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Cg: how to index a float4 vector?

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Hi, In Cg shader, if we have an float4 vector, for example: float4 vec; //declare a float4 vector and i want to use a variable to index into vector's component, for example: int index=1; //variable index; float component = vec[index]; of course the Cg complier returns an error here. So i wonder if there are alternative methods in Cg to index specific component of an vector? (i don't expect the answer to be: create an array, convert vector to array, then index the array :) ), thanks!

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IIRC GLSL allows what you described, but again i may be wrong. Anyway, one way i can think of is by using a dot product in order to extract the correct component.
Something like that (extract the y component as in your example) :


float component = dot(vec, float4(0,1,0,0));


If 'index' is a uniform variable you can pass the mask vector (in the above example the float4(0,1,0,0)) as a uniform, instead of the index. If index is a vertex attribute you can get the correct mask vector from an array of 4 vectors. Something like this :


float4 mask[4] = {{1,0,0,0}, {0,1,0,0}, {0,0,1,0}, {0,0,0,1}};
float component = dot(vec, mask[vertex.index]);


I'm not sure if the array syntax is correct, but you get the idea.

Hope that helps.

HellRaiZer

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thanks HellRaiZer:) good method.
but still a problem when the indexed component is used as left value,
for example:
vec[index]= something
any other methods? (i hate to use "switch" in shaders...)

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I think something like this should work (again an example for the y-axis):


// vec[index] = something;
float4 mask = float4(0,1,0,0);
float4 inverseMask = float4(1,0,1,1);
vec = vec * inverseMask + mask * something;



HellRaiZer

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