Shaping an image to fit in a polygon

Richmond richmondmathewson at
Sat Oct 16 16:00:49 EDT 2021

I'm a lazy slob, and because of that I would just do this sort of thing 
outwith LiveCode,
with, say, GIMP.

If you could explain your rationale behind trying to go "the long way 
round" in LiveCode
I might be both more sympathetic and more helpful.

Best, Richmond.

On 16.10.21 22:17, David Epstein via use-livecode wrote:
> I am trying to create an image that will appear to have the shape of some arbitrary polygon.  The idea is to set the image's alphaData so that pixels within the polygon are opaque, and those outside the polygon are transparent.  The scripts below create a rectangular image of whatever is under the polygon's enclosing rectangle, and then adjust the alphaData of that new image.  But while the new image gets trimmed a bit, the result does not match the shape of the designated polygon.  Any thoughts?
> David Epstein
> on action gID ## gID is the short id of the polygon
>    import snapshot from rectangle globalRect(the rect of control id gID)
>    put the short id of image (the number of images) into imID
>    set the width of image id imID to the width of grc id gID
>    set the height of image id imID to the height of grc id gID
>    set the loc of image id imID to the loc of grc id gID
>    set the polyCrop of image id imID to gID
> end action
> function globalRect theRect
>    return globalLoc(item 1 to 2 of theRect),globalLoc(item 3 to 4 of theRect)
> end globalRect
> setProp polyCrop polyID
>    -- crop target image's alphadata to show only points within grc id polyID
>    put the filled of grc id polyID into fillState
>    set the filled of grc id polyID to true
>    put the rect of the target into r
>    put numToChar(255) into P ## opaque
>    put numToChar(0) into T ## transparent
>    repeat with y = 1+item 2 of r to item 4 of r ## SEE NOTE*
>      repeat with x = 1+item 1 of r to item 3 of r
>        put x,y into myPoint
>        if within(grc id polyID,myPoint) then put P after hold else put T after hold
>      end repeat
>    end repeat
>    set the filled of grc id polyID to fillState
>    set the alphaData of the target to hold
> end polyCrop
> * NOTE:  the "1+" is these statements is my effort to step through the exact number of pixels in the width and height of the image.  The number of values in the series (item 1 of the rect of the image) to (item 3 of the rect of the image), inclusive, is 1+ the width of the image.  (But I am not really sure whether to add 1 to the first value or subtract 1 from the last value).
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