MATLAB trilinear interpolation

I'm working with large 3D data and have to interpolate in 3D space. With Matlab's interp3 function my computer hangs because of high memory requirement of vectorized interp3. So i decided to write a mex file to sequencially interpolate the data. There is a speed gain up to 5x.
This function acts as matlab's interp3 function when interp3 is set to linear. However there are some differences. First of all trilinear requires monotonically spaced input grid such as produced by meshgrid. Also trilinear uses zero as extrapolaiton value (can be changed as an input parameter).
This is my first mex code and can be improved such as this code can be en equivalent of interp3.