pyopengl-users

[PyOpenGL-Users] glCallList & external data dependency

[Prashant S. <ani...@ya...>]

While doing couple of experiments with 'List' system I have got couple of doubts:

glNewList(...,GL_COMPILE)
glNewList....
print 'just for the test'
glEndList()

When I initialize the list, the print statement executes. Later calling this list doesn't print the 'print' statement. I did this to test whether I can include some
condition based drawing between 'glNewList' & 'glEndList'. These condition are based on some global variables. It seems 'List' instruction behaves like
hard coded. Am I right here?



      

Re: [PyOpenGL-Users] glCallList & external data dependency

[Mike C. F. <mcf...@vr...>]

Prashant Saxena wrote:
> While doing couple of experiments with 'List' system I have got couple 
> of doubts:
>
> glNewList(...,GL_COMPILE)
> glNewList....
> print 'just for the test'
> glEndList()
>
> When I initialize the list, the print statement executes. Later 
> calling this list doesn't print the 'print' statement. I did this to 
> test whether I can include some
> condition based drawing between 'glNewList' & 'glEndList'. These 
> condition are based on some global variables. It seems 'List' 
> instruction behaves like
> hard coded. Am I right here?
Display lists work by copying all of the *GL* commands you execute, 
including the data they execute across to the card's internal memory (or 
the GL's internal memory if your card doesn't have enough RAM).  As a 
result they cannot access your local (main memory) data.  The display 
list does not include any *Python* calls at all, only the GL-level calls.

As a note: display lists tend to be very slow on modern hardware 
(compared to array-based geometry).  They'll be faster than doing 
per-vertex rendering in Python, but you will likely want to use 
array-based operations rather than display lists if you want reasonable 
performance.

HTH,
Mike

-- 
________________________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://www.vrplumber.com
  http://blog.vrplumber.com

Re: [PyOpenGL-Users] glCallList & external data dependency

[Greg E. <gre...@ca...>]

Prashant Saxena wrote:
> I did this to test 
> whether I can include some
> condition based drawing between 'glNewList' & 'glEndList'. These 
> condition are based on some global variables. It seems 'List' 
> instruction behaves like
> hard coded. Am I right here?

Yes. A display list records a sequence of OpenGL calls and
the parameter values they had at the time of those calls,
and that's all. There is no way of performing conditional
branching within a display list.

If you want different sequences of operations each time,
you will need to record a separate display list for each
possible sequence, or break the sequence up into several
display lists that you can mix and match to get the
effect you want.

-- 
Greg

Re: [PyOpenGL-Users] glCallList & external data dependency

[Greg E. <gre...@ca...>]

Mike C. Fletcher wrote:

> As a note: display lists tend to be very slow on modern hardware 
> (compared to array-based geometry).  They'll be faster than doing 
> per-vertex rendering in Python, but you will likely want to use 
> array-based operations rather than display lists if you want reasonable 
> performance.

Maybe, maybe not. Speed of executing Python code may still
be the bottleneck in a Python-driven game, so one display
list call could be faster than several Python calls to
set up and execute a series of array-based operations.

As always, you should measure rather than making assumptions
where optimisation is concerned.

-- 
Greg