OpenGL ES 2.0: Object rotation problem

Question

Hi everyone !

I know the following question have been asked many times on different forums, but after viewing different solutions, it still did not fix my problem, or maybe I did not get what they mean
I am trying to model a pyramid using OpenGL ES 2.0 on the iPhone.
I would like to allow the user to rotate this pyramid using the touch screen, so he can view every face of the shape.
I am using my own matrix utilities, inspired by the library given in the book OpenGL ES 2.0 Programming Guide.
I have a simple vertex shader that takes 2 uniforms and 2 attributes.
Here is my vertex shader :

uniform mat4 modelViewMatrix;
uniform mat4 projectionMatrix;
attribute vec4 vertex;
attribute vec4 color;
varying vec4 colorVarying;

void main()
{
 colorVarying = color;
 gl_Position = projectionMatrix * modelViewMatrix * vertex;
}

So, during my rendering, I create 2 matrices : the modelView and the projection matrix.

Here is how my projection matrix is initialized :

[pMatrix frustumWithAngleOfView:70.0f
                          ratio:(float)backingWidth / (float)backingHeight
                          zNear:1.0f
                           zFar:100.0f];
[program use];
glUniformMatrix4fv(unif_projectionMatrix, 1, GL_FALSE, pMatrix.vertices);
[program unuse];

My pyramid vertices are set as follow :

GLfloat pyramid[] = {
  //Base
  -1.0, -1.0, 1.0, 1.0,
  -1.0, -1.0, -1.0, 1.0,
  1.0, -1.0, -1.0, 1.0,
  1.0, -1.0, 1.0, 1.0,
  //Front face
  0.0, 1.0, 0.0, 1.0,
  -1.0, -1.0, 1.0, 1.0,
  1.0, -1.0, 1.0, 1.0,
  //Right face
  0.0, 1.0, 0.0, 1.0,
  1.0, -1.0, 1.0, 1.0,
  1.0, -1.0, -1.0, 1.0,
  //Back face
  0.0, 1.0, 0.0, 1.0,
  1.0, -1.0, -1.0, 1.0,
  -1.0, -1.0, -1.0, 1.0,
  //Left face
  0.0, 1.0, 0.0, 1.0,
  -1.0, -1.0, -1.0, 1.0,
  -1.0, -1.0, 1.0, 1.0

 };

Finally, here is my drawing code :

[EAGLContext setCurrentContext:context];

glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebuffer);
glViewport(0, 0, backingWidth, backingHeight);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// Use shader program
[program use];

// Apply transformations on the ModelView matrix
[mvMatrix identity];
[mvMatrix translateX:0 Y:0 Z:-6.0];
[mvMatrix rotateX:rotatex Y:rotatey Z:rotatez];

// Set the ModelView matrix uniform in the v-shader
glUniformMatrix4fv(unif_modelViewMatrix, 1, GL_FALSE, mvMatrix.vertices);
// Set the vertex attribute in the v-shader
glVertexAttribPointer(attr_vertex, 4, GL_FLOAT, GL_FALSE, 0, pyramid);
glEnableVertexAttribArray(attr_vertex);

// Draw
glDrawArrays(GL_TRIANGLE_FAN, 0, 4);
glDrawArrays(GL_TRIANGLES, 4, 3);
glDrawArrays(GL_TRIANGLES, 7, 3);
glDrawArrays(GL_TRIANGLES, 10, 3);
glDrawArrays(GL_TRIANGLES, 13, 3);

glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER];

Everything seems to work fine, my pyramid is here, translated backward on the Z axis.
The problem shows up when I start to rotate the shape, it looks like it is rotating around the user, not around itself.
So for example, when I touch the screen from left to right, I see the pyramid going out of the screen at the right edge, and then come back into screen from the left edge, as if it had done a full circle around me :p.

I am pretty much sure about my matrix management code, so I decided to do not post it here to avoid too much code in this question. I can give it if necessary.

Have you any idea about what I did wrong ?

Thanks a lot !

Tom

Answer 1

It sounds like the problem is in the matrix management code - specifically, that the code to update mvMatrix (the modelViewMatrix) is actually altering projectionMatrix instead.