normalize, textureXD textureXDProj

[V-man]

This might be a stupid question but what is the defined behavior of normalize?

I'm assuming

normalize(float thing) = 1.0
normalize(vec2 thing) = thing/length
normalize(vec3 thing) = thing/length
normalize(vec4 thing) = thing/length or maybe
normalize(vec4 thing) = vec4(vec3(thing)/length_of_vec3, 1.0)

and if a vector is of length = 0.0, result is a NULL vector.

=================================
Part deux

What about textureXD vs textureXDProj.

In ARBfp, I used TXP to take the effect of projection into account. TEX gave incorrect results.

In GLSL, texture2D works.

Which to use and when?

[mrbill]

Originally posted by V-man:
This might be a stupid question but what is the defined behavior of normalize?

Not a stupid question. Normalizing a zero length vector is undefined.

I'm looking at the documentation (even of several shading languages) and all are ambiguous here. I thought I'd get bailed out on inversesqrt( 0.0 ), but no....

Which to use (texture2D or texture2Dproj) and when?

Perhaps the best way to think of it is this way:

vec4 texture2DProj( sampler2D sampler, vec3 coord )
{
    return texture2D( sampler, coord.st/coord.p );
}
// Or...
vec4 texture2DProj( sampler2D sampler, vec4 coord )
{
    // coord.p is ignored!
    return texture2D( sampler, coord.st/coord.q )
}

Which do you need? Well, that depends.

If you know your q component is always 1.0, you might as well do the non-projective fetch. And you often *do* know this. For example, in fixed function if you are providing texture coordinates with glMultiTexCoord2f[v] or glMultiTexCoord3f[v], you are not using texgen, and your texture matrix is identity.

Otherwise, it still depends. What does your algorithm require? A projective or non-projective fetch?

-mr. bill

[V-man]

I think your pseudo code for normalize is correct.

vec4 thing;
normalize(thing);

is done like so

r *= invsqrt(dot4(thing, thing));

so one has to be careful on the type of "thing".

=============================================

I guess I was mistaken about ARB_fragment_program.

If tex coord is (x, y, 0, 1) then
TEX and TXP give the same results.
I'm talking about plain 2D texturing a la glTexCoord2f

I could have sweared TEX and TXP didn't give the same result when I was learning that stuff... hmmm

and it looks like it's the same with texture2D and texture2DProj

So the effect of the projection matrix is taken into account automatically I suppose.
Just to be clear, I'm talking about perspective correct texturing here.

[Humus]

Originally posted by V-man:
normalize(vec4 thing) = thing/length or maybe
normalize(vec4 thing) = vec4(vec3(thing)/length_of_vec3, 1.0)

I haven't read up in the spec, but it should definitely be the first one. It just makes more sense, plus that normalizing a four-component vector is useful functionality that there's no reason to leave out from the spec. The second normalizing behavior can be done anyway with vec4(normalize(thing.xyz), 1.0);