Transformation hierarchy causes object to jump in space? - c++

Question

I have two objects that I want to parent together so that Tri is a child of Torus. When I do so and multiply the matricies together by adding the parents modelView to the childs, the child jumps in space initially, over to the right and up by a few units. Where do I insert an offset into this, and how do I calculate it?
obj = make_shared<Object>(*this);
obj->rename("tri");
obj->type->val_s = "tri";
obj->t->val_3 = glm::vec3(-4.f, 1.5f, 0.f);
allObj.push_back(obj);
obj = make_shared<Object>(*this);
obj->rename("torus");
obj->type->val_s = "obj";
obj->t->val_3 = glm::vec3(3.f, 2.f, 0.f);
allObj.push_back(obj);
//Matrix
scaleM = glm::scale(glm::mat4(), s->val_3);
rotationM = glm::toMat4(r_quat);
glm::vec3 usablePivot = t->val_3 - pivot->val_3;
glm::mat4 localAxis1M = glm::translate(glm::mat4(), usablePivot);
glm::mat4 localAxis2M = glm::translate(glm::mat4(), -usablePivot);
translationM = glm::translate(glm::mat4(), t->val_3);
modelM = translationM * localAxis2M * rotationM * scaleM * localAxis1M;
//View
usableView = myGL->ViewM;
//Projection
usableProjection = myGL->ProjectionM;
//MVP
if (parent == "world") { MVP = usableProjection * usableView * modelM; }
else { MVP = usableProjection * usableView * parentTo->modelM * modelM; }

Answer 1

Matrix order in mine OpenGL apps:
sub-object matrices
the lowest in ownership hierarchy are first
they are local to their parents
their have offset to point (0,0,0) of parent space
if not then and they are local to world then this is the jump reason
in that case the sub-objects are not really sub objects
and handle them as normal object without parent
object to world
is transform matrix of objects without parent
they are local to your scene world
then goes camera view
it is inverse of camera space
where Z-axis is view direction
last is projection
like gluPerspective(...)
this one is usually stored in GL_PROJECTION matrix on fixed pipeline
instead of GL_MODELVIEW
Clipping is done by OpenGL separately via glViewport
(your usable view I think)
Look for more info here
you can check the content of your matrices
look at positions 12,13,14 where the offset vector is stored
do not forget that this vector is local to parent coordinate system!!!
so it is more likely rotated by it ...
also a good idea is to draw axis lines for each tested matrix in its parent space
to see if they are correct
I use red,gree,blue lines for x,y,z - axis
just extract origin of coordinate system [12,13,14]
and draw line from it to the same point + a*axis vector
a is line length (big enough so you see a line not a point)
axis vectors are at positions x=[0,1,2], y=[4,5,6], z=[8,9,10]
do not forget to set matrices to parent coordinate system !!!
if you handle matrices your self via GLSL then do not forget that
direction vectors like Normals are transformed without offset
so get the whole transform matrix (all multilicated together without projection and sometimes also camera)
set the offset to zero [12,13,14]=(0.0,0.0,0.0)
and multiply by this matrix