# Polygon Position

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On 21/02/2010 at 05:47, xxxxxxxx wrote:

User Information:
Cinema 4D Version:   11.5
Platform:   Windows  ;
Language(s) :     C++  ;

---------
How do I determine the vector positions of the points that make up a given polygon?

~Shawn

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On 21/02/2010 at 06:00, xxxxxxxx wrote:

Have a look into const CPolygon* PolygonObject::GetPolygonR(void). It gives you an array of all polygons (CPolygon). Each Polygon contains the point indexes of the points it's made from.
Then use const Vector* PointObject::GetPointR(void) to get an array with all points contained in the object. To do your calculation, use the point indexes you got from the CPolygon.

Cheers,
Jack

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On 21/02/2010 at 06:03, xxxxxxxx wrote:

Thanks Jack,   I appreciate your response.

~Shawn

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On 21/02/2010 at 12:23, xxxxxxxx wrote:

So here's what I have so far.

``````

LONG lngPolygonCount = objPoly->GetPolygonCount();
LONG lngPointCount = objPoly->GetPointCount();
const CPolygon * selectedPoly = objPoly->GetPolygonR();
const Vector* points = objPoly->GetPointR();
LONG lngI;

for (lngI=0; lngI < lngPolygonCount; lngI++)
{
if (bsPoly->IsSelected(lngI))
{
GePrint ("First Polygon " + LongToString(lngI) + " is selected.");
lngA = selectedPoly[lngI].a;
lngB = selectedPoly[lngI].b;
lngC = selectedPoly[lngI].c;
lngD = selectedPoly[lngI].d;

}
}

``````

this gives me the index for each of the points...

so now would I do something like this....?

Vector pointLocation = Vector(points[lngA].x,points[lngA].y,points[lngA].z);

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On 22/02/2010 at 05:46, xxxxxxxx wrote:

What you do now depends on what you consider to be the "polygon's position". Is it a position centered between the polygon's points? In that case, add the positions of points a, b and c, then divide by 3.0. If it's not only a triangle but a quad (check if point c == d to find out about that), add a, b, c and d, then divide by 4.0.

Or is it the centered (or otherwise weighted) position on a triangle face? Then use barycentric coordinates (AFAIK this does only work with triangles).

Vector pointLocation = Vector(points[lngA].x,points[lngA].y,points[lngA].z);

This is not necessary. Since the points are stored as vectors (point positions) the following is sufficient: Vector pointLocation = points[lngA];

Cheers,
Jack

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On 22/02/2010 at 05:53, xxxxxxxx wrote:

Thanks Jack,  that helps a lot.  I think I've got it figured out thanks!

Cheers,
~Shawn

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On 22/02/2010 at 16:05, xxxxxxxx wrote:

Is it possible to get the rotation of a a particular polygon?

~Shawn

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On 22/02/2010 at 17:28, xxxxxxxx wrote:

or better yet,   how would I align my first object to the normal of the polygon that is selected?

~Shawn

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On 23/02/2010 at 02:38, xxxxxxxx wrote:

What you want to do is not clearly defined. Basically you have to create a rotation matrix for the polygon. For this you need at least two vectors. One is the normal, the other one has to be defined by you. For instance you could choose the vector pointing from the polygon's midpoint to its first vertex. Or you choose a global vector.

cheers,
Matthias

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On 23/02/2010 at 02:54, xxxxxxxx wrote:

I want to move one object and have it sit on the polygon of another object and have it rotate to the normal of that polygon so that the object is sitting flush with that polygon.   SO I will need the center between all of the points of that polygon (which I have) and a point in space that points directly outward from that polygon?  There is no function in the sdk that provides the normal of a polygon?

so the vector from the center to the first point would create a plane upon which I could determine rotation?

how would I get that?

Here's how I determine the center of the polygon

``````
//Caclulate the polygon position.
if (selectedPoly[lngI].c == selectedPoly[lngI].d)
{
polyLocation = (points[lngA] + points[lngB] + points[lngC]) / 3; //Polygon is a Triangle

}
else
{
polyLocation = (points[lngA] + points[lngB] + points[lngC] + points[lngD]) / 4; //Polygon is not a Triangle
}

``````

how would I set up the rotation matrix with this info if points[lngA]  is the first point of the polygon?

Thanks,

~SHawn

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On 23/02/2010 at 15:44, xxxxxxxx wrote:

Okay so I want to set up a rotation matrix for the selected polygon.  do I do this by mulitplying the vector of one point by the vector of a second point in the polygon?

