> I realize that I'm confused about whether you are working on Stuart's
> clouds in C++ or clouds in Nasal.
They're the same now - there's an interface to generate a cloud from Nasal
which is what I'm using, whereas Basic Weather uses and xml format for layer
declaration, but for rendering purpose there is no difference.
> How are you generating the tracer? I gather that it
> doesn't use any textures.
If a flag is set in the cloud definition, after a cloud (consisting of a number
of sprites) has been generated, a tracer is inserted at the same position in
the center of the cloud. Currently only works if you insert clouds from Nasal.
Formally a tracer is a cloud with just a single sprite, but a cloud with a top
shade factor set to zero (i.e. a pitch-black top) - which is why we know in the
shader it can't be a real cloud.
> Your shader seems like an expensive
> way to toggle between tracers and real sprites; it would be much
> better to not render the sprite geometry at all in your tracer pass.
I basically don't - if a vertex not belonging to a tracer arrives in
tracer.vert, all the vertex shader does is to move it out of the view frustrum
(come to think of it, the if-statement in the fragment shader shouldn't even be
necessary - sorry, this is just mess in progress). Also the effect currently
declares a texture which I'm not using because at some point I wanted to test
round tracers.
> You should use a <color-mask type="vec4d">0 0 0 0</color-mask>
> declaration to turn off color writes when rendering the tracers, since
> you don't want them to actually appear in the output. That will save
> you some fill costs.
Thanks, that's very helpful.
> As for not seeing much improvement when the terrain is occluded, I can
> think of several possibilities. You said that "I've tested that I am
> not vertex-shader dominated by discarding half the pixels of the
> terrain in my field of view;" how did you do that?
By putting
if (gl_FragCoord.y > 300) {discard;}
early into the terrain fragment shader.
> but this
> can only be done if the shader can't affect the Z output value.
> Writing the fragment depth would do that (urban shader), but the
> classic offenders are "discard" and alpha testing.
Thanks for the pointers - I'll have a look. How come it works for clouds then?
The normal cloud passes all use alpha testing...
* Thorsten
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