Version 5 of Transparent Toplevel

Someone was enquiring on the Tcl'ers Chat if Tk could do transparent toplevels. It can't natively, but you can simulate them using a canvas. Here is some code which should work on Microsoft Windows systems. It simply gets the wallpaper picture by querying the registry, and then draws the appropriate part of it as a background on the canvas. There are some problems, which should be addressed:

Only works on windows (unix/mac code would be welcome)
Assumes scaled wallpaper (you can check the WallpaperTile value to see if it's actually tiled, and possibly other registry settings - I'll leave that as an exercise...)
Only scales the image by integer amounts - this is a limitation of Tk's image scaling code.
You have to pack child widgets into $path.c - the canvas. You could use snit or some other code to make this more transparent (no pun intended).

 # Creates a "transparent" window on windows...
 package require Tk
 package require registry
 package require Img

 proc transtop {path args} {
     uplevel 1 [list toplevel $path] $args
     # Create a canvas in this toplevel
     pack [canvas $path.c] -fill both -expand 1
     # Find out the desktop wallpaper image
     set file [registry get HKEY_CURRENT_USER\\Control\ Panel\\Desktop \
         Wallpaper]
     set im [image create photo -file [file normalize $file]]
     set bg [image create photo]
     set scalex [expr {int(double([winfo screenwidth $path]) / 
                 double([image width $im]) + 0.9)}]
     set scaley [expr {int(double([winfo screenheight $path]) / 
                 double([image height $im]) + 0.9)}]
     $bg copy $im -zoom $scalex $scaley
     set id [$path.c create image 0 0 -anchor nw -image $bg]
     updateBgImage $path.c $id
     bind $path <Configure> [list updateBgImage $path.c $id] 

     return $path
 }

 proc updateBgImage {c id} {
     set x [expr {0 - [winfo rootx $c]}]
     set y [expr {0 - [winfo rooty $c]}]
     $c coords $id $x $y
 }

 if {$argv0 eq [info script]} {
     # Main
     transtop .t
     bind .t <Destroy> { exit }
     wm withdraw .
 }

FW notes this is by Neil Madden, who seems to have forgotten to ;) And also notes that TkTrans is the standard full implementation.

NEM D'oh! Yes, it is by me. Is TkTrans a compiled extension, or pure Tcl code? Either way, if you can use it instead of this code, I would. Note also, that using transparency in an application is likely to be a hideous usability error. Having a semi-transparent background would probably be better. Also, note that if you pack other widgets (text widgets, buttons, labels etc) onto the canvas, they will not be transparent...

GS (031206) Is there a way to save a screenshot in a file from the clipboard ? If it is possible, we can use the cwind package do a snapshot of the desktop with:

 package require cwind
 ::cwind::send |SNAP|

And after we can put the file into the canvas.

I have made a test by hand saving the image desktop with MsPaint. The result is funny.