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OpenGL gluProject/gluUnProject

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Guest Anonymous Poster
use glOrtho projection for rendering axises

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That's not exactly helpful. Need a little more explanation than that, either pseudo code or whatever.

And if you mean draw it in 2D using glOrtho, then no, I don't think that is correct.

I think it's drawn in 3D, because the axes has depth, as you can see by the last image showing
the axes being rendered straight on. As well as the axes are clipped by the camera view, when
the camera is pointing somewhere else.

So, if these axes are drawn in 3D, how to determine the lengths of these axes in 3D?

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glOrtho isn't just used for 2D, it's an orthographic projection. Basically this means that objects don't get smaller when they get further away. But if that's not it, then there's another alternative.
It might be that it's simply done by calculating the distance to the object (can be done by retrieving the modelview matrix and checking the translation) and then just scaling based on this distance. Does that make sense?

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No, I don't believe the axes are drawn in glOrtho, because then the axes lines would be distorted.
(e.g. the lines wouldn't match with its perspective view grid lines). I am 99.9% certain it is drawn
in 3D, I just don't know how.

Quote:

It might be that it's simply done by calculating the distance to the object (can be done by retrieving the modelview matrix and checking the translation) and then just scaling based on this distance. Does that make sense?


No, not really. :)

The lines are drawn in 3D because:
1. Axes are clipped appropriately.
2. Axes are distorted appropriately.
3. Axes lines show depth as the camera is rotated around them.

The problem is how to calculate their length in 3D, while keeping their screen space 2D length relatively constant.

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Quote:
Original post by raydog
No, I don't believe the axes are drawn in glOrtho, because then the axes lines would be distorted.
(e.g. the lines wouldn't match with its perspective view grid lines). I am 99.9% certain it is drawn
in 3D, I just don't know how.

Ok, so it's drawn with perspective projection.

Quote:
Quote:

It might be that it's simply done by calculating the distance to the object (can be done by retrieving the modelview matrix and checking the translation) and then just scaling based on this distance. Does that make sense?


No, not really. :)

Ok... :)
Say you have the arrows showing the axis defined at the origin (i.e. so that the arrows meet at (0,0,0).)
You do the viewing transformation (gluLookAt or whatever) and then possibly a modelling transformation (if the arrows can be moved.)
Get the modelview matrix with glGetDoublev or glGetFloatv.
The position of the object relative to the viewer will be stored in the matrix, indices 12, 13 and 14. Use this to calculate the distance from the viewer to the arrows.
Call glScalef with this distance as scale.
Draw the arrows.
You will still see an effect of the perspective projection, but the arrows won't get smaller as they get farther away. Right?

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If I understand you correctly, you're suggesting that I use the distance from the camera's eye to the origin
of the local axis as function of scale?

I don't think that would work so good, as there can many be different local axes everywhere, and
they all would be at different distances from the camera's eye, giving me different scales.

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>>If I understand you correctly, you're suggesting that I use the distance from the camera's eye to the origin
of the local axis as function of scale?<<

yes in all the screenshots there is perspction (but very small i guess a FOV 40 degrees) so it seems to be in orthogonal projection.
so as someone suggested, draw the world (grid objects etc) and then for the 3 color axis just measure the distance from the origin to the camera's position and then draw the length of the lines of the axis based on that

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Quote:
Original post by raydog
I don't think that would work so good, as there can many be different local axes everywhere, and
they all would be at different distances from the camera's eye, giving me different scales.

So what? You will scale them differently, but since they are at different distances I fail to see what the problem is.

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Quote:
Original post by Rasmadrak
why not simply "AxisLength *= DistanceToCamera"

or something similar...?

Yes, that would be a scale.

raydog, why don't you simply try it? See how it looks - if it doesn't look good, we'll try to figure out something else.

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Each Camera has its own Rendering Pass... hence it will produce a different Axis length without affecting any other Camera.

Scaling with perspective and then un-scaling is a bit wastefull, how about just using orthographic projection in the first place?

Raydog, I think that you are mis-understanding some concepts of 3d graphics here: The objects that are drawn don't have to have any connection to actual 'things', you can arbitrarily scale or otherwise mess with coordinates as they enter the pipeline however you wish and it only applies to that one rendition (in fact this is the whole point behind shaders).

Hope that helps, ~SPH

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I did try it out:
FLOAT d = Sqrt_Distance( camera_position, axis_position );
d *= 0.125f; // too big, scale it down

// render axis line
glVertex3f( axis_position.x,
axis_position.y,
axis.position.z );

glVertex3f( axis_position.x + (d * axis_dir.x),
axis_position.y + (d * axis_dir.y),
axis_position.z + (d * axis_dir.z) );


But, I see noticeable change in axis length as I zoom in and out. It's not rock solid constant.
It's very bad up close too.

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btw, I do need to use this in a perspective view, so that's probably where the errors are coming from.
Every 3d modelling program I know uses something like this, lightwave, 3dsmax, etc... when you select
an object.

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heres what i do

float vector_len = LENGTHOFVECTOR( g_Program.scene.camera.camPOS - point );
float scaled_line_len = vector_len / line_length;

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There could be a precision problem (rounding/truncating) in your calculation of 'd' and so the length may vary. So instead of multiplying the axes length with the distance of the camera from the axes, I multiply it with the zoom factor. The length then remains "rock solid constant"

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I don't know what you mean by zoom factor. This is a perspective view. The camera I use has
a world space position, and that's what I use to setup the view. I use floating-point precision.

zedzeek,
If LENGTHOFVECTOR returns the square root distance, and line_length is a constant, then
that's exactly what I'm doing. d *= 0.125f, is just an optimization for d /= 8;

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Well, I guess that's as good as it gets.

If this is how everyone does it, then it is a lot faster then going through all that gluUnProject crap. :O

Thanks for the help.

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