Question

I need to draw a line between two meshes I’ve created. Each mesh is associated with a different model matrix. I’ve been thinking on how to do this and I thought of this:

glMatrixMode(GL_MODELVIEW);
glLoadMatrixf(first_object_model_matrix);
glBegin(GL_LINES);
glVertex3f(0, 0, 0); // object coord
glMatrixMode(GL_MODELVIEW);
glLoadMatrixf(first_object_model_matrix);
glVertex3f(0, 0, 0); // ending point of the line
glEnd( );

But the problem is that I can’t call glMatrixMode and glLoadMatrixf between glBegin and glEnd. I’m also using shaders and the programmable pipeline, so the idea of turning back to the fixed pipeline with my scene rendered isn’t exciting.

Can you:

• Suggest me precisely how to draw a line between two meshes (I have their model matrix) with shaders.

or

• Suggest me how to write code similar to the one above to draw a line having two meshes model matrices.

Answer 1

Calculate the line’s two points by multiplying each one with one of your model matrices. The following is pseudo-code. Since you’re using Qt, you could use its built-in maths libraries to accomplish this effect.

vec3 line_point_1 = model_matrix_object1 * vec4(0, 0, 0, 1);
vec3 line_point_2 = model_matrix_object2 * vec4(0, 0, 0, 1);
// Draw Lines