texture interpolating

I have a 1d texture and a quad with four vertices. How is the tex color interpolated in the inner reagion of the element. Is there a paramter to define?

Interpolation is linear on the triangles, (it is possible to define a quad with non linear texture and so you see a seam or kink in the interpolation which may be why you are asking). You can change this linear interpolation using 4D texture coords, s,t,r,q where q ia used to weight the interpolation (it adjusts the perspective correct interpolation). For a 1D texture t and r will be ignored, just s and q will be used.