leech
leech

Reputation: 378

Rotating cubes in OpenGL

I want to have a cube that rotates around the center. This is something i can do with this transformation:

model = glm::rotate(identity, glm::radians(-100.0f * time), glm::vec3(0.0, 1.0, 0.0)); //rotate around y-axis
model = glm::translate(model, glm::vec3(8.0, 0.0, 0.0));
model = glm::rotate(model, glm::radians(-100.0f * time), glm::vec3(1.0, 0.0, 0.0)); //self-rotation

Now I want a cube that will rotate around the first cube as it is rotating around y-axis. Imagine that the first cube is the Earth and the second cube is the moon. I tried this, but it didn't work.

test = glm::rotate(test, glm::radians(-100.0f * time), glm::vec3(0.0, 1.0, 0.0));
test = glm::translate(test, glm::vec3(3.0, 0.0, 0.0));
test = glm::translate(test, glm::vec3(8.0, 0.0, 0.0));
test = glm::rotate(test, glm::radians(-100.0f * time), glm::vec3(0.0, 1.0, 0.0));
test = glm::translate(test, glm::vec3(-8.0, 0.0, 0.0));
test = glm::rotate(test, glm::radians(-100.0f * time), glm::vec3(1.0, 0.0, 0.0)); // self-rotation

Any ideas what I'm doing wrong? Here is a picture of the rotations: Sun - Earth - Moon Rotations

Upvotes: 2

Views: 1161

Answers (2)

Spektre
Spektre

Reputation: 51835

Your comment suggest you have this order of operations:

mvp = m_projection*m_view*m_model
vertex' = mvp*vertex

meaning m_view is inverse matrix of view and m_model is direct matrix of actual mesh.

So you should set m_projection and m_view once and then just updating the m_model.

I do not use GLM (have my own libs for math) however IIRC they mimic old fixed pipeline matrix math. So when I do this (C++/VCL/OpenGL/GLSL ... I know the points should be VBO/VAO I just wanted test quickly):

//---------------------------------------------------------------------------
//    ang   ,ang speed,body r,orbit r
//    [deg] ,[deg/s]   [unit],[unit]
float                  rs=1.0;          // star
float a0=0.0,da0= 50.0,r0=0.5,R0= 7.0;  // planet
float a1=0.0,da1=200.0,r1=0.2,R1= 1.0;  // moon
float a2=0.0,da2=250.0,r2=0.2,R2= 1.5;  // moon
float a3=0.0,da3= 20.0,r3=0.5,R3=10.0;  // planet
float a4=0.0,da4=150.0,r4=0.2,R4= 1.0;  // moon
float a5=0.0,da5=180.0,r5=0.2,R5= 1.5;  // moon
float b =0.0,db =50.0;                  // common self rotation
//---------------------------------------------------------------------------
void gl_draw()
    {
    GLint ix;
    GLfloat mp[16],mv[16],mm[16],m0[16];
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    float aspect=float(xs)/float(ys);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(60.0/aspect,aspect,0.1,100.0);
    glGetFloatv(GL_PROJECTION_MATRIX,mp);
    glMatrixMode(GL_TEXTURE);
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glTranslatef(0.0,0.0,-25.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mv);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glGetFloatv(GL_MODELVIEW_MATRIX,mm);

    glDisable(GL_DEPTH_TEST);
    glDisable(GL_TEXTURE_2D);
    glDisable(GL_CULL_FACE);
//  glEnable(GL_CULL_FACE);

    // GLSL sphere shader
    glUseProgram(prog_id);
    ix=glGetUniformLocation(prog_id,"m_projection"); glUniformMatrix4fv(ix,1,false,mp);
    ix=glGetUniformLocation(prog_id,"m_view");       glUniformMatrix4fv(ix,1,false,mv);
    ix=glGetUniformLocation(prog_id,"m_model");

    // sun
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glRotatef(b,0.0,0.0,1.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mm); glUniformMatrix4fv(ix,1,false,mm);
    glBegin(GL_POINTS); glColor3f(1.0,1.0,0.0); glVertex4f(0.0,0.0,0.0,rs); glEnd();
    // planet
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glRotatef(a0,0.0,0.0,1.0);
    glTranslatef(R0,0.0,0.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,m0);
    glRotatef(b,0.0,0.0,1.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mm); glUniformMatrix4fv(ix,1,false,mm);
    glBegin(GL_POINTS); glColor3f(0.0,0.7,1.0); glVertex4f(0.0,0.0,0.0,r0); glEnd();
    // moon
    glLoadMatrixf(m0);
    glRotatef(a1,0.0,0.0,1.0);
    glTranslatef(R1,0.0,0.0);
    glRotatef(b,0.0,0.0,1.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mm); glUniformMatrix4fv(ix,1,false,mm);
    glBegin(GL_POINTS); glColor3f(0.4,0.4,0.4); glVertex4f(0.0,0.0,0.0,r1); glEnd();
    // moon
    glLoadMatrixf(m0);
    glRotatef(a2,0.0,0.0,1.0);
    glTranslatef(R2,0.0,0.0);
    glRotatef(b,0.0,0.0,1.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mm); glUniformMatrix4fv(ix,1,false,mm);
    glBegin(GL_POINTS); glColor3f(0.4,0.4,0.4); glVertex4f(0.0,0.0,0.0,r2); glEnd();
    // planet
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glRotatef(a3,0.0,0.0,1.0);
    glTranslatef(R3,0.0,0.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,m0);
    glRotatef(b,0.0,0.0,1.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mm); glUniformMatrix4fv(ix,1,false,mm);
    glBegin(GL_POINTS); glColor3f(0.0,0.7,1.0); glVertex4f(0.0,0.0,0.0,r3); glEnd();
    // moon
    glLoadMatrixf(m0);
    glRotatef(a4,0.0,0.0,1.0);
    glTranslatef(R4,0.0,0.0);
    glRotatef(b,0.0,0.0,1.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mm); glUniformMatrix4fv(ix,1,false,mm);
    glBegin(GL_POINTS); glColor3f(0.4,0.4,0.4); glVertex4f(0.0,0.0,0.0,r4); glEnd();
    // moon
    glLoadMatrixf(m0);
    glRotatef(a5,0.0,0.0,1.0);
    glTranslatef(R5,0.0,0.0);
    glRotatef(b,0.0,0.0,1.0);
    glGetFloatv(GL_MODELVIEW_MATRIX,mm); glUniformMatrix4fv(ix,1,false,mm);
    glBegin(GL_POINTS); glColor3f(0.4,0.4,0.4); glVertex4f(0.0,0.0,0.0,r5); glEnd();

