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Re: IDL and OpenGL [message #17299 is a reply to message #17065]

Wed, 29 September 1999 00:00

ushomirs
Messages: 14
Registered: May 1993

Junior Member

I think what SGI folks meant is that IDL routines are **not** compiled
into **machine** code, the way C programs are. So even an IDL routine
is "compiled", it is transformed into a bytecode for it's own virtual
machine. So unlike compiled C code, where the processor executes your
program directly, the processor executes the code for the "IDL virtual
machine", which in turn reads your program (or the "compiled" bytecode)
and interprets it step by step. So there is an extra level of
indirection involved in running IDL code. that's what the SGI people
were refering to.

If IDL code were truly compiled in the same sense as C code, then
this loop
FOR i=0,9 do foo[i]=2.*foo[i]

would take exactly as long as

foo=2.*foo

It doesn't, because the IDL "virtual machine" has to interpret every
iteration of the FOR loop, but it can just map the vector operation onto
a machine code for loop.

Now, in case of OpenGL, this distinction is a wash. So long as most of
the time is spent inside OpenGL, some slowness associated with
interpreting rather than executing machine code directly doesn't matter.

greg

THere's some discussion of this in IDL's external development guide.

In this respect IDL is much more like Java, where th

In article <37F22743.52AC5811@ssec.wisc.edu>,
Liam Gumley <Liam.Gumley@ssec.wisc.edu> wrote:
> Richard Tyc wrote:
>> Our group was having a meeting with some SGI folks yesterday and
some
>> interesting points were brought up which I hope some IDL experts
could shed
>> some light on. We were discussing the performance of IDL vs. OpenGL
source
>> based app.. Their point was that IDL is an interpreted language and
as such
>> is not optimized (or tuned as they put it) for OpenGL and thus runs
>> significantly slower than a custom C app using OpenGL.
>
> IDL does indeed run in an interpreting mode (at the command line),
where
> each statement is interpreted and executed separately. However once
you
> 'compile' an IDL procedure, it is no longer 'interpreted' each time it
> is called.
>
> Perhaps someone in the know could give us a brief description of how
IDL
> procedures are transformed from source code to CPU instructions.
>
> Cheers,
> Liam.
>
> --
> Liam E. Gumley
> Space Science and Engineering Center, UW-Madison
> http://cimss.ssec.wisc.edu/~gumley
>

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[Message index]

		IDL and OPENGL By: enric on Mon, 06 September 1999 00:00
		Re: IDL and OpenGL By: ushomirs on Wed, 29 September 1999 00:00
		Re: IDL and OpenGL By: Richard Tyc on Wed, 29 September 1999 00:00
		Re: IDL and OpenGL By: ushomirs on Wed, 29 September 1999 00:00
		Re: IDL and OpenGL By: Liam Gumley on Wed, 29 September 1999 00:00
		Re: IDL and OpenGL By: Karl Schultz on Fri, 08 October 1999 00:00
		Re: IDL and OpenGL By: Karl Schultz on Wed, 31 October 2007 09:29
		Re: IDL and OpenGL By: Steve Houston on Wed, 07 November 2007 06:28
		Re: IDL and OpenGL By: David Fanning on Wed, 31 October 2007 05:31
		Re: IDL and OpenGL By: Gaurav on Tue, 30 October 2007 22:38
		Re: IDL and OpenGL By: Gaurav on Tue, 30 October 2007 22:31
		Re: IDL and OpenGL By: Robbie on Tue, 30 October 2007 15:17
		Re: IDL and OpenGL By: Steve Houston on Wed, 07 November 2007 06:52
		Re: IDL and OpenGL By: David Fanning on Tue, 30 October 2007 05:50

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