+) / 2) + 5;
    $top            = ($full->height() / 2) + 120;
    $full->stringFT('blue', "$ENV{'DOCUMENT_ROOT'}/cgi-bin/Image/water
+mark.ttf", 80, 0.18, $left, $top, $copyright); 

    open my $fh, '>', "$ENV{'DOCUMENT_ROOT'}/artwork/full/$data{'id'}.
+jpg";
    print $fh $full->jpeg(100);
    close $fh;
    
    my $thumb = $image->resize(300, 1000);
    open $fh, '>', "$ENV{'DOCUMENT_ROOT'}/artwork/thumbs/$data{'id'}.j
+pg";
    print $fh $thumb->jpeg(32);
    close $fh;
    
    print "Content-type: text/plain\n\n";
    print $data{'id'};
    exit 0;
}

[download]
Can you suggest either a better way to accomplish what I am trying to do or a relatively simple way to detect how dark the background is, select a colour for the text that will show up on that background and then actually create text in that colour?

As always, any other advice on improving my code would be welcome.

In reply to Adding a watermark to an image with GD::Image by Bod

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