Re: [osg-users] Finding the edge of a frustum, or intersection with the edge of a frustum

[Brad Colbert]

Hi,

I thought I would post my solution for finding the intersection of circles with 
a rectangle.  Used to test were circles are intersecting the edge of my Ortho 
projection.


Code:
bool isPointOnSegment(const osg::Vec3d& p1, const osg::Vec3d& p2, const 
osg::Vec3d& point)
{
// First test that the point is co-linear
osg::Vec3d v1 = p2 - p1;
osg::Vec3d v2 = point - p1;

// Cross product
double z = v1.x() * v2.y() - v1.y() * v2.x();

if(z)
return false;

// Dot product will tell us if it's between the points
double dp1 = v1 * v1;
double dp2 = v1 * v2;

// On end
if((dp1 == 0) || (dp1 == dp2))
return true;

// On Segment
else if((dp1 > 0.0) && (dp2 < dp1))
return true;

return false;
}

//
template  int sgn(T val) {
return (T(0) < val) - (val < T(0));
}
std::list findLineCircleIntersections(const osg::Vec3d& p1, const 
osg::Vec3d& p2, const osg::Vec3d& center, const double r)
{
std::list points;
double dx, dy, A, B, C, dis, t;

dx = p2.x() - p1.x();
dy = p2.y() - p1.y();

A = dx * dx + dy * dy;
B = 2 * (dx * (p1.x() - center.x()) + dy * (p1.y() - center.y()));
C = (p1.x() - center.x()) * (p1.x() - center.x()) +
(p1.y() - center.y()) * (p1.y() - center.y()) -
r * r;

dis = B * B - 4 * A * C;

if(dis < 0.0)
{
// No intersection
}
else
{
// Two solutions.
double t1 = (float)((-B - sqrt(dis)) / (2 * A));
double t2 = (float)((-B + sqrt(dis)) / (2 * A));

double x1 = p1.x() + t2 * dx;
double y1 = p1.y() + t2 * dy;
osg::Vec3d point1(x1, y1, p1.z());

if (dis == 0.0) // Tangent
{
// Validate
if(isPointOnSegment(p1, p2, point1))
points.push_back(point1);
}
else if (dis > 0.0)
{
double x2 = p1.x() + t1 * dx;
double y2 = p1.y() + t1 * dy;
osg::Vec3d point2(x2, y2, p1.z());

// Two intersections
if(isPointOnSegment(p1, p2, point1))
points.push_back(osg::Vec3d(x1,y1,p1.z()));
if(isPointOnSegment(p1, p2, point2))
points.push_back(osg::Vec3d(x2,y2,p1.z()));
}
}

return points;
}

std::list findRectangleCircleIntersections(const osg::Vec3d& s1p1, 
const osg::Vec3d& s1p2,
   const osg::Vec3d& s2p1, 
const osg::Vec3d& s2p2,
   const osg::Vec3d& s3p1, 
const osg::Vec3d& s3p2,
   const osg::Vec3d& s4p1, 
const osg::Vec3d& s4p2,
   const osg::Vec3d& 
center, const double r)
{
std::list points;

std::list s1points = findLineCircleIntersections(s1p1, s1p2, 
center, r);
points.insert(points.end(), s1points.begin(), s1points.end());

std::list s2points = findLineCircleIntersections(s2p1, s2p2, 
center, r);
points.insert(points.end(), s2points.begin(), s2points.end());

std::list s3points = findLineCircleIntersections(s3p1, s3p2, 
center, r);
points.insert(points.end(), s3points.begin(), s3points.end());

std::list s4points = findLineCircleIntersections(s4p1, s4p2, 
center, r);
points.insert(points.end(), s4points.begin(), s4points.end());

return points;
}




Cheers,
Brad

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Read this topic online here:
http://forum.openscenegraph.org/viewtopic.php?p=76910#76910





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Re: [osg-users] Finding the edge of a frustum, or intersection with the edge of a frustum

[Glenn Waldron]

Brad,
You take the plane that defines the left side of the view frustum; then you
intersect your line with that plane.

I find this is easiest to do in normalized clip space (which goes from
[-1..1] in each dimension). The left plane will be defined as
point=(-1,0,0), normal=(1,0,0). You can convert your line's points into
clip space by multiplying each point by the (modelview matrix * projection
matrix).

I successfully used this approach for graticule labeling in osgEarth. Good
luck!

Glenn Waldron / osgEarth


On Fri, Nov 8, 2019 at 3:44 PM Brad Colbert  wrote:

> Hi,
>
> I'm using an ortho frustum to draw a map.  On the map I'm drawing a kind
> of range ring / sector pattern, starting from an origin using lines.  I
> want to draw text at the edge of the viewport where the lines intersect
> with the edge (well, just inside so the text is viewable).
>
> The problem I'm having is I can't quite figure out how to tell where the
> lines end within the frustum.  Any ideas would be welcomed.
>
> Picture for context is attached.
>
> Thank you!
>
> Cheers,
> Brad
>
> --
> Read this topic online here:
> http://forum.openscenegraph.org/viewtopic.php?p=76886#76886
>
>
>
>
> Attachments:
> http://forum.openscenegraph.org//files/mapview_458.png
>
>
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