问题
having a bad coding day.
Right I need to make a 3D cube that spins around etc via user interaction. Hey no biggy.
All the examples to make a 3D cube seem to use glOrthof and when I demo one to people they say its not 3D.
The problem is that glFrustumf seems to put me in the cube instead of in front of me. I cant move it back using glTransform because it re-uses the ModelView Matrix (I even tried manually modifying that)
/* save current rotation state */
GLfloat matrix[16];
glGetFloatv(GL_MODELVIEW_MATRIX, matrix);
/* re-center cube, apply new rotation */
glLoadIdentity();
glRotatef(self.angle, self.dy,self.dx,0);
/* reapply other rotations so far */
glMultMatrixf(matrix);
So questions are.
To do a 3D cube must I use glFrustumf and if so, how the hell do I step back 5 but still re-use the model matrix (it keeps the cube spinning in what ever direction the user moves it)
回答1:
I'm not sure what you mean by glOrthof() "not being 3-D". The rotating cube example I have here (using both OpenGL ES 1.1 and 2.0 for rendering of the textured cube) seems to work on 3-D, and I use glOrthof() in the OpenGL ES 1.1 side of the renderer. Shading and other effects can be applied independently of the glOrthof() usage.
In that example, I don't read back the model view matrix to manipulate the cube. Instead, I keep a copy of the matrix locally and modify that using some Core Animation helper functions. In addition to the CATransform3DRotate() that I perform on the cube, you should be able to throw in a CATransform3DTranslate() to displace it in a certain direction, while still being able to spin it.
I keep a local copy of the model view matrix for performance (reading back the model view matrix halts the rendering pipeline on OpenGL ES 1.1), and for compatibility with 2.0 (where you need to send the matrix as a uniform to the shaders).
Also, in an answer to your later question (which might get closed), you can't just arbitrarily change values within the model view matrix and expect to see linear displacements from that. You need to get the math right, and matrix math was never one of my strong points. I find it best to let a transform operation (like those provided in Core Animation) do the math for you when manipulating matrices.
来源：https://stackoverflow.com/questions/4749786/iphone-opengl-glorthof-vs-glfrustumf-is-glorthof-not-3d