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# opengl perspective and orthogonal

By using the function
glortho( 0,0,maxX,maxY,0,maxZ) I can define the viewing volume in my viewport....
I have set the option in the user interface window  to select either ortho view or perspective view..... My coordinate axes x,y,z  lie at the lower left corner of the graphical window in orthographic view...

the problem is if the user selects the perspective view....the picture disappears and it is somewhere in the graphics window.....  and the coordinate axes comes to the center.....
glperspective(45, w/h,1.0, 12000);

is there anything I have  to improve the relation between ortho and perspective....

Thanking you

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jsbsudha
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3 Solutions

Commented:
>> and the coordinate axes comes to the center.....

If that's the only problem, you can simply do a translation ... no ?
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Commented:
either do what i8 suggested or put the origin {0,0,0} in ortho-view as well in the center of the screen, this can be done by:

glortho( -width*0.5f, width*0.5f, height*0.5f, -height*0.5f, 0, maxZ)

ike
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Commented:
but anyway a good solution would be to give your client/customer a button in the gui to center the object(s) into the window .. otherwise they're gonna be frustrated very soon, searching for the object .. ;)
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Graphics ExpertCommented:
Ortho is parallel projection, thus resulting in constant size of projected objects, regardless the distance between objects and the projection plane.

Perspective is conic projection, thus resulting in variable size of projected objects, dependent of distance between objects and the projection plane.
The figure compares the methods.

Althought obvious, the construction methods differences are reffered to suggest pre-sets such that the results are coherent in terms of projected sizes.

Probably the distance between objects and projection plane are large enough to make conic projection so small when compared to the parallel projection (where distance doesn't influentiate in the projection size). By adjusting proper distances, both methods can result in approximated similar sizes.

Jose
ortho-perspective.GIF
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Graphics ExpertCommented:
As you have different positions of the orign (lower left for ortho and center for perspective), what you see in ortho is the upper right quadrant (just positive x's and y's) and in perspective, all the four quadrants.

Also, the combination of:
distance object-viewport   +
small viewport   +
different orign positions
can produce what you describe. In the below figure, we notice that the blue object doesn't appear in the viewport (in red) when the method is perspective.

Jose
ortho-perspective2.GIF
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Author Commented:
thank you for explaining me clearly about the perspective and orthogonal....  I  would like to consider your answer..... Before that my application suddenly shows the runtime exception error...I am in urgent condition to solve the error......I am going to post that question now....
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Author Commented:
what is the solution for it?I want to make my perspective projection  to start from lower left corner.......like my orthographic projection.....
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Commented:
See the first few posts ;)
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Author Commented:
I don't want to move my orthographic view to center........
may be I can do translation but.........please wait

i willc ome again
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