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I am trying to rotate an object about one of its side and already tried the common approach as found on the forums:

translate(-P); rotate(); translate(P);

In OpenGL (reversing the order of translations/rotations), I used the following code:

glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glPushMatrix(); glTranslatef(-50, 50, 0); glRotatef(rotationCoord, 0, 1, 0); glTranslatef(50, -50, 0); glBegin(GL_QUADS); glVertex3f(-50.0, 50.0, 0); glVertex3f(50.0, 50.0, 0); glVertex3f(50.0, -50.0, 0); glVertex3f(-50.0, -50.0, 0); glEnd(); glPopMatrix();

However, the rectangle that I am drawing doesn't seem to be rotating around one side as pivot. (trying to set the left side as the pivot point and rotate around it). I made a screen capture vid to show what kind of rotation I am getting right now. Here's the video:

http://youtu.be/VgEZ_rsG3xU

How do I set the pivot for this object so that it rotated around that point?

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  • The choice of pivot is odd. The y-axis translation component has no effect, since you're rotating in the xz-plane. Is this what you intended? Commented Jul 13, 2013 at 13:03
  • @MarceloCantos : I didn't intend to rotate the around the Y-axis for now. Just X-axis. But the left hadn side of the rectangle seems to be moving whereas the pivot should be stationary. right? Commented Jul 13, 2013 at 13:06
  • glRotatef(…, 0, 1, 0) is a rotation around the y-axis (aka xz-plane), not the x-axis. Commented Jul 13, 2013 at 13:08
  • @MarceloCantos : Oops. I had to mention in the above comment that rotaiton has to be only around Y-axis for now, not X-axis. Apologies for the confusion. That said, with rotatio around Y-axis as well as it is happening now, the pivot should be stationary about which I am not sure why. Commented Jul 13, 2013 at 13:11
  • Try reversing the order: T(P) R() T(-P). This is clutching at straws, but it would be good to at least eliminate the possibility. Commented Jul 13, 2013 at 13:30

1 Answer 1

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The problem comes from the 

glTranslatef(0, 0, -10.0);

you're calling after the rotation. That means the (0,0,-10) is applied to the object coordinates (+-50,+-50,0) before they are transformed by anything else, so the quad is offset before it's being rotated.

Here's the code I used for testing, maybe you get something out of it:

#include <glm/glm.hpp> #include <glm/ext.hpp> using namespace glm; void display(void) { glClearColor(0,0,0,0); glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); GLfloat aspect = (float)g_Width / g_Height; mat4 eyeToCamera = perspective(45.f, aspect, g_nearPlane, g_farPlane); // projection mat4 cameraToWorld = lookAt(vec3(0, 0, -150), vec3(), vec3(0, 1, 0)); // view vec3 pivot(50, -50, 0); mat4 worldToObject = translate(pivot) * rotate(rotationAngle, vec3(0, 1, 0)) * translate(-pivot); // model glLoadMatrixf(value_ptr(eyeToCamera * cameraToWorld * worldToObject)); glBegin(GL_QUADS); glVertex3f(-50.0, 50.0, 0); glVertex3f(50.0, 50.0, 0); glVertex3f(50.0, -50.0, 0); glVertex3f(-50.0, -50.0, 0); glEnd(); glutSwapBuffers(); }

Note that I don't use any matrix stack functions, and I only set the GL_MODELVIEW matrix.

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4 Comments

I needed to translate the translate the back by -10 first because I have the projection matrix applied as above (which is needed for one of my subsequent operations that is to come later). I tried reducing the z value of the rectangle to change how far the rectangle is drawn and to see if that is creating a problem with the rotation. But even if the rectangle is at -0.1, I still don't get a fixed pivot. Because of OpenGL's post-multiplication, that translate by -10 has to be there after rotation so as to make the rectangle fully visible inside the perspective projection.
I am not sure why that translate by -10 is causing or how should I go around for fixed pivot with a rectangle still being shown in my perspective projection.
@user1240679 Put the glTranslatef(0, 0, -10.0) call before the rotation, i.e. directly after calling glPushMatrix above. That way the translation is applied after the object is rotated.
Superb! Thanks. A lil off-topic doubt to be cleared here. I have the vector positions of a rectangle (which is rotated in the real world - two Z vertices at Z=0, other two at Z=200). I want to get the rotation matrix from these vertices and apply it as I did above in my post (but instead of glRotatef will be using glMultMatrix). The question is here btw: stackoverflow.com/questions/17585424/… If you could just give a read to comments discussion below that would give the context. Thanks

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