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Distorted orthographic world


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#1 c_leigh   Members   -  Reputation: 122

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Posted 13 April 2000 - 11:49 AM

Let''s say my OpenGL window is 640x480, and I have an orthographic view setup using glOrtho() with a distance of 1000 from left to right, 10 from top to bottom, and 1000 from near to far. At (0, 0, 0) I have a sphere rendered. How can I keep my scene from looking like it has an oval in the center, instead of a sphere? Thanks, Chris O.

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#2 deathlok   Members   -  Reputation: 122

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Posted 13 April 2000 - 12:06 PM

In order to draw a correct sphere, you''ll most likely need to change your glOrtho() settings to reflect the aspect ratio of your screen.
If your screen is 640:480, your aspect ratio is (640/480), thus your Ortho distances will have to reflect that ratio. Assuming you want to keep 1000 for your left/right distance, you''ll need 750 for your top/bottom distance.
Because: 640/480 = 1000/750

Hope that helps.

Deathy

#3 c_leigh   Members   -  Reputation: 122

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Posted 13 April 2000 - 12:52 PM

Maybe I'm going about this the wrong way then.

What I've done in my program is create spheres of planets in our solar system, all accurately placed. For example, the sun is at (0, 0, 0), Mercury is at (57894375.69, 0, 0), Venus is at (108159260.01, 0, 0), and so on. The x coordinates are the distances, in kilometers, that the planets are away from the sun. In setting up a glOrtho() view as I showed in my prior post, I was attempting to view all the planets and sun in on the screen at one time, only in my program the glOrtho() settings would have been quite larger than the values I used for the example.

So, now my question is, how should I view my model of the solar system in OpenGL, keeping in mind that the farthest object from the Sun is Pluto at a distance of 5913603801.1.

Thanks!
Chris O.

Edited by - c_leigh on 4/13/00 6:59:50 PM

#4 Bad Monkey   Members   -  Reputation: 145

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Posted 13 April 2000 - 01:14 PM

Hi c_leigh,

I've always found that its best to use a perspective projection when modelling the solar system... it just looks more realistic

You should use an orthographic projection for stuff like CAD and apps where you where you want a more accurate representation of a small space , or you're doing a blueprint-type representation... so this is not really true of a solar system (but its up to you).

The way I would set up a projection for a solar system model would be:

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(theta, screenWidth/screenHeight, 0.1, 100.0);

glViewport(0,0,screenWidth,screenHeight);

Theta is the field of view (in degrees)... have a play with this to see what you like best (I think it has to be under 180). screenWidth and screenHeight are obviously, in your case, 640 and 480. The next number is the distance from the viewer to the front clipping plane... (as a general rule, I keep this small). The final number is the one you'll have to change a fair bit I think... its the distance from the viewer to the far clipping plane. Because you're using such huge numbers for your vertex positions, you'll need this to be pretty huge as well, so that when you move back to see all your planets, they don't get clipped away.

Which brings me to being able to see them all at once... to see all your planets at once, I would translate the viewer back along the z-axis until you can see them all (you'll have to tweak this yourself... this also will be a pretty large number). So in your draw function:

glClear(....);

//make sure your matrix mode is modelview, OK

//translate viewer/camera
glTranslatef(0.0,0.0, -zDistance);

glPushMatrix(); //save matrix state
DrawSolarSystem();
glPopMatrix(); //restore matrix state

glFlush() / SwapBuffers() / whatever


I hope that was helpful... gotta go now, my boss wants to see my finished project (prepare for a major crash session... only happens when you give a demo )

-------------
squirrels are a remarkable source of protein...

Edited by - Bad Monkey on 4/13/00 7:18:14 PM

#5 Anonymous Poster_Anonymous Poster_*   Guests   -  Reputation:

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Posted 13 April 2000 - 01:39 PM

i would make the distances between the planets much smaller but still to scale.

there is no need to use their *true* distances since you are scaling everything to fit within your monitor.

think about it



#6 Demon Lord   Members   -  Reputation: 122

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Posted 14 April 2000 - 10:48 AM

I think you should scale your planetary system in astronomical units (Earth -> Sun = 1) instead of kilometers.
My opinion.

Eric Laberge




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