[Kst] Lines and Arrows

George Staikos staikos at kde.org
Sat Feb 4 14:38:03 CET 2006

Bugs 121068 and 121108 really are the same problem resulting in different 
symptoms.  On a micro-level they look completely different and are fixed 
differently, but in the grand scheme of things I think they're just a 
mis-implementation of the view object interface.  A view object is specified 
by its properties, its paint, and its geometry.  The paint and the geometry 
are entirely separate things, and the paint must be contained within the 
geometry.  The geometry is something used at the program level, though in 
many cases it aligns directly with something at the user-interface level.  In 
the case of plots, labels, and boxes, the geometry and the bounds of the 
object should be identical.  However, in the case of lines, arrows, and 
ellipses, this is not necessarily the case.  This does not mean that the 
geometry is meaningless, nor does it mean that the object can paint outside 
the geometry.  The problem with line (and arrow) is that it has "from" and 
"to" properties, and those properties are presently assumed to be two 
opposite corners of the bounding box.  This works for a line of thickness = 1 
(or 0), but nothing else.  In other cases the line is drawn outside the 
bounding box.

I think the correct thing is that lines should compute 'from' and 'to' based 
on the geometry of the object, and not store them.  When the user tries to 
adjust a line by setting one of those two properties, it should compute the 
bounding box/geometry required for the line, clip it to the 
parent/view/whatever we want), and then adjust the 'from' or 'to' property 
accordingly.  Basically it should do the opposite of what I did originally, 
which is to draw the largest possible line inside the bounding box.  This new 
idea gives the following properties:

1) 'from' and 'to' are exactly that and not just "close" for large widths.
2) painting is always inside the geometry
3) lines can still be manipulated as expected
4) lines of all widths work

One interesting side effect: making a line thicker will either make the object 
geometry larger or shorten the line - or both.

How does this apply to 121108?  The manipulation routines need to behave 
accordingly:  lines are on a from-to basis, not a bounding box basis like 
other objects.  they have a bounding box, but it doesn't entirely make sense 
from a UI point of view.  parenting needs to change in different ways in this 

I think this maps up to Barth's options #1 and #3 combined.

George Staikos
KDE Developer				http://www.kde.org/
Staikos Computing Services Inc.		http://www.staikos.net/

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