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

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