Quote:

can anyone tell me when it's wise to use float3 and when float4? .... why a 4-dimensional vector for space?



Position is calculated in four dimensions because some technical reasons...essentially, the 3d position vector has to be transformed to homogenous perspective space, and a fourth component, w, is needed..

from Dr. Math:
"For technical reasons, you need a fourth coordinate to do this
efficiently, but the idea is this: The 3D point you would call
(x, y, z) is represented by any 4D "homogeneous" vector like this:
(ax, ay, az, a), where a is non-zero. We usually pick a to be
1, so it is represented by (x, y, z, 1). But if after multiplying
your input 4D vector by the matrix you get something like this:
(2, 11, 18, 2), the a is 2, so this corresponds to (2/2, 11/2, 18/2)
in 3 dimensions."

In any case, normally you dont need to worry about it, just output a float4 vector for you final transform ... you just cast the positon input as a 4d vector and do all the mul using that, and there you go.. mul(inpos,matWorldViewproj); where inpos is a 4d vector..

As far as other uses of vectors, always be aware of what components you really need.. if all you care about in a vector is the xyz or rgb values, you should use a float3, unless you need to use these vectors to operate on a different float4.

For instance, this operation is illegal: c1.xyzw*=c2.xyz; ...where c1 is a float4 and c2 is float3)

to properly do this operation do this instead:
c1.xyz*=c2.xyz; or you can do it in a dumber way like this
c1.xyzw*=float4(c2.xyz,1); ..we just recast the float3 c2 into a float 4, adding a one to the w component, meaning it will unchanged by the multiplication.. if you wanted an addition, just 0 instead of 1...

There is an exception to this rule however, you can always multiply a vector by a scalar.. therefore: c1.xyzw*=2.0f; ...is legal..this just mulitplies each component of the vector by the scalar value...

Interestingly, in shaders, scalars can be subsituted for vectors in any case.. such that: float value=c1.rgb; is legal.. this is the easy way to get a monotone value from a color vector.. which is very useful in many shaders.

The basic rule is to be aware of what the numbers your vectors represent actually mean, and use the appropriate dimensions.

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