> One can assume that > a vec4 varying is no more expensive than a vec3. (...) > I'm not sure it's useful to think of each component of a varying > vector as a "varying" i.e., three vec3 varying values use up three > varying "slots," and so do 3 float varying values
I dunno... Just counting the number of operations, mathematically the best case scenario for interpolating a general vector across a triangle is in Cartesian coordinates where each coordinate interpolates as an independent number, so the cost of a vec4 would be the same as the cost for 4 floats. In any other case, like curved coordinates or Minkowski space, a Jacobian comes to bite and the vector is more expensive than just 4 scalar numbers. Now, what I don't know if there's some fancy hardware trick which makes a Cartesian vec4 as cheap as a float. In this case, we could use this by combining every four varying float into one varying vec4 and get the same job done for 25% of the cost. But... ... the thing I did try is that in adapting the urban shader to atmospheric scattering I ran out of varying slots, I needed two more varying float. I solved this by deleting one varying vec3 (the binormal) and computing it as the cross product - and that gave me the two slots I needed (and presumably one left, but I didn't try that). So this would suggest that indeed each vector component counts the same as a varying float. > One reason to pass this as a varying is that on old hardware, GeForce > 7 and earlier, it is very expensive to change a uniform that is used > by a fragment shader. It forces the shader to be recompiled. So, this > is actually a well-known optimization for old machines. Okay, I didn't know that... But pretty much all weather and environment-dependent stuff (ground haze functions, the wave amplitude for the water shader, overcast haze for the skydome,...) makes use of slowly but continuously changing uniforms (I think gl_LightSource is technically also a uniform), so it doesn't really make sense to have this old machine friendly code in one place in the shader but not in other places in the same shader. > Also, I want to point out that, in your example, lightdir is in the > local coordinate system of the terrain, if in fact you are shading > terrain. I would call "world space" the earth-centric coordinate > system in which the camera orientation is defined. gl_Vertex is in some coordinate system which I've usually encountered as 'world space' in shader texts as opposed to gl_Position which is supposed to contain the vertex coordinates in 'eye space'. I realize that gl_Vertex is *not* in the global (xyz) coordinates of Flightgear Earth, although I don't know how the two relate. Somehow once in the shader world, z is always up... Just a matter of semantics? > I don't think that any varyings -- except for the fragment coordinates > -- are mandatory, except perhaps on very old hardware. I remember at least one text claiming that once you use ftransform(), gl_FrontColor and gl_BackColor get values as well - but that source or myself may be mistaken. Cheers, * Thorsten ------------------------------------------------------------------------------ Don't let slow site performance ruin your business. Deploy New Relic APM Deploy New Relic app performance management and know exactly what is happening inside your Ruby, Python, PHP, Java, and .NET app Try New Relic at no cost today and get our sweet Data Nerd shirt too! http://p.sf.net/sfu/newrelic-dev2dev _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
