transparency in opengl (using FLTK)

Question

I'm drawing some 3D structures in a Fl_Gl_Window in FLTK's implementation of opengl. This images are drawn and rotated so the code looks something like

glTranslatef(-xshift,-yshift,-zshift);
glRotatef(ang1,ang2,ang3);
glTranslatef(xshift,yshift,zshift);
glColor4f((120.0/256.0),(120.0/256.0),(120.0/256.0),0.2);
for (int side=0;side<num_sides;side++){
    glBlendFunc(GL_SRC_ALPHA,GL_ONE_MINUS_SRC_ALPHA);
    glEnable( GL_BLEND );
    glBegin(GL_TRIANGLES);


    //draw shape

    glEnd();
    glDisable(GL_BLEND);
}

and it almost works apart from at different angles the transparency doesn't work properly. For example, if I draw a cube from one side it will look transparent all the way through without being able to discern the two sides but from the other one side will appear darker as it is supposed to. It's as if it calculates the transparency too 'early' as in before the rotation. Am I doing something wrong? Should I move the rotation to below the transparency effects (i.e. before them in execution) or does the order of the triangles matter?

Answer 1

The order of the triangles matters. To get the desired effect for transparency you need to render the triangles in back to front order because the hardware blending works by reading the color for the fragment in the depth buffer and blending it with the fragment currently being shaded. That's why you are getting different results when you rotate your cube since you are not changing the order of the triangles in the cube. You may also want to look into Order Independent Transparency techniques.

Depending on how many triangles you have sorting them every frame can get really expensive. One approximation technique is to presort the triangles along the x, y, and z axes and then choose the sorted ordered that most closely matches your viewing direction. This only works to a certain extent. One popular type of order independent transparency technique is depth peeling. Here's a tutorial with some code for implementing it: http://mmmovania.blogspot.com/2010/11/order-independent-transparency.html?m=1. You might also want to read the original paper to get a better understanding of the technique: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.18.9286&rep=rep1&type=pdf.