The answer is given by the definition of 'terms':

    terms = (row < 4) ? row : 4;

In pratice, that means that
  - with row<4 the loop is

      for (i = 0; i <= row; i++) {
       ...
      }
 
- with row>=4 the loop is
      for (i = 0; i <= 4; i++) {
       ...
     }





Alastair M. Robinson wrote:
> Hi,
>
> I may be missing something obvious here, but I'm trying to understand
> the workings of the Gaussian Blur plugin, since I need to implement
> something similar myself, and either there's something screwy here, or
> there's something obvious I'm missing.
>
> In gimp-2.4.4/plugins/common/gauss.c, the gauss_iir() function,
> at line 960 we have four pointers initialised - two to the beginning of
> their respective buffers, and two to one tuple before the end.
>
>            sp_p = src;
>            sp_m = src + (height - 1) * bytes;
>            vp = val_p;
>            vm = val_m + (height - 1) * bytes;
>
> But then the inner loop does this:
>    for (b = 0; b < bytes; b++)
>      {
>        vpptr = vp + b; vmptr = vm + b;
>        for (i = 0; i <= terms; i++)
>          {
>            *vpptr += n_p[i] * sp_p[(-i * bytes) + b] - d_p[i] * vp[(-i *
> bytes) + b];
>            *vmptr += n_m[i] * sp_m[(i * bytes) + b] - d_m[i] * vm[(i *
> bytes) + b];
>          }
>          ...
>
> On the first run through, with b=0, the index [(-i * bytes) + b] will be
> negative for all but the first iteration, yet it's used with the sp_p
> and vp pointers, which point to the beginning of their buffers, thus
> accessing memory outside the buffer.  Or am I missing something here?
>
> And while we're on the subject, can anyone point me to an explanation of
> the maths behind this IIR approximation of the Gaussian filter?  I
> understand Gaussian blurring well enough to implement a
> convolution-based version, but I want to implement a
> local-contrast-stretch filter - basically a gentle large-radius
> unsharp-mask, which would require unfeasibly large convolution matrices
> to do it that way.
>
> All the best,
> --
> Alastair M. Robinson
>
>
> _______________________________________________
> Gimp-developer mailing list
> Gimp-developer@lists.XCF.Berkeley.EDU
> https://lists.XCF.Berkeley.EDU/mailman/listinfo/gimp-developer
>
>   

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