I suspect this is one of those cases where vector operations don't buy you anything; indeed, they probably force a lot of extra work.

Once you have your "segmented image" -- ie. each object colored differently -- then a single left to right, top-to bottom pixel scan will discover the bounding boxes of all the objects.

In pseudo code:

use constant { LEFT=>0, RIGHT=>1, TOP=>2, BOTTOM=>3 };

my @bounds;
for my $y ( 0 .. $height -1 ){
    for my $x ( 0 .. $width - 1 ) {
        my $pixel = $image->pixel( $x, $y );
        
        $bounds[ $pixel ][ LEFT   ] = $x if $x < ( $bounds[ $pixel ][ 
+LEFT   ] // 1e99 );
        $bounds[ $pixel ][ RIGHT  ] = $x if $x > ( $bounds[ $pixel ][ 
+RIGHT  ] // 0 );
        $bounds[ $pixel ][ TOP    ] = $y if $y < ( $bounds[ $pixel ][ 
+TOP    ] // 1e99 );
        $bounds[ $pixel ][ BOTTOM ] = $y if $y > ( $bounds[ $pixel ][ 
+BOTTOM ] // 0 );
    }
}

[download]

Unfortunately I cannot help you with applying that to PDL because I gave up on trying to find an efficient way to apply a piece of perl code to all the elements of a piddle.

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to Re: PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" by BrowserUk
in thread PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" by vr

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