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Diagonal selection in Script-Fu using gimp-free-select?

April 11, 2009 - 15:56 — Charles Belov

Is it possible to create a one-pixel diagonal selection using gimp-free-select?

That is, for a 100 x 100 pixel image, the following pixels would be selected:

0,0
1,1
2,2
3,3
...
98,98
99,99

I've attached a simple test case in which I attempt to create such a selection over a white image and fill the selection in black, but it appears nothing is being selected, as the whole image turns out black. I expected a jagged diagonal line from the upper left corner to the lower right corner.

I'm guessing I'm misunderstanding something about the floatarray, as supposedly gimp-free-select can accept a minimum of 2 points.

The attached script-fu goes into a new Debug menu when registered.

AttachmentSize
diagonal-select.scm1.99 KB
diagonal-select4a.scm2.14 KB
diagonal-select4b.scm2.14 KB
diagonal-select4c.scm2.36 KB
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Plugins & Scripts
April 11, 2009 - 21:35 — RobA

but two points can't define a selection. There is no "inside". playing with the free select (lasso) tool, you can't get a valid selection with only two points. you need 3 (to get a triangular selection) or 4 (for a rectangular selection). -Rob A>
April 12, 2009 - 00:56 — Charles Belov

GIMP allows selecting a single row or column, so there doesn't necessarily have to be an inside. And if we can't define a path by two points, then why does gimp-free-select allow for a minimum of two points to define a selection? From parameters: num-segs INT32 Number of points (count 1 coordinate as two points) (num-segs >= 2) Anyway, if I replace the two points with four points my test case does work, so thanks. I've added a second test case diagonal-select4b.scm "Test diagonal gradient with 4 points (alternative B)" (also in the Debug menu) which works as I want it. I've also added diagonal-select4a.scm "Test diagonal gradient with 4 points (alternative)" which almost works except that 1 point in the middle is missing. The only difference with the 4b file is that the selected line is 1 pixel shorter. I've filed a bug concerning this missing pixel. But in any case, now I've got working code so I'm happy.
April 12, 2009 - 04:13 — RobA

because the free select tool (and the lasso tool) allow for points defining their selection at the fractional pixel level. So to define a 1 px horizontal or vertical selection with free select I would expect you need to use 4 points where the y or x respectively has a delta of 1 px between the two pairs of points. -Rob A>
October 20, 2011 - 19:43 — saulgoode

Just paint directly on the selection channel in white and those pixels will be selected.

(gimp-selection-none image) ; fill selection with black 
(gimp-context-set-foreground '(255 255 255)) ; white = fully selected
(set! selection (car (gimp-image-get-selection image))) ; the selection is a channel
(gimp-context-set-brush "Circle (01)") ; one-pixel brush
(set! stroke (make-vector 4 0))
(vector-set! stroke 0 0) ; x1 = 0
(vector-set! stroke 1 0) ; y1 = 0
(vector-set! stroke 2 100) ; x2 = 100
(vector-set! stroke 3 100) ; y2 = 100
(gimp-pencil selection 4 stroke) ; draw a one-pixel white line from x1,y1 to x2,y2

Note that if you are just using numeric constants for the coordinates (not variables), you can reduce the last six lines to just:

(gimp-pencil selection 4 #(0 0 100 100))

I left out the grunt work.

EDIT: fixed up code formatting using 'pre' tags.

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