| Re: idl obj gaphics and opengl - how similar? [message #32885] |
Fri, 15 November 2002 15:31 |
Karl Schultz
Messages: 341
Registered: October 1999
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Senior Member |
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"David Fanning" <david@dfanning.com> wrote in message
news:MPG.183de0975833db61989a1d@news.frii.com...
> R.G. Stockwell (sorry@noemail.now) writes:
>
>> I was just wondering...
>> how similar are the IDL object graphics functions,
>> and the OpenGL library?
IDL Object Graphics is built on top of OpenGL, but OG wasn't specifically
designed to be similar to OpenGL. OG can theoretically be built on top of
other graphics packages. It has a VRML "driver", so that is an existance
proof of the possibility, although VRML is pretty similar to OpenGL anyway
(most VRML viewers are written in OpenGL). The idea here is to be able to
move OpenGL to another graphics system if we had to. A few years ago, the
life expectancy of OpenGL on IDL platforms wasn't very optimistic. But now
OpenGL is pretty solid, especially now that it is integrated into XFree86
and no other graphics package is making any bids for the cross-platform
package of choice. We try to be careful about exposing too much
OpenGL-specific function in IDL OG, in case we someday need to move to
another package that might not have these features.
The amount of similarity between an IDL OG object and OpenGL depends on the
object. The IDLgrPolyline object really doesn't do too much more than store
the vertex and related data. It's Draw method basically just submits the
vertex and other data to OpenGL . OTOH, an object like IDLgrSurface has
tons of code to implement all that surface functionality, including stuff
like Lego mode and all that. At the end of the day, it just submits lines
and polygons to OpenGL, but there was a bit of work involved to come up with
these lines and polygons. In other words, there is no "surface" OpenGL
procedure. The IDL "surface" needs to be broken down into small convex
polygons for OpenGL .
>> i.e. if one knows how to do objects gaphics in IDL,
>> will they be (reasonably easily) be able to do similar
>> things in c and opengl?
OpenGL is a very procedural graphics library and therefore doesn't really
have the concept of objects. So a LOT of the code in OG is in the object
implementation, which is primarily storing the graphical data and the
graphic attributes related to the data. A big part of OG is also in the
"Draw" code which draws all the Views, Models, and atoms in your graphics
tree. These things aren't strictly necessary for a C/OpenGL program to get
just something on the screen, but are needed to present a framework for more
complex scenes. It is also all there so you can just call Draw to draw the
entire scene without walking through all your data each time.
So, yeah, if you don't need the object structure and have fairly simple
data, it isn't that hard to get an image on the screen with C/OpenGL . You
have to set up your view and coordinate system, based on your data. You
have to walk through your data, passing OpenGL the information on a vertex
by vertex basis. Pretty easy so far. But if you want to get interactive or
start drawing things like surfaces or contours, you're in for a lot of code.
> I don't really know the answer to this, because
> I've never tried to write an OpenGL-type of
> program. But I do spend quite a bit of time with
> my nose in an OpenGL-oriented graphics book trying
> to figure out what the heck is going on in object
> graphics.
Yes, this can help a lot.
> My impression is that as low-level as IDL's object
> graphics API is, it is still at a higher level than
> OpenGL. At least I'm always thinking to myself, "Oh,
> I see how IDL does that!", and I'm always glad I don't
> have to do it myself.
Right. Perhaps the biggest things that IDL OG adds to OpenGL are the
"graphics tree" (Views, Models, etc), the higher function primitives, and
persistant/retained nature of the objects.
Karl
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| Re: idl obj gaphics and opengl - how similar? [message #32900 is a reply to message #32885] |
Fri, 15 November 2002 10:11  |
Pavel A. Romashkin
Messages: 531
Registered: November 2000
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Senior Member |
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Now, this is no gentle humor anymore. This is plane sarcasm! :-)
BTW, if you look at a few posts by Karl, you will find some pointers at
how is OGL used in IDL OG. And I would not say I have a clue other than
"I don't want to know".
Cheers,
Pavel
"R.G. Stockwell" wrote:
>
> Hey, when that IDL to C++ translator is finished (from
> another poster), that would be a great way to
> generate C++ OpenGl code. ;)
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| Re: idl obj gaphics and opengl - how similar? [message #32902 is a reply to message #32900] |
Fri, 15 November 2002 08:41 |
R.G. Stockwell
Messages: 363
Registered: July 1999
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Senior Member |
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David Fanning wrote:
> R.G. Stockwell (sorry@noemail.now) writes:
>
>
>> I was just wondering...
>> how similar are the IDL object graphics functions,
>> and the OpenGL library?
>>
>> i.e. if one knows how to do objects gaphics in IDL,
>> will they be (reasonably easily) be able to do similar
>> things in c and opengl?
>
>
> I don't really know the answer to this, because
> I've never tried to write an OpenGL-type of
> program. But I do spend quite a bit of time with
> my nose in an OpenGL-oriented graphics book trying
> to figure out what the heck is going on in object
> graphics.
>
> My impression is that as low-level as IDL's object
> graphics API is, it is still at a higher level than
> OpenGL. At least I'm always thinking to myself, "Oh,
> I see how IDL does that!", and I'm always glad I don't
> have to do it myself.
>
> Cheers,
>
> David
>
Ah, openGL sounds deep and tedious. Perfect!
Your description is much as I expected, in that IDL
is similar, but higher level.
Hey, when that IDL to C++ translator is finished (from
another poster), that would be a great way to
generate C++ OpenGl code. ;)
Thanks,
bob stockwell
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| Re: idl obj gaphics and opengl - how similar? [message #32904 is a reply to message #32902] |
Fri, 15 November 2002 07:48 |
James Kuyper
Messages: 425
Registered: March 2000
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Senior Member |
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"R.G. Stockwell" wrote:
>
> Hi All,
> I was just wondering...
> how similar are the IDL object graphics functions,
> and the OpenGL library?
>
> i.e. if one knows how to do objects gaphics in IDL,
> will they be (reasonably easily) be able to do similar
> things in c and opengl?
C itself doesn't have any graphics capabilities. There are several
different graphics systems that have C interfaces, (and OpenGL is one of
them), but you need to specify the library, not the C language, as the
point of reference.
That said, I bow out: I know very little about using those libraries. I
can say that IDL typically operates at a higher level than those
libraries, with the typical ease-of-use vs. performance trade-offs that
apply when you change levels that way.
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| Re: idl obj gaphics and opengl - how similar? [message #32913 is a reply to message #32904] |
Thu, 14 November 2002 15:32 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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R.G. Stockwell (sorry@noemail.now) writes:
> I was just wondering...
> how similar are the IDL object graphics functions,
> and the OpenGL library?
>
> i.e. if one knows how to do objects gaphics in IDL,
> will they be (reasonably easily) be able to do similar
> things in c and opengl?
I don't really know the answer to this, because
I've never tried to write an OpenGL-type of
program. But I do spend quite a bit of time with
my nose in an OpenGL-oriented graphics book trying
to figure out what the heck is going on in object
graphics.
My impression is that as low-level as IDL's object
graphics API is, it is still at a higher level than
OpenGL. At least I'm always thinking to myself, "Oh,
I see how IDL does that!", and I'm always glad I don't
have to do it myself.
Cheers,
David
--
David W. Fanning, Ph.D.
Fanning Software Consulting, Inc.
Phone: 970-221-0438, E-mail: david@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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