    glUseProgram(0);

    glFlush();
    SwapBuffers(hdc);
    }
//---------------------------------------------------------------------------
void __fastcall TForm1::Timer1Timer(TObject *Sender)
    {
    // this is periodicaly called by timer
    gl_draw();
    float dt=0.001*float(Timer1->Interval); // timer period in seconds
    a0=fmod(a0+da0*dt,360.0);
    a1=fmod(a1+da1*dt,360.0);
    a3=fmod(a3+da3*dt,360.0);
    a4=fmod(a4+da4*dt,360.0);
    a5=fmod(a5+da5*dt,360.0);
    b =fmod(b +db *dt,360.0);
    }
//---------------------------------------------------------------------------

I got this output (using my sphere shader):

preview

The shader just takes point x,y,z,r as a sphere 3D center and radius, emit BBOX quad and render inscribed sphere with normal shading. It also uses color and your 3 matrices.

So if I see it right you should do something like this:

model = identity; 
model = glm::rotate(model, glm::radians(b), glm::vec3(0.0, 1.0, 0.0));
// render star
model = identity; 
model = glm::rotate(model, glm::radians(a0), glm::vec3(0.0, 1.0, 0.0));
model = glm::translate(model, glm::vec3(R0, 0.0, 0.0));
model0= model;
model = glm::rotate(model, glm::radians(b), glm::vec3(0.0, 1.0, 0.0));
// render planet
model = model0;
model = glm::rotate(model, glm::radians(a1), glm::vec3(0.0, 1.0, 0.0));
model = glm::translate(model, glm::vec3(R1, 0.0, 0.0));
model = glm::rotate(model, glm::radians(b), glm::vec3(0.0, 1.0, 0.0));
// render moon
model = model0;
model = glm::rotate(model, glm::radians(a2), glm::vec3(0.0, 1.0, 0.0));
model = glm::translate(model, glm::vec3(R2, 0.0, 0.0));
model = glm::rotate(model, glm::radians(b), glm::vec3(0.0, 1.0, 0.0));
// render moon
model = identity; 
model = glm::rotate(model, glm::radians(a3), glm::vec3(0.0, 1.0, 0.0));
model = glm::translate(model, glm::vec3(R3, 0.0, 0.0));
model0= model;
model = glm::rotate(model, glm::radians(b), glm::vec3(0.0, 1.0, 0.0));
// render planet
model = model0;
model = glm::rotate(model, glm::radians(a4), glm::vec3(0.0, 1.0, 0.0));
model = glm::translate(model, glm::vec3(R4, 0.0, 0.0));
model = glm::rotate(model, glm::radians(b), glm::vec3(0.0, 1.0, 0.0));
// render moon
model = model0;
model = glm::rotate(model, glm::radians(a5), glm::vec3(0.0, 1.0, 0.0));
model = glm::translate(model, glm::vec3(R5, 0.0, 0.0));
model = glm::rotate(model, glm::radians(b), glm::vec3(0.0, 1.0, 0.0));
// render moon

If this is not working then your got some other mismatch between matrix order and used math or the GLM behaves differently than I expect. I used common angle for self-rotations so you just add indexes for your bodies ...

Also beware old GL rotations use [deg] so if GLM want [rad] you need to convert the angles and angular speed constants ...

If you want to have something more precise/related to real world or better visually see this:

Upvotes: 2

Mudkip Hacker
Mudkip Hacker

Reputation: 175

object A rotating around B order: self rotation of A, translate to offset of A to B, rotate about B, translate by B position. Since B is the sun which I assume is in your world origin, and everything is on the xz plane (Cartesian not OpenGL coords), this becomes:

model = glm::rotate(identity, glm::radians(-100.0f * time), glm::vec3(1.0, 0.0, 0.0)); //self-rotation
model = glm::translate(model, glm::vec3(8.0, 0.0, 0.0)); // earth is 8 away from sun, where unrotated about sun is 8 units east
model = glm::rotate(model, glm::radians(-100.0f * time), glm::vec3(0.0, 1.0, 0.0)); //rotate about sun
// no need for another translation since sun is at origin

Then for a object whose parent is the earth, you first resolve that relationship before you resolve the parent to world relationship:

model = glm::rotate(identity, glm::radians(-100.0f * time), glm::vec3(1.0, 0.0, 0.0)); //self-rotation of moon
model = glm::translate(model, glm::vec3(-4.0, 0.0, 0.0)); // moon is 4 away from Earth, where unrotated about Earth is 4 units west
model = glm::rotate(model, glm::radians(-100.0f * time), glm::vec3(0.0, 1.0, 0.0)); //rotate about Earth
// translate to Earth origin, which is calculate from above code block

Disclaimer, I don't use glm-math, this is just the order of transformation geometrically, and I don't know if it translates directly to glm::x

Upvotes: 0

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