On Thu, 26 Apr 2007 08:38:42 +0200, Sven Neumann <[EMAIL PROTECTED]> wrote:

> Hi,
>
> On Wed, 2007-04-25 at 09:08 +1000, Campbell Barton wrote:
>> just did a simple test
>>
>> * 16x16 rgb image
>> * select half (vertically in my case) and fill in
>> * run the Gaussian blur, 1 blur pixel
>> * Hold  Ctrl+F to rerun the filter many times
>>
>> - Image will eventually turn black
>> - Happens for RLE and IIR
>
> As explained already, that's slightly off-topic as we were discussing
> the Blur tool, not the Gaussian Blur plug-in. But since you are bringing
> it up, let's have a look at the plug-in as well...
>

It seems that the initial report has pointed out a similar error in the
filter. I said some time back that the use of gint() rather than round is
fairly indemic in gimp and is an area that needs following up.

The errors are not aweful, slap in the face errors but it seems very many
operations are degrading the image somewhat.

> What you describe sounds like a very artifical test.

That is a comment that you often put forward in replies to bugs but it is
necessary to develop specific test cases to pinpoint and quantify a
problem.

There is little point in trying to blur a "real" image like a photo and
then say it "it seems a bit darker".

Nearly all tests are necessarily artifical.


> As I outlined already, small rounding errors are hard to avoid.

Trying to eliminate truncations of 8 bit data would be a big help. The
only thing that would be hard is finding them all ;)

> So it is not too
> unlikely that, even for a rather stable algorithm, you can create a test
> that, when repeated often enough, will give an error. Your test image is
> very small, so the contribution from the border is large. But of course,
> an ideal blur algorithm should deal correctly with that.
>

> If we look at a larger test image filled with stripes of white and black
> color, the plug-in seems to do its job rather well. So I don't think
> there's urgent need to review the plug-in.

I described a large image not affected by border issues and posted a
histogram showing the mean 112 grey level . Since it is not too disimilar
  from your test case it's surprising that the results are so different.

Maybe you could post your test image somewhere so that we can compare the
results and see why there is a clear difference.

/gg


> But of course we will
> appreciate any patches that improve the result without hurting the
> performance.
>
>
> Sven
>
>
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