image_scale in practice

method stencil_render {sn image_zoom} {
    my variable stendat stencil_alpha stencil_alpha_mode stencil_zoom
    if {[info exists stencil_zoom($sn)]} {
      if { $stencil_zoom($sn) eq $image_zoom } {
        if {![catch {image height stencil.$sn}]} {
          ###
          # If the image exists at the same magnification, return it
          ###
          return stencil.$sn
        }
      }
    }
    set scaley 1.0

    set info [my stencil_info $sn]
    dict with info {}

    set file [my stencil_file $sn]
    if {![file exists $file]} {
      set file [tk_getOpenFile -parent . -filetypes [irm::image_types]]
      if {$file==""} {
        return
      }
      irm::image_package $file
      if {[catch {image create photo stencil -file $file} err]} {
        taotk::dialog  -icon error -type ok -parent . -message $err
        return
      }
    }
    if { $image_zoom > 4.0 } {
      ###
      # Crop image
      ###
      set ans [::taotk::dialog -type okcancel -message "
Stencil $sn is zoomed in to a factor of $image_zoom : 1.
This may tie up a lot of system memory.

Press cancel to abort."]
      if { $ans == "cancel" } {
        puts "Stencil magnification exceeded ($image_zoom)"
        ###
        # Clip a window
        ###
        return {}
      }
    }
    image create photo stencil.raw -file $file
    image create photo stencil.$sn

    set wid [image width stencil.raw]
    set hgt [image height stencil.raw]
    set newwid [expr {int($wid * $image_zoom)}]
    set newhgt [expr {int($hgt * $image_zoom * $scaley)}]
    set stendat($sn) [list wid $wid hgt $hgt zoom $image_zoom scale $image_zoom]

    # Alpha blending is slow on most platforms
    if {[my cget stencil_alpha_mode]} {
      ::shapes::image_scale stencil.raw $newwid $newhgt stencil.$sn [my cget stencil_alpha]
    } else {
      ::shapes::image_desat_scale stencil.raw $newwid $newhgt stencil.$sn [my cget stencil_alpha]
    }
    image delete stencil.raw
    set stencil_zoom($sn) $image_zoom
    return stencil.$sn
  }