ulis, 2002-11-24
I gave this explanation on comp.lang.tcl and saw a new related question. The first subject was : scrolling to an item.
Here are some explanations about scrolling.
sx1 vx1 ix1 ix2 vx2 sx2
| | | | | |
+-------------------------------------------------+
| |
| +--------------------+ |
| | | |
| | +------+ | |
| | | item | | |
| | +------+ | |
| | | |
| | visible area | |
| +--------------------+ |
| |
| scroll region |
+-------------------------------------------------+
The scroll region is where are all your items.
For a canvas generally you set the scroll region with:
.cnv config -scrollregion [.cnv bbox all]
sx1 & sx2 are the x coordinates of the scroll region.
sx1, the beginning, sx2, the end.
You get sx1 & sx2 with .cnv cget -scrollregion.
The visible area is the visible surface of the canvas displayed on screen.
vx1 & vx2 are the x coordinates of the visible area.
vx1, the beginning, vx2, the end.
You know vx1 & vx2 because vx1 is 0
and you fixed vx2 with -width $vx2.
ix1 & ix2 are the x coordinates of the item.
ix1, the beginning, ix2, the end.
You can get ix1 & ix2 with [.cnv bbox item_tag].
Scrolling is varying the position of the visible area within the scroll region.
If you want to see the beginning of your item you need to have ix1 between vx1 & vx2.
For a canvas, the operation to use is:
.cnv xview moveto $fraction
where $fraction is ($vx1 - $sx1) / ($sx2 - $sx1)
(the percent of the scroll region before the visible area).
So, if you want ix1 == vx1
(the beginning of the item at the beginning of the visible area)
for a canvas you need to do:
.cnv xview moveto [expr {double($ix1 - $sx1) / ($sx2 - $sx1)}]A canvas example:
# create the canvas & the item
canvas .c -width 100 -height 100 \
-scrollregion {-400 -400 400 400} -bg white -bd 1 -relief sunken
.c create text 200 200 -text hidden -tags hidden
# the 'view' proc
proc view {tag} \
{
lassign [.c bbox $tag] x1 y1 x2 y2
lassign [.c cget -scrollregion] s1 t1 s2 t2
.c xview moveto [expr {double($x1 - $s1) / ($s2 - $s1)}]
.c yview moveto [expr {double($y1 - $t1) / ($t2 - $t1)}]
}
# some stuff for the demo
proc hide {} \
{
.c xview moveto 0.0
.c yview moveto 0.0
}
button .v -text view -command {view hidden}
button .h -text hide -command {hide}
grid .c -columnspan 2
grid .v .h