Lutz-Peter Hooge wrote:
> I posted a patch adding bicubic interpolation to image_maps
> to .binaries.programming.

The matrix stated in the description (from image.cpp)

 >* DESCRIPTION
 >*    calculates the interpolation between f[1] and f[2] at position x
 >*    using the cubic function given by:
 >*
 >*                       ( [-0.5  1.5 -2.5  0.5 ]   [f[0]] )
 >*    [x^3, x^2, x, 1] * ( [ 1.0 -2.5  2.0 -0.5 ] * [f[1]] )
 >*                       ( [-0.5  0.0  0.5  0.0 ]   [f[2]] )
 >*                       ( [ 0.0  1.0  0.0  0.0 ]   [f[3]] )
 >*

does not correspond to the code

 >static DBL interp_cubic(DBL f[4], DBL x)
 >{
 >        DBL val;
 >        val=(-0.5*f[0] + 1.5*f[1] - 1.5*f[2] + 0.5*f[3])*x*x*x
 >           +( 1.0*f[0] - 2.5*f[1] + 2.0*f[2] - 0.5*f[3])*x*x
 >           +(-0.5*f[0]            + 0.5*f[1]           )*x
 >           +(            1.0*f[1]                      );
 >        return(val);
 >}
 >

It seems that the code implements the other matrix:

                        ( [-0.5  1.5 -1.5  0.5 ]   [f[0]] )
     [x^3, x^2, x, 1] * ( [ 1.0 -2.5  2.0 -0.5 ] * [f[1]] )
                        ( [-0.5  0.5  0.0  0.0 ]   [f[2]] )
                        ( [ 0.0  1.0  0.0  0.0 ]   [f[3]] )

Is it just a typing mistake or ?


 >A few things to discuss:
 >
  >3) Currently the matrix with determines the coefficients for the
 >cubic interpolation function is hardcoded. Would it make sense
 >to let the user specifiy a different one?

It would be more convenient to have a choice, of course with some
default matrix hardcoded.

Gleb

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