glsl - Clipping-planes in OpenGL ES 2.0

Question

I need to clip a few hundred objects under a clipping plane in OpenGL ES 2.0 and would appreciate ideas from people more experienced with this subset of OpenGL.

In OpenGL ES 1.x there is glClipPlane. On the desktop you have glClipPlane, or gl_ClipDistance in your shader. Neither of these two are available in OpenGL ES 2.0. It seems that this kind of functionality disappeared completely with 2.0.

It seems the only way to do this is either A) run the plane equation in the fragment shader, or B) write a very complex vertex shader that positions vertices on the plane if they are behind it.

(A) would be slow compared to glClipPlane, since "regular" clipping is done after the vertex shader and before the fragment shader, each fragment would still have to be partially processed and discarded.

(B) would be very hard to make compatible between shaders, since we can't discard vertices we have to align them with the plane and adjust attributes for those that are "cut". It's not possible to interpolate between vertices in the shader without sending all vertices in a texture and sample it, which would be extremely expensive. Often it would probably be impossible to interpolate the data correcly anyway.

I've also thought of aligning the near plane with the clipping plane which would be an efficient solution.

And drawing a plane after rendering the whole scene and checking for depth-fail will not work either (unless you are looking close to perpendicular to the plane).

What works for a single object is to draw the plane to the depth buffer and then render the object with glDepthFunc(GL_GREATER), but as expected it does not work when one of the objects is behind another. I tried to build on to this concept but eventually ended up with something very similar to shadow volumes and just as expensive.

So what am I missing? How would you do plane clipping in OpenGL ES 2.0?

Answer 1

Here is two solutions I've found on Vuforia SDK forums.

1. Using shaders by Harri Smatt:

   uniform mat4 uModelM;
   uniform mat4 uViewProjectionM;
   attribute vec3 aPosition;
   varying vec3 vPosition;
   void main() {
     vec4 pos = uModelM * vec4(aPosition, 1.0);
     gl_Position = uViewProjectionM * pos;
     vPosition = pos.xyz / pos.w;
   }
   

   ---

   precision mediump float;
   varying vec3 vPosition;
   void main() {
     if (vPosition.z < 0.0) {
       discard;
     } else {
       // Choose actual color for rendering..
     }
   }
   

2. Using quad in depth buffer by Alessandro Boccalatte:

   • disable color writing (i.e. set the glColorMask(false, false, false, false);)
   • render a quad which matches the marker shape (i.e. just a quad with the same size and position/orientation of the marker); this will only be rendered into the depth buffer (because we disabled color buffer writing in the previous step)
   • enable back the color mask (glColorMask(true, true, true, true);)
   • render your 3D models