[GRASS-user] integration of sample.c algorithms into r.resample

[Glynn Clements]

Dylan Beaudette wrote:

> > > note that the slope computed from the bicubic algorithm looks very similar to
> > > the NN slope map -- all computed via the new r.bilinear code. I was under the
> > > impression that the cubic output would be similar to the bilinear,
> > > but "smoother"...
> >
> > The bicubic interpolation formula was taken directly from
> > lib/gis/sample.c; it wouldn't surprise me if it was wrong.

[snip]

> > Try changing the relevant lines to:
> >
> >         DCELL c0 = (u * (u * (u * (c30 - 3*c20 + 3*c10 - c00) + (-c30 + 2*c20  - 5*c10 + 2*c00)) + (c20 - c00)) + 2*c10)/2;
> >         DCELL c1 = (u * (u * (u * (c31 - 3*c21 + 3*c11 - c01) + (-c31 + 2*c21  - 5*c11 + 2*c01)) + (c21 - c01)) + 2*c11)/2;
> >         DCELL c2 = (u * (u * (u * (c32 - 3*c22 + 3*c12 - c02) + (-c32 + 2*c22  - 5*c12 + 2*c02)) + (c22 - c02)) + 2*c12)/2;
> >         DCELL c3 = (u * (u * (u * (c33 - 3*c23 + 3*c13 - c03) + (-c33 + 2*c23  - 5*c13 + 2*c03)) + (c23 - c03)) + 2*c13)/2;
> >         DCELL c  = (v * (v * (v * (c3  - 3*c2  + 3*c1  - c0 ) + (-c3  + 2*c2   - 5*c1  + 2*c0 )) + (c2  - c0 )) + 2*c1 )/2;

> Thanks for the sample code Glynn. I changed your r.bilinear with the
> above, but the output is clearly not correct. I do not have time
> tonight to dig into the equations, however here is a link to the
> output from the above:
> 
> http://169.237.35.250/~dylan/temp/raster_sampling/bicubic_v2_broken.png

Right; that definitely isn't correct.

-- 
Glynn Clements <glynn at gclements.plus.com>