To get the RGB of a pixel with PerlMagick, you can use: 
($red,$green,$blue,$opaque) = split(/,/,$image->Get("pixel[$x,$y]"));

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Update: After some thougth I came up with this to solve your first problem: 
#!/usr/bin/perl -w

use strict;
use Image::Magick;

# Simuate background webcam picture... :)
my $image1 = Image::Magick->new;
$image1->Set(size=>'100x100');
$image1->ReadImage('xc:black');
$image1->Draw(stroke=>'green', primitive=>'rectangle', points=>'5,5 95
+,95');

# Textbackground image
my $image2 = Image::Magick->new;
$image2->Set(size=>'100x20');
$image2->ReadImage('xc:blue');

# Make composite image of background and a 60.5 % transparent text-bac
+kground
$image1->Composite(compose=>'Blend', image=>$image2, x=>0, y=>0, opaci
+ty=>60.5);

# Add un-transparent text atop on background
$image1->Annotate(font=>'@c:/windows/fonts/arial.ttf', pointsize=>15, 
+stroke=>'red', text=>'Whoo',x=>30,y=>15,fill=>'red');

# Write image
$image1->Write('x.png');

[download]
Update 2: It seems that newer versions (>5.25) of IM have replaced the Blend command with Dissolve and wants the opacity to be an integer...

/brother t0mas

In reply to Re: Image Manipulation Package for setting Opaqueness? by t0mas
in thread Image Manipulation Package for setting Opaqueness? by keith

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