A few volume rendering hints.
1.
Cannot discard writing into render-targets selectively in an MRT frame-buffer. This arises, for example, when two targets are used to store two custom denormalized depth buffers. A frame-buffer has a depth attachment as well, but it is used in a regular sense of a depth buffer.
2.
Volume rendering can use analytic volume buffer or a set of depth textures. To limit volume to a custom 3D shape requires preliminary step to render that shape into two depth attachments, one with inverted normals and inverted depth test:
// Enable depth test, inverted, front to back.
glDepthMask(GL_TRUE);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_GREATER);
// Render only back faces.
glEnable(GL_CULL_FACE);
glCullFace(GL_FRONT);
// Clear the buffers.
// Note that the most distant shape will overwrite all others.
glClearDepth(0.0);
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//... Render the shape of the volume, i.e. a cloud.
// Enable depth test, back to front.
glDepthMask(GL_TRUE);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);
// Render only front faces.
glEnable(GL_CULL_FACE);
glCullFace(GL_BACK);
// Clear the buffers.
glClearDepth(1.0);
glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//... Render the same shape of the volume.
3.
In a fragment shader calculate the distance to a rasterized point. Do it in fragment instead of in a vertex, hoping for speedup due to hardware interpolation. If two vertices spread out evenly in a view space, their depths will be equal and much bigger than the distance to a front clipping plane. This results in artifacts when moving towards a volume and crossing it.
4.
A depth buffer attachment can now be shared between with other frame-buffers. It works for simple convex shapes as volumes that can be rendered from inside and outside. The real work is to be done elsewhere to modulate this shape with other detail, i.e. noise.
