Re^2: Detect boundaries within .png's

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;
[download]

Then do the same for $img->xchg(1,2) for the columns, and hey presto, there's your bounding box!

Comment on Re^2: Detect boundaries within .png's - and cropping Select or Download Code

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Re^3: Detect boundaries within .png's - and cropping by etj (Priest) on Apr 25, 2022 at 01:57 UTC
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;` [download]	[reply] [d/l] [select]
Re^4: Detect boundaries within .png's - and cropping by etj (Priest) on May 03, 2022 at 20:35 UTC
This has now been released with PDL 2.079! See separate announcement for more.	[reply]