Image scaling

Richard Suchenwirth 2003-02-25 - Photo images can be resized by adding the -zoom or -subsample switches when copying an image. Here is a wrapper that takes only a factor and selects the appropriate switch. The image is scaled in place (so when it is displayed in a widget, updating goes automatically), the temporary image t is freed when no more needed. The following enhanced version is 3 LOC more, but also does flipping around x and/or y axis:

 proc scaleImage {im xfactor {yfactor 0}} {
    set mode -subsample
    if {abs($xfactor) < 1} {
       set xfactor [expr round(1./$xfactor)]
    } elseif {$xfactor>=0 && $yfactor>=0} {
        set mode -zoom
    }
    if {$yfactor == 0} {set yfactor $xfactor}
    set t [image create photo]
    $t copy $im
    $im blank
    $im copy $t -shrink $mode $xfactor $yfactor
    image delete $t
 }

Usage examples: adding the following lines gives iFile: a little file system browser scaling capacities on the image shown on the "File" page:

 .m add casc -label Image -menu [menu .m.image -tearoff 0]
 .m.image add comm -label "Zoom x 3" -command {scaleImage $g(i) 3}
 .m.image add comm -label "Zoom x 2" -command {scaleImage $g(i) 2}
 .m.image add comm -label "Zoom x 0.5" -command {scaleImage $g(i) 0.5}
 .m.image add comm -label "Zoom x 0.33" -command {scaleImage $g(i) 0.33}
 .m.image add separator
 .m.image add comm -label "Flip LR" -command {scaleImage $g(i) -1 1}
 .m.image add comm -label "Flip TB" -command {scaleImage $g(i) 1 -1}
 .m.image add comm -label "Flip both" -command {scaleImage $g(i) -1 -1}

For robustness, one might disable this menu when no image is displayed. Experience shows that enlarging bigger photo images may let the little machine run out of memory - time to throw away some fat MP3 files...

dzach 16-Sep-2005: Following a suggestion of suchenwi in the Tcler's chat, here is a proc to uniformly scale an image in place, using a ratio of integers r1/r2:

    proc ratscale {img r1 r2} {
        image create photo tmp_img
        tmp_img copy $img -zoom $r1
        $img blank
        $img copy tmp_img -subsample $r2
        image delete tmp_img
    }

However for large rationals (say 200/255) this may be a memory killer, since it will first zoom the image by 200 and then subsample it (1 every 255).

Image scaling also helps in the GIF transparency problem on iPAQ - this workaround works:

put an instance of the image in a widget (e.g. a text)
zoom up
subsample down again

Now transparent pixels are in the widget background color (white), no more random and black, for all instances, and certainly look better than before.

 foreach i $g(images) {
   $g(text) image create end -image $g($i)
   scaleImage $g($i) 3
   scaleImage $g($i) .33
 }

RS 2006-02-13: Here's a variation that takes an image and a percentage (see Greatest common denominator for gcd), and returns an accordingly scaled image:

 proc image% {image percent} {
   set deno      [gcd $percent 100]
   set zoom      [expr {$percent/$deno}]
   set subsample [expr {100/$deno}]
   set im1 [image create photo]
   $im1 copy $image -zoom $zoom
   set im2 [image create photo]
   $im2 copy $im1 -subsample $subsample
   image delete $im1
   set im2
 }
 proc gcd {u v} {expr {$u? [gcd [expr $v%$u] $u]: $v}}

For fast arbitrary rotation (and scaling) see: Enhanced photo image copy command

The combination of photo image zooming and the Img extension let us code A little magnifying glass in just a few lines.

hypnotoad Has a C based Tk Image scaler that works with arbitrary sizes: Image Scaling in C

Category Image Processing