[gameprogrammer] Re: OpenGL... Where are the calculations made?

On Wed, May 7, 2008 at 10:21 AM, richard sabbarton <
richard.sabbarton@xxxxxxxxx> wrote:

> Hi Bob,
>
> Thanks... That is exactly what I needed to know/hear.  I have just
> started working on a 3D Space Shooter type project and there are many
> reasons for me to make calculations based on the current camera/player
> position.  I also need to make the calculations for other ships as
> well.
>
> Each ship (including camera/player) is defined by its location,
> direction vector and up vector (to avoid gimbal lock).  Each movement
> will require motion along or rotation around an axis based on the
> orientation.  Additionally, I will need to be able to find a point
> relative to the ships orientation and origin.  An example would be the
> starting point of a missile fired.  It may be below and to the side of
> the players position and not at the position itself.


May I make a suggestion? Instead of define the location and orientation of
your ships the way you are doing it, define them using a 4x4 matrix. Keep
all the ship orientation and position information in the form of a 3D
transformation matrix. All the same information is in the matrix, but you
will find that there are any number of libraries for rotating and
translating those matrices. The matrix routines will usually take advantage
of vector multiplication instructions in modern CPUs so they are very fast.
And, doing things like finding a point offset from the current location of
the ship is as simple as copying the matrix and translating it. The
translation will be relative to the ships current location and the new
matrix can be used to represent the position and orientation of a missile,
for example.

Modern CPUs have tremendous floating point and vector arithmetic
capabilities.

Bob Pendleton


>
>
> Regards
>
> Richard
>
> 2008/5/7 Bob Pendleton <bob@xxxxxxxxxxxxx>:
> >
> >
> >
> > On Wed, May 7, 2008 at 2:32 AM, richard sabbarton
> > <richard.sabbarton@xxxxxxxxx> wrote:
> > > Hi Guys,
> > >
> > > I have another question regarding OpenGL and I am hoping you can clear
> > > something up for me.  If I run matrix manipulation functions using
> > > OpenGL (e.g. glTranslate(), glRotate(), gluLookAt(), etc.) I am
> > > assuming that the calculations required to perform these functions are
> > > performed by the GPU on the Graphics adapter rather than the CPU.  Is
> > > this correct?  Does that mean that the glMatrix is stored in the
> > > memory of the graphics adapter or in my system RAM?  I am asking
> > > because I assume that the GPU can process these calculations quicker
> > > and more efficiently than the CPU can.
> >
> > It is implementation dependent, but for any modern graphics add
> >
> > >
> > >
> > > For example, if I want to calculate a cross-product or face normal I
> > > can use sin(), cos(), etc to perform the calculations but, IMO, these
> > > can be quite costly as far as CPU time.  So what if, instead of using
> > > sin() & cos() I was to use glXxxx functions instead.  I could load an
> > > identity matrix and then use gluLookAt() to look at point A and supply
> > > point B as an Up Vector. I can then translate 1.0 along the X axis
> > > and pluck my normalised cross-product from the matrix.  No sin(),
> > > cos() or sqrt() in sight.  Do you think this will be quicker?  Also, I
> > > am not sure about the overhead of pulling the values from the Matrix
> > > as (another assumtion) would this involve copying the matrix from the
> > > Graphics Memory into the System Memory?
> >
> > Do not do that. The cost of moving the data back and forth from system
> > memory and the GPU is very high and is much more costly than just doing
> the
> > computation you want in the CPU. If the math your are doing is slowing
> you
> > project down, then you should first see if the math can be done faster in
> > the CPU. You can often skip many steps in your math and speed it up a
> lot.
> > In some cases you can't and then you might want to look at shader
> > programming to solve your problem.
> >
> > Here is a suggestion: instead of asking if and idea is a good idea for
> > solving a low level problem, step back a bit and ask us how to solve your
> > high level problem. We can be a lot more helpful when we know what you
> are
> > trying to do rather than just who you are trying to do it.
> >
> > Bob Pendleton
> >
> > >
> > >
> > > Regards
> > >
> > > Richard
> > >
> > > ---------------------
> > > To unsubscribe go to http://gameprogrammer.com/mailinglist.html
> > >
> > >
> > >
> >
> >
> >
> > --
> >
> > + Bob Pendleton: writer and programmer
> > + email: Bob@xxxxxxxxxxxxx
> > + web: www.GameProgrammer.com
> > + www.Wise2Food.com
> >
> > +--------------------------------------+
>
> ---------------------
> To unsubscribe go to http://gameprogrammer.com/mailinglist.html
>
>
>


-- 

+ Bob Pendleton: writer and programmer
+ email: Bob@xxxxxxxxxxxxx
+ web: www.GameProgrammer.com
+ www.Wise2Food.com

+--------------------------------------+

Other related posts: