Re: Blur tool darkens image