How do you sample an image at a computed coordinate in the context of a Blender Internal node-based texture? I am experimenting with procedural textures, and I'd like to create a texture like

x = A.dot(orco-P0)
y = B.dot(orco-P0)
output = image[x][y]


It is not THAT hard to compute dot products in nodes (although it is a lot of clicking).  The roadblock I am hitting is that the Image input has no input for coordinates.

How can I accomplish my mission in texture nodes?  (This is using Blender Internal in case that wasn't clear from the subject line)
The calculations belong in the material not the texture.

The nodes for a texture define the colour for every available point as if it was an image.

The texture node in a material requests the colour from the texture at a specific point. The vector input to the texture node defines the point requested from the texture.