[vtk-developers] A weird 6.x specific bug with ModifiedEvents

[David Gobbi]

On Mon, Apr 14, 2014 at 9:35 AM, Prabhu Ramachandran
<prabhu at aero.iitb.ac.in> wrote:
> Hi everyone,
>
> I just discovered a weird bug with VTK 6.1 and 6.0 that doesn't happen with
> VTK 5.x.  Here is a tiny Python script:
>
> ##################################################
> import vtk
> def f(o, e):
>     print e, o.GetAmbientColor(), o.GetColor(),
>     print o.GetDiffuseColor(), o.GetSpecularColor()
>
> p = vtk.vtkProperty()
> p.AddObserver('ModifiedEvent', f)
> p.SetColor(0,0,0)
> print p.GetColor()
> ##################################################
>
> If you comment out the AddObserver line, everything works correctly but with
> it, you can never set the color of the property!  This doesn't seem to be
> specific to Python and happens with a small C++ example as well.

I can confirm, I saw exactly what you reported.  Then I changed the Set
method to SetAmbient(0.1234) and the problem disappeared, everything
worked fine.

Then, I tried with both SetColor and SetAmbient, and saw something that
looks Very Bad:

##################################################
import vtk
def f(o, e):
    print e, "Color:", o.GetColor()
    print e, "Ambient:", o.GetAmbient()

p = vtk.vtkProperty()
p.SetColor(0,0,0) # clear value
p.SetAmbient(0) # clear value
p.AddObserver('ModifiedEvent', f)
print "SetColor(0.25,0.5,0.125)"
p.SetColor(0.25,0.5,0.125)
print "SetAmbient(0.1234)"
p.SetAmbient(0.1234)
##################################################

The output was:

SetColor(0.25,0.5,0.125)
ModifiedEvent Color: (0.0, 0.0, 0.0)
ModifiedEvent Ambient: 0.0
SetAmbient(0.1234)
ModifiedEvent Color: (0.027461278262417663, 0.054922556524835325,
0.013730639131208831)
ModifiedEvent Ambient: 0.1234

So as far as I can tell, catching the ModifiedEvent results in some sort
of memory corruption.  I'm compiling with llvm-gcc on OS X 10.6.

  David