perrya

07-12-2013, 01:53 PM

I have a problem where I need to change the point on which a rotate operation occurs using GLM. The standard response on the Internet is to do the following;

glPushMatrix();

glTranslatef(x, y, z);

glRotatef(angle, rot_axis_x, rot_axis_y, rot_axis_z);

Draw();

glPopMatrix();

However, I am using OpenGL and everything revolves around a matrix. So my code looks like this;

glm::vec3 position;

float angle, turrent, barrel;

glm::vec3 direction;

glm::vec3 scale = glm::vec3(1.0f);

glm::mat4 MVP;

...

MVP = glm::translate(MVP,position);

MVP = glm::rotate(MVP,angle,direction);

MVP = glm::scale(MVP,scale);

setMVP(MVP);

clazz.drawWheels();

clazz.drawBody();

MVP = glm::translate(MVP,glm::vec3(0,0,1.1));

MVP = glm::rotate(MVP,turrent,direction);

setMVP(MVP);

clazz.drawTurrent();

demoElevation();

MVP = glm::rotate(MVP,elevation,glm::vec3(1.0f, 0.0f, 0.0f));

setMVP(MVP);

clazz.drawBarrel();

...

It's a tank! And I have managed to draw the body nicely. And I have managed to turn the turrent nicely. One thing I can't seem to do is elevate the barrel properly. It doesn't seem to have the right rotation point. But this begs the question, how can I change the rotation point before I do a rotate using OpenGL 3.2 MVP (model view perception) matrixes?

Doing a;

MVP = glm::translate(MVP,glm::vec3(0,0,.5));

MVP = glm::rotate(MVP,elevation,glm::vec3(1.0f, 0.0f, 0.0f));

setMVP(MVP);

clazz.drawBarrel(); only moves the barrel further out, it does not change the centre point at which a rotate is made from.

New to OpenGL and wonders who out there can shed some light on this for me.

Thanks,

Perry

glPushMatrix();

glTranslatef(x, y, z);

glRotatef(angle, rot_axis_x, rot_axis_y, rot_axis_z);

Draw();

glPopMatrix();

However, I am using OpenGL and everything revolves around a matrix. So my code looks like this;

glm::vec3 position;

float angle, turrent, barrel;

glm::vec3 direction;

glm::vec3 scale = glm::vec3(1.0f);

glm::mat4 MVP;

...

MVP = glm::translate(MVP,position);

MVP = glm::rotate(MVP,angle,direction);

MVP = glm::scale(MVP,scale);

setMVP(MVP);

clazz.drawWheels();

clazz.drawBody();

MVP = glm::translate(MVP,glm::vec3(0,0,1.1));

MVP = glm::rotate(MVP,turrent,direction);

setMVP(MVP);

clazz.drawTurrent();

demoElevation();

MVP = glm::rotate(MVP,elevation,glm::vec3(1.0f, 0.0f, 0.0f));

setMVP(MVP);

clazz.drawBarrel();

...

It's a tank! And I have managed to draw the body nicely. And I have managed to turn the turrent nicely. One thing I can't seem to do is elevate the barrel properly. It doesn't seem to have the right rotation point. But this begs the question, how can I change the rotation point before I do a rotate using OpenGL 3.2 MVP (model view perception) matrixes?

Doing a;

MVP = glm::translate(MVP,glm::vec3(0,0,.5));

MVP = glm::rotate(MVP,elevation,glm::vec3(1.0f, 0.0f, 0.0f));

setMVP(MVP);

clazz.drawBarrel(); only moves the barrel further out, it does not change the centre point at which a rotate is made from.

New to OpenGL and wonders who out there can shed some light on this for me.

Thanks,

Perry