~Shawn

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On 23/02/2010 at 15:49, xxxxxxxx wrote:

Here's what I have so far......

``````

C4DObjectList * objList = C4DObjectList::Alloc();
if (!objList) return FALSE;

//DECLARATIONS & DEFINITIONS
if (!doc) return FALSE;

BaseObject *op = doc->GetActiveObject();
if(!op) return TRUE;

PolygonObject *objPoly = ToPoly(op);
if (!objPoly) return FALSE;

BaseSelect *bsPoly = objPoly->GetPolygonS();
BaseSelect *bsPoint = objPoly->GetPointS();
BaseSelect *bsEdge = objPoly->GetEdgeS();

Real dx, dy, len = data.GetReal(1, 0.0);
BaseContainer device;
Real mousex = msg.GetLong(BFM_INPUT_X);
Real mousey = msg.GetLong(BFM_INPUT_Y);
win->MouseDragStart(KEY_MLEFT, mousex, mousey, MOUSEDRAG_NOMOVE);

LONG lngEditorMode = doc->GetMode();

AutoAlloc<ViewportSelect> vps;
if(!vps) return FALSE;

LONG left, top, right, bottom, width, height;
bd->GetFrame(&left, &top, &right, &bottom);
width = right - left + 1;
height = bottom - top + 1;

if(!vps->Init(width, height, bd, op, doc->GetMode(), TRUE, VIEWPORT_IGNORE_HIDDEN_SEL)) return FALSE;

LONG lngX = mousex;
LONG lngY = mousey;

//DRAW CIRCLE TO SHOW HOTSPOT

while (win->MouseDrag(&dx, &dy, &device) == MOUSEDRAG_CONTINUE)
{
if (dx == 0 && dy == 0)
continue;

lngX += dx;
lngY += dy;

vps->ShowHotspot(win, lngX, lngY);
}

vps->ShowHotspot(win, lngX, lngY);

//SELECT THE POLYGON THAT IS CLICKED

BaseContainer state;
LONG lngKey = state.GetLong(BFM_INPUT_QUALIFIER);
Bool shiftDown = GetInputEvent(BFM_INPUT_KEYBOARD, state);
LONG lngPolygonCount = objPoly->GetPolygonCount();
LONG lngPointCount = objPoly->GetPointCount();
const CPolygon * selectedPoly = objPoly->GetPolygonR();
const Vector * points = objPoly->GetPointR();
LONG lngI;

Matrix opMatrix  = op->GetMg();

switch (lngEditorMode)
{
//POLYGONS MODE
case Mpolygons:
{
if(vpPoly)
{
//FIRST SELECTION
if (secondSelection == FALSE)
{
bsPoly->DeselectAll();
bsPoly->Select(vpPoly->i);
firstSelection = vpPoly->op;
secondSelection = TRUE;
}

//SECOND SELECTION
else if (secondSelection == TRUE)
{
bsPoly->DeselectAll();
bsPoly->Select(vpPoly->i);

for (lngI=0; lngI < lngPolygonCount; lngI++)
{
if (bsPoly->IsSelected(lngI))
{
lngA = selectedPoly[lngI].a;
lngB = selectedPoly[lngI].b;
lngC = selectedPoly[lngI].c;
lngD = selectedPoly[lngI].d;

//Caclulate the polygon position.
if (selectedPoly[lngI].c == selectedPoly[lngI].d)
{
polyLocation = (points[lngA] + points[lngB] + points[lngC]) / 3; //Polygon is a Triangle

}
else
{
polyLocation = (points[lngA] + points[lngB] + points[lngC] + points[lngD]) / 4; //Polygon is not a Triangle
}

//Align 1st Selection to the polygon of 2nd Selection (X AXIS)
if (polyLocation.x > 0)
{
firstSelection->SetRot(CalcFaceNormal(points,selectedPoly[lngI]) * opMatrix);

}

if (polyLocation.x < 0)
{
firstSelection->SetRot(CalcFaceNormal(points,selectedPoly[lngI]) * opMatrix);

}

//Align 1st Selection to the polygon of 2nd Selection (Y AXIS)
if (polyLocation.y > 0)
{
firstSelection->SetRot(op->GetRot());
}

if (polyLocation.y < 0)
{
firstSelection->SetRot(op->GetRot());
}

//Align 1st Selection to the polygon of 2nd Selection (Z AXIS)
if (polyLocation.z > 0)
{
firstSelection->SetRot(op->GetRot());
}

if (polyLocation.z < 0)
{
firstSelection->SetRot(op->GetRot());
}

bsPoly->DeselectAll();
}
}
secondSelection = FALSE;
}

op->Message(MSG_CHANGE);
}
``````

I am trying to get (firstSelection) to rotate to the normal of the selected polygon on (op).    I am using CalcFaceNormal() but that seems to return incorrect results.  Does anyone know how I could get this to rotate properly?

Thanks,

~SHawn

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On 24/02/2010 at 02:43, xxxxxxxx wrote:

I recommend to read up on Vector and Matrix arithmetics.

http://www.flipcode.com/documents/matrfaq.html

Also the CINEMA 4D R9.5 COFFEE documentation has a very good tutorial on using matrices.

Especially vector cross products are essential to build your own cutom rotation matrices.

I can not debug or write your plugin, sorry.

cheers,
Matthias

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On 24/02/2010 at 02:48, xxxxxxxx wrote:

Thanks for the link Matthias.   LOL...  I don't want you to write it for me.   Where's the fun in that.

~Shawn

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On 24/02/2010 at 07:31, xxxxxxxx wrote:

Great tutorial on Matrices.  Thanks for that.

~Shawn

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On 25/02/2010 at 03:47, xxxxxxxx wrote:

Is there a c++ equivalent to the function you mention in the tutorial GetMulP()?

or would I just use

firstSelection->SetPos(opMatrix * points[lngA]);

first selection being my object,  and points[lngA] being the vector location of the point I am trying to move the object to.

Thanks,

~Shawn

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On 25/02/2010 at 05:47, xxxxxxxx wrote:

Yes, multiplying a vector with a matrix transforms the vector into the matix' space.

The Matrix class has several overloaded operators for matrix-matrix and matrix-vector arithmetics.

cheers,
Matthias

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On 26/02/2010 at 09:38, xxxxxxxx wrote:

Okay... SO I think I kindof understand a rotation matrix now.   I need to find the vector from the center of the polygon which I did with

``````
polyLocation = (points[lngA] + points[lngB] + points[lngC] + points[lngD]) / 4;
``````

This gives me a vector that represents the very center of the polygon

Then I need another point along the same plane which I use points[lngA].  This is the first point in the polygon.

So mathematically that gives me A & B.   Assuming my equation is C = A x B.     So to get the point which represents the Z axis or the point hovering directly above the polygon face, I simply multiply

``````
Vector rotC =  polyLocation * points[lngA];
``````

And this should give me my rotation matrix correct?

So now my question is how do I then rotate my object to match that  rotation matrix?  Here is what I am trying.

``````
//ROTATION MATRIX

//GetPoints
Vector p1 = polyLocation;
Vector p2 = points[lngA];

//Contruct Matrix
Matrix rotMatrix;

rotMatrix.off = p1; //The base of the matrix
rotMatrix.v1 = !(p2 - p1); //X axis points toward the second point
rotMatrix.v2 = !(p2-p1%Vector(0,1,0)); //Y Axis is perpendicular to the X axis
rotMatrix.v3 = rotMatrix.v1 * rotMatrix.v2;

Vector scale = Vector(Len(rotMatrix.v1), Len(rotMatrix.v2), Len(rotMatrix.v3)); //Get Scale
Vector pos = rotMatrix.off; ///Get the Position

//Create a vector which contains the rotation matrix.
Vector rot1 = MatrixToHPB(rotMatrix);

firstSelection->SetPos(opMatrix * polyLocation);
firstSelection->SetRot((rot1));

``````

Please let me know if I am way off base here and am going about this completely wrong so that I know not to continue in this way.    Thanks lot for your help.

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On 01/03/2010 at 06:22, xxxxxxxx wrote:

Thanks for all your help Jack.  I have officially filled up your PM box.   I didn't notice that what you sent me was different.  Thanks for that.

~Shawn

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On 03/03/2010 at 17:15, xxxxxxxx wrote:

Hey folks.  So I believe that I have created the rotation matrix correctly.  And when I click the polygon on the second object the object moves to that polygon correctly, however it does not rotate correctly.  Sometimes it rotates a bit but not the way it should.  I want the firstSelection object to rotate so that it is sitting flush with the polygon no matter what the rotation of the polygon is.    Does anyone see any reason in the following code why this wouldn't be happening?

Any help would be greatly appreciated.  This one's a bit frustrating.

``````for (lngI=0; lngI < lngPolygonCount; lngI++)
{
if (bsPoly->IsSelected(lngI))
{
lngA = selectedPoly[lngI].a;
lngB = selectedPoly[lngI].b;
lngC = selectedPoly[lngI].c;
lngD = selectedPoly[lngI].d;

//Caclulate the polygon position.
if (selectedPoly[lngI].c == selectedPoly[lngI].d)
{
polyLocation = (points[lngA] + points[lngB] + points[lngC]) / 3; //Polygon is a Triangle

}
else
{
polyLocation = (points[lngA] + points[lngB] + points[lngC] + points[lngD]) / 4; //Polygon is not a Triangle
}

//Align 1st Selection to the polygon of 2nd Selection (X AXIS)
if (polyLocation.x > 0)
{
//ROTATION MATRIX
//GetPoints
Vector p1 = polyLocation;
Vector p2 = points[lngA];

Vector scale = Vector(Len(opMatrix.v1), Len(opMatrix.v2), Len(opMatrix.v3)); //Get Scale

//Contruct Matrix
Matrix rotMatrix;

rotMatrix.off = p1; //The base of the matrix
rotMatrix.v3 = !(p1); //Z Axis is along the normal
rotMatrix.v1 = !(p2 - p1); //X axis points toward the second point
rotMatrix.v2 = rotMatrix.v1%rotMatrix.v3;//Y Axis is perpendicular to the X axis
rotMatrix.v1 = !(rotMatrix.v3%rotMatrix.v2);//recreated second axis..   using cross product.

rotMatrix.v1 = !(rotMatrix.v1 * scale.x);
rotMatrix.v2 = !(rotMatrix.v2 * scale.y);
rotMatrix.v3 = !(rotMatrix.v3 * scale.z);

Vector rot1 = MatrixToHPB(rotMatrix);

firstSelection->SetMg(rotMatrix);