xna 4.0 - How to access the nth element in a 2d texture from a pixel shader in XNA

How can I access the nth element in a texture2d from a pixel shader? For example if I wanted to get the 5th vector4 from 10 x 10 texture2d

Answer

2D textures usually aren't addressed in terms like the "nth element". The whole point of a 2D texture is that you want to access it using 2D coordinates. If you want to send a 1D array to the shader I'd use a 1D texture.

That being said, you can compute it by something like this:

int x = n % textureWidth;
int y = n / textureWidth;                            // integer division
float u = (float(x) + 0.5) / float(textureWidth);    // map into 0-1 range

float v = (float(y) + 0.5) / float(textureHeight);
float4 result = tex2D(sampler, float2(u, v));

(This assumes you already know how to set up a sampler parameter in the shader and bind a texture to it from the application side.)