Simple zooming and scaling in a canvas

Arjen Markus I needed a script to move and scale the contents of a canvas. RS suggested that this can be achieved with a few event bindings. He, of course, was right as usual. It is slightly more complicated than one-line commands, but still, it can hardly be easier.

Notes:

In the actual script I use so-called world coordinates to transform metric coordinates to and from pixel coordinates. So the procedures get more complicated to update the transformation data.
If the number of items gets large (a few hundred or a few thousand, as in my case), you can notice a delay between the updates. I use a slightly different method for this situation - control-arrow-keys to shift it a known number of pixels. But the principle stays the same.

# Experiment with scale/move items in a canvas
#
proc moveItems { x y } {
    global xc yc
    
    if { [info exists ::xc] } {
        .c move all [expr {$x-$xc}] [expr {$y-$yc}]
    }   
    set xc $x
    set yc $y
}

proc scaleItems { type x y } {
  
    if { $type eq "+" } {
        .c scale all $x $y [expr {sqrt(2.0)}] [expr {sqrt(2.0)}]
    } else {
        .c scale all $x $y [expr {1.0/sqrt(2.0)}] [expr {1.0/sqrt(2.0)}]
    }  
}

pack [canvas .c -bg white] -fill both

.c create oval        0   0 100 100 -fill red   -outline black -width 4
.c create rectangle 200 200 300 300 -fill green -outline yellow
.c create line 0  200  200 0 -fill black -width 10
.c create line 0 -200 -200 0 -fill blue  -width 6

set xc 0
set yc 0
focus .c 
bind .c <Button-1>               {set xc %x; set yc %y} 
bind .c <B1-Motion>              {moveItems    %x %y}
bind .c <KeyPress-a>             {scaleItems + %x %y}
bind .c <KeyPress-b>             {scaleItems - %x %y}
bind .c <Control-KeyPress-plus>  {scaleItems + %x %y}
bind .c <Control-KeyPress-minus> {scaleItems - %x %y}