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OpenGL: How do I get the coordinates of a specific point after transformation?

Suppose I have a point at (250,125,-20). After the following transformation,

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(100.0, 50.0, 0.0);
glRotatef(-25.0, 0.0, 1.0, 0.0);

How can I get the value of current coordinates of that point? Need I write a subroutine to multiply a matrix to a vector? Are there any built-in solutions?

Solution

  • You can't get the coordinates for a specific vertex (point) after a transformation, however for this particular case you can get the ModelViewMatrix after the translate/rotate is applied.

    glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(100.0, 50.0, 0.0);
glRotatef(-25.0, 0.0, 1.0, 0.0);  

glGetFloatv(GL_MODELVIEW_MATRIX , *your_matrix*);
//print your matrix to check if that is the desired transformation coordinates

    There is no magic tape in OpenGL, you will have to write your own framework e.g: for every objects in your world a class where you hold the vertices and what data you find relevant.