Keith Vetter 2007-09-10 : One cute effect I've always wanted to do was to fade one image into another. Unfortunately, tcl's image handling abilities are not one of its strong points, and the straight-forward method for fading is too slow for this.

But then I thought of a clever hack of exploiting the -alpha attribute of a toplevel window. Place two toplevels exactly on top of each other, each displaying a different image, and vary the first one's alpha value from 0 to 1 while varying the second ones from 1 to 0. (It turns out you also need a third toplevel with a neutral background under everything to keep the desktop from peeking through.)

Nice little hack. It would be harder, but not impossible, to generalize to non-trivial situations.

Note, my display flickers as you move the scale--not sure if it's my machine or an OS problem.

JAG, 11-Sep-2007 - Keith, that's a pretty neat hack - nicely done. FYI, running on WinXP-Pro the fade seems pretty smooth, though I do get an occasional odd "flash" in the display. KPV yep, that's what I get.

David Easton, 11-Sep-2007 - Can't you do this with just the 2 images if the background image is kept opaque and the front image varies from 0 to 1?

KPV - You're probably right. Just tried it out (simply comment out the alpha setting for window .t1) and looks about the same. However, my approach is probably a bit simpler if I were to extend this so that it can fade between a series of images.

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uniquename 2013aug18

For those readers who do not have the time/facilities/whatever to setup the code below and execute it, here are several images that indicate what the following code can do.

I had to make one small change to get the code to work. The 'earth.gif' and 'earthris.gif' files were not in the Tk directory that this code expected them to be in. I changed the statement

   set idir [file join $::tk_library .. .. demos Tk[info tclversion] images]

to

   set idir "/usr/share/doc/tk8.5/examples/images"

Then when the code executed, the first of the following 3 images appeared.

I moved the 'slider' of the 'scale' widget to minus-10 to get the second image. Then I moved the slider to minus-100 to get the 3rd image.

As the discussion above indicates, there are really 3 aligned windows involved here. You can see in the code below that they are named '.t0', '.t1', and '.t2'.

The following image shows that I could separate the windows into two. It appears that I could pull '.t2' off of '.t1', but it seems that '.t0' and '.t1' remain aligned. I could not pull t0 and t1 apart, but you can see the window titles 'Image Fading' and 'Image #1' seem to be blended together on the title bar of the img1 window. In fact, whenever I released the mouse cursor, window t2 would snap back over the combined window t0-t1.

I found that if I moved the slider to minus-100 and then dragged the upper window to another part of the screen, the window would 'lock'. Once I let go of the mouse cursor, I could not grab the window again to move it. And the window-manager menu to Minimize/Maximize/Close the window was not available. Fortunately, the entire desktop was not frozen. I could kill the Tk script process to get the locked window off of the desktop.

It is interesting that the '-alpha' feature works on my Linux installation (Ubuntu 9.10, which, I believe, uses the Gnome 2.x 'metacity' window manager). I thought I had tried to a 'wm' command before to see if transparency effects work on this Linux, but it did not work.

Apparently, 'wm -alpha' works --- but this technique of fading images by using multiple windows and the alpha-channel feature of window managers is not 'safe' to use --- judging by the window 'locks' I experienced.

I think I would implement 'image fading' in a different way --- but it was interesting to me to find that 'wm -alpha' works with my Ubuntu 9.10 window manager.

(For those who are curious about the partial icons appearing at the bottom right of the last image, those are the icons for Filezilla, gnome-screenshot, and mtpaint --- 3 applications that I use quite often.)

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 ##+##########################################################################
 #
 # ImageFade.tcl -- hack to get two images fading into one another
 # by Keith Vetter, September 2007
 #
 
 package require Tk
 
 set S(title) "Image Fading"
 set S(perc) 100

 # Idir is the tk demo directory
 set idir [file join $::tk_library .. .. demos Tk[info tclversion] images]
      
 proc DoDisplay {} {
    global S
    
    wm withdraw .
    foreach id {0 1 2} {
        set w .t$id
        toplevel .$w
        label $w.l -image ::img::img$id
        pack $w.l -side top -fill both -expand 1
        wm protocol $w WM_DELETE_WINDOW exit
        wm resizable $w 0 0
        wm title $w "Image \#$id"
        wm geom $w +10+10
        raise $w
        if {$id == 0} {
            scale $w.s -from -100 -to 100 -orient h -variable S(perc)
            pack $w.s -side bottom -expand 1
            wm title $w $::S(title)
        } else {
            wm transient $w .t0
        }
    }
    foreach t [trace info variable S(perc)] {
        eval trace remove variable S(perc) $t
    }
    trace variable S(perc) w Tracer
    update
    bind .t2 <Configure> {WindowMove}
 }
 ##+##########################################################################
 # 
 # Grabs two images from the tk demo directory
 # 
 proc FindPhotos {} {
    global idir

    set img1 [file join $idir earth.gif]
    set img2 [file join $idir earthris.gif]
    if {! [file exists $img1]} {
        set idir2 [file nativename [file normalize $idir]]
        set msg "Cannot find tk demo images\nearth.gif and earthris.gif."
        append msg "\n\nLooking in:\n  $idir2"
        tk_messageBox -icon error -message $msg -title "Error"
        exit
    }
    
    image create photo ::img::img1 -file $img1
    image create photo ::img::img2 -file $img2
    image create photo ::img::img0 \
        -width [image width ::img::img1] -height [image height ::img::img1]
 }
 ##+##########################################################################
 # 
 # We want to make sure that our 3 windows stay in sync with
 # each other when the top one moves.
 # 
 proc WindowMove {} {
    set pos "+[winfo x .t2]+[winfo y .t2]"
    wm geom .t0 $pos
    wm geom .t1 $pos
 }
 proc Tracer {var1 var2 op} {
    global S
 
    set alpha2 [expr {($S(perc)+100)/200.0}]    ;# Convert -100-100 to 0-1.0
    set alpha1 [expr {1 - $alpha2}]
    wm attribute .t1 -alpha $alpha1
    wm attribute .t2 -alpha $alpha2
    update
 }
 FindPhotos
 DoDisplay 
 return

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