[osg-users] Camera intrinsics

benedikt naessens benedikt.naessens at spaceapplications.com
Wed Apr 6 04:36:40 PDT 2011


Keith Parkins wrote:
> After looking at this again, I am unclear as to whether you have built
> the projection matrix from the intrinsic parameters. I was assuming that
> you had made it by hand. To do that you would do something like this:
> 
> //-----------------------------------------------------------------------------// 
> // The _intrinsic variable holds the five values for the intrinsic
> // matrix. The intrinsic * our pixel transform to get the projections
> // matrix. The intrinsic values (itr[]) are given as five doubles
> // such that: 
> //
> //    | itr[0] itr[1] itr[2]  |
> //    |   0    itr[3] itr[4]  |
> //    |   0      0      0     |
> //
> //    | alpha_u gamma u_0  |
> //    |   0    alpha_v v_0 |
> //    |   0      0      0  |
> //
> //-----------------------------------------------------------------------------//
> 
> void 
> -Camera::calcProjection() {
> double alpha_u, alpha_v;
> 
> // calc alphas
> alpha_u = _intrinsic[0];
> double cot_theta = - _intrinsic[1]/_intrinsic[0];
> double sin_theta = sqrt(1/(1+cot_theta*cot_theta));
> alpha_v = _intrinsic[3] * sin_theta;
> 
> double skew = _intrinsic[1];
> 
> double u0, v0;
> u0 = _intrinsic[2]; v0 = _intrinsic[4];
> 
> double left = -u0 / alpha_u * _near;
> double bottom = (_screen_height-v0) / alpha_v * _near;
> double right = (_screen_width - u0) / alpha_u * _near;
> double top = -v0 / alpha_v * _near;
> 
> _projection[0] = 2 * _near / (right - left);
> _projection[4] = 2 * _near * skew / ((right - left) * alpha_u);
> _projection[5] = 2 * _near / (top - bottom);
> _projection[8] = -(right + left)/(right - left);
> _projection[9] = -(top + bottom)/(top - bottom);
> _projection[10] = (_far + _near) / (_far - _near);
> _projection[11] = 1;
> _projection[14] = -2 * _far * _near/(_far-_near);
> 
> |}
> 
> 
> /lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
> 
> 

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Post generated by Mail2Forum[/quote]

There seems to be a mismatch between the projection matrix generated by e.g. makePerspective and the projection matrix elements you suggest here.

When I do the comparison between the two, it seems that: 
_projection[5], _projection[8], _projection[9] and _projection[11] have opposite signs. 

I'm also a bit surprised that the projection matrix generated by OSG (makePerspective) has -1 instead of 1 for _projection[11]. Maybe that has something to do with the inversion of the Y axis (0,0 in the bottom left instead of the top left) ?

Can you give me a suggestion why these differences exist ?

Thanks !
Benedikt.

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