How can I render "two-sided" clouds like in Minecraft?

Question

The clouds in Minecraft are semi-transparent and are rendered on both sides. If you fly into the cloud you can see inside of the cloud. If I render clouds the inside faces would be visible on the outside. How can I prevent that? Z-Ordering the faces and render near to far with depth test on? There must be a better, easier way. Could the accum or stencil buffers be used somehow?

UPDATE : I think the crucial point everyone is forgetting is that these clouds and semi-transparent and "blended" into the scene as each face is rendered. If two faces are rendered on top of each other the "white" texture will double up which is undesired. The faces also seem to have slight lighting variations which would rule out using a stencil.

UPDATE2: Just another note. Each cloud is 12x12x4 blocks (roughly). Larger clouds are just a group of the base clouds stuck together.

Here's showing clouds from above. They are translucent and the chunks below can be seen.

And here's the view from inside them.

Answer 1

Set culling to cull front faces first, then render your clouds. This will render the backfaces. After that, switch culling to cull backfaces, and render the clouds, which will render the front faces on top of the back faces.

This will prevent rendering your backfaces on top of the front faces, but still keep back faces visible all times.

Update: From the pictures, I see that the backfaces are only visible when you are inside. Just switch the order of the above instructions: Render front faces first, backfaces later so they are discarded by the z buffer if you are outside. Just render with culling disabled in that case.

Answer 2

Sounds like the clouds have both inside and outside faces. Make your cloud double sided but don't turn off backface culling.

Answer 3

I think there's a special shader for that. To lighten the alpha calculation, it might only consider the first alpha from the mesh from the POV, and blend everything else behind it that has no alpha (or that is not clouds).

From the inside of a cloud, other clouds are no longer visible.

Answer 4

I think the only solution is to do a 2 pass rendering (as Janos said). But the clouds first need to be Z-Ordered. Then render the outside faces of all clouds (back culling), and then render all inside faces (front culling). This will prevent seeing the inside faces from outside, and the sorting will prevent seeing a cloud behind another cloud. Remember the clouds and semi-transparent and "blended" into the scene as each face is rendered.

UPDATE : "Each" cloud must be rendered in 2 passes individually. Do outside faces, then inside faces. Then repeat that for each cloud. Clouds must still be z-ordered.

Here is some pseudo code:

void renderClouds() {
  Cloud clouds[] = getClouds();
  clouds = sortClouds(clouds);
  for(int a=0;a<clouds.length;a++) {
    glCullFace(GL_BACK);
    clouds[a].render();  //render outside faces
    glCullFace(GL_FRONT);
    clouds[a].render();  //render inside faces
  }
}

That works for me now. But I'm open to better ideas.

Answer 5

I suggest another approach:

First you render your world and sky, etc. just like you would do normally.

Then you have to sort the clouds (or the vertices of the clouds) from closest to the camera to furthest from the camera. You then test if the camera is inside a cloud, if this is the case, you will have to turn off back face culling so that the insides of the cloud will be rendered.

Then you render your clouds (from near to far) and if you have depth test enabled you will not have any clouds overlapping.

This is my theory. If it is wrong, please let me know so I can learn from it.