[HJG] I have some images on a canvas, and I want to get the color of the pixel under the cursor.

Right now, this works only for an image placed on the canvas at 0,0 :
----
  package require Img

  proc int x { expr int($x) }

  proc ShowPixel {x y i} {
    set c "- - -"
    catch { set c [$::img1 get $x $y] }
    wm title . "$x $y #$i : $c"
  }

  set fn gosmall.gif
  set img1 [image create photo -file $fn]

  pack [canvas .c]
  .c create image  0  0 -image $img1 -anchor nw -tag Copy1
  .c create image 80 80 -image $img1 -anchor nw -tag Copy2

  bind .c <Motion> { ShowPixel [int [%W canvasx %x]] [int [%W canvasy %y]] [%W find withtag current] }
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The RGB-values of the pixel under the cursor are shown only for the first copy of the image...

I can query the coordinates of an item/image on the canvas with ''bbox'' 
(to adjust coordinates within the image vs. the screen-coordinates given by the cursor), 
but how do I pass the item (as opposed to its id) to the proc ShowPixel ?

[RS]: Pass in the widget name. For images you can retrieve the name as ''-image'' attribute. Sketch (missing the transposition of image coordinates yet):

 proc getColor {canvas x y} {
    set color ""
    set id [$canvas find withtag current]
    switch -- [$canvas type $id] {
       image {set color [[$canvas itemcget $id -image] get $x $y]}
       line - polygon - rectangle - oval {
            set color [$canvas itemcget $id -fill]
       }
    }
    return $color
 }

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[Category Graphics] - [Category Image Processing]