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Detecting whether selection is rectangular?

April 28, 2009 - 07:41 — Charles Belov

Are there any GIMP functions to detect whether a selection is purely rectangular (as opposed to round, rounded, feathered, non-contiguous, etc.)?

I mean something relatively simple, not involving iterating through every pixel in the selection bounds.

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April 29, 2009 - 13:37 — saulgoode

Alternative using channels

The following sets a boolean variable 'rect?' if the selection is rectangular. It assumes a selection exists -- test the value of '(car (gimp-selection-bounds...' at the beginning if you need to ensure this. I left out the variable declarations. (set! channel (car (gimp-selection-save image))) (set! bounds (cdr (gimp-selection-bounds image))) (gimp-rect-select image (car bounds) (cadr bounds) (- (caddr bounds) (car bounds)) (- (cadddr bounds) (cadr bounds)) CHANNEL-OP-REPLACE FALSE 0) (gimp-channel-combine-masks (car (gimp-image-get-selection image)) channel CHANNEL-OP-SUBTRACT 0 0) (set! rect? (= (car (gimp-selection-bounds image)) 0)) (gimp-selection-load channel) (gimp-image-remove-channel image channel)
April 29, 2009 - 18:02 — RobA

Nice!

It is nice to see alternate ways to accomplish the same thing! (You sure love channels!) -Rob A>
April 28, 2009 - 15:33 — RobA

Solution to determining if a selection is rectangular

You could use the histogram function to get the pixel count of the selection and compare that to the width x height. (cadddr (gimp-histogram HISTOGRAM-VALUE 0 256)) will return the number of pixels in the current selection (or the whole layer, if no selection) and (gimp-selection-bounds ) returns a list if (non-empty x1 y1 x2 y2) so x2 - x1 times y2 - y1 would provide the rectangular area. I tested this from the scheme console and it appears to work: (let* ( (bounds (gimp-selection-bounds 1)) (area (cadddr (gimp-histogram 2 HISTOGRAM-VALUE 0 256))) ) (= (* (- (list-ref bounds 3) (list-ref bounds 1)) (- (list-ref bounds 4) (list-ref bounds 2))) area) ) -Rob A>
May 17, 2009 - 10:07 — Charles Belov

Works for some definitions of rectangle

This one works on some conditions and not others, although I think it would be suitable for my purposes since (1) I don't expect to try this on a transparent layer (2) having a mask where none is needed wastes some bytes, but doesn't impact the visible results.
  • No selection:
    • Background: no mask (correct)
    • Non-transparent layer: no mask (correct)
    • Transparent layer: no mask (correct)
  • Rectangular selection:
    • Background: no mask (correct)
    • Non-transparent layer: no mask (correct)
    • Transparent layer: mask (incorrect but harmless)
  • Rectangular selection inverted:
    • Background: mask (correct)
    • Non-transparent layer: mask (correct)
    • Transparent layer: mask (correct)
  • Rectangular selection feathered:
    • Background: mask if feathered 1 pixel or more (correct enough)
    • Non-transparent layer: mask if feathered 1 pixel or more (correct enough)
    • Transparent layer: mask if feathered at all (correct)
  • Rectangular selection with pixel punched out:
    • Background: mask (correct)
    • Non-transparent layer: mask (correct)
    • Transparent layer: mask (correct)
  • Rectangular selection with internal pixel faded:
    • Background: mask (correct down to even 1% opacity for the faded pixel; sweet!)
    • Non-transparent layer: mask (correct down to even 1% opacity for the faded pixel; sweet!)
    • Transparent layer: mask (correct down to even 1% opacity for the faded pixel; sweet!)
  • Oval selection:
    • Background: mask (correct)
    • Non-transparent layer: mask (correct)
    • Transparent layer: mask (correct)
so I'll probably use this because the code is simpler, although it is helpful to know the other method as well.
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