[Clusterviz-devel] pan-scale, gluPerspective, refactoring

[<K.W...@gm...>]

Hi,
yesterday I tried again to figure out the right scale value and I failed
again. I think I will wait until I know more about projection matrices and
OpenGL before I try again. However, here is what I found out:

If you call:
gluPerspective(angle, aspect, near, far)
P= glGetFloatv(GL_PROJECTION_MATRIX)

then for all reasonable values of near and angle, it holds

P(0,0,0) == (0,0,-1)
P(tan_degree(angle/2)*aspect, 0,0) == (1,0,-1)
P(0,tan_degree(angle/2),0) == (0,1,-1)

I assumed here that tan interprets its arguments as degree, so using
math.tan would involve some scaling. The right hand sides are results of the
matrix-by-vector-multiplication. I used this function to do it, but I am not
sure if it is the right way:

def apply_matrix(A,v):
  if len(v)==3: v.append(1.)
  res=[]
  for zeile in A:
    res.append(sum([a*vi for (a,vi) in zip(zeile,v)]))
  if res[-1]!=0:
    for i in range(len(res)): res[i]= res[i]/res[-1]
  return res[:-1]

I have no clue how to use the projection matrix to synchronize the mouse
movement and the translation of the scene. I thought the view plane is
perpendicular to the z-axis, its center being at z=-near, but since you said
it's only a clipping parameter, I know I was wrong... 

====
I spotted in the TODO file you'd like an orthogonal perspective, too. I
think this would require a rewrite of the zoom function. Maybe using
gluLookAt instead? And then the scale parameter of the pan function would be
different, too.
====
What I like to do on Sunday, is to divide the clusterdisplay class into two
classes, one for the window-handling (keyboard and mouse binding,
reshaping,..) and one for drawing the data points, cluster centres etc.
Please stop me if you don't want this.
====
One small change to trackball.py I comitted: Now gluPerspective is called at
the program start and when resizing the window, too. Hope everything went
right with svn.

Regards,
Karsten.
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