Intriguingly, there's no cropping type functionality in core PDL. A bit of a ponder suggests a good approach would be to (for rows), assuming an image of (rgba,x,y):
$non_blank_rows = ($img->slice('(3)') != 0)->orover;
($low, $high) = $non_blank_rows->which->minmaximum;
Then do the same for $img->xchg(1,2) for the columns, and hey presto, there's your bounding box! | [reply]
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It turns out that making this broadcast properly over multiple images was impressively hard.
which had a hardcoded limitation that it would only work on a 1-dim ndarray (fixed by imitating PDL::VectorValued's strategy of having a "how many set for each vector" output ndarray).
Then actually using that with minmaximum also posed a problem, since the number of values per image might vary (in other words, the outputs might be ragged) - fixed that by having which fill the unset index-values with -1, and then setting that as the badvalue for the output ndarray, so minmaximum would ignore them.
After that, the rest was easy! The new PDL::Image2D::crop works on a mask, so the above snippet would call
$x1x2y1y2 = ($img->slice('(3)') != 0)->crop;
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This has now been released with PDL 2.079! See separate announcement for more.
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