[Gmsh] Patch: minimal arrow size

[Jiří Hnídek]

Hi Christophe,
thanks for reply.

Christophe Geuzaine píše v Pá 05. 12. 2008 v 13:56 +0100:
> Jir(í Hnídek wrote:
> > Hi,
> > I created small patch of GMSH source code. This patch is created from 2
> > days old cvs source snapshot (I mentione this, because noticed lot of
> > new code in last cvs snapshot). This patch adds option to change minimal
> > arrow size. Hopefully, this screenshot describes more:
> > 
> > http://dl.getdropbox.com/u/369894/GMSH_minimal_arrow_size.png
> > 
> > Why is it useful for us? We try to do visualization of water flow in a
> > underground (simulation of poisoned water, simulation of
> > decontamination, etc). There are areas with large flow speed (rifts) and
> > very small flow speed (massif). Flows in massif are sometimes so small,
> > that are invisible with logarithmic value scale. This option could help
> > us see and understand processes in underground better :-).
> > 
> 
> Hi Jiri - Thanks for the patch. This could indeed be very interesting: 
> we'll have a look.
> 
> Another thing you could try is to select the "logarithmic" value scale 
> mode. That's the mode we usually use when we visualizae vector fields 
> with large variations. How does it work on your dataset?

Logarithmic value scale works fine for some types of data set, but for example in this case it doesn't work too good:

Linear value scale:
http://dl.getdropbox.com/u/369894/Linear_vector_field.png

Logarithmic value scale:
http://dl.getdropbox.com/u/369894/Logarithmic_vector_field.png

Logarithmic value scale win minimal arrow size:
http://dl.getdropbox.com/u/369894/Logarithmic_vector_field_with_min_arrow_size.png

Why logarithmic value scale does not work for us too good in this case?
It is influenced by code in function drawVectorArray(...):

scale = opt->ArrowSize / l * log10(l / opt->TmpMin) /
log10(opt->TmpMax / opt->TmpMin);

This returns zero values for vectors near to the smallest vector in a
vector field. Most of vectors in our vector fields are near to the
minimal vector, because flow in massif is small. Why are so small flows
so important for us? Because such flows are in large volumes and amount
of flowed water could be significant.

> > If you will find my patch interesting, then let me know, I can recreate
> > it against new cvs source snapshot. I will try to ask again. Do you have
> > any public cvs repository? This would help me very much. Are you
> 
> Not public, but we create accounts for frequent contributors.

Ok, I will try to convince you, that I deserve account :-). How many
patches should I send to be accepted as contributor? I'm Blender
developer and Blender developer community gives subversion write access
to developer, who contributed 3 good patches. Do you have similar
politic?

> > interested in any cooperation? I have some ideas to improve GMSH. What
> 
> Of course! Don't hesitate to contact us with ideas, remarks, etc.

I would like to focus at OpenGL. It seems you use OpenGL immediate mode
in many cases. I would like to rewrite it with OpenGL display lists or
vertex arrays. I know it isn't so easy in project like GMSH. We have
same problems in Blender. We still use OpenGL immediate mode, becuase
Blender has old design (should be rewritten in blender2.5 though).

BTW: I created wiki page (czech language) for my colleagues. They should
write there some ideas about improving GMSH:

https://www.nti.tul.cz/wiki/WikiUser:Jiri.Hnidek/GMSH_Ideas

> Cheers,
> 
> Christophe

Cheers,

Jiri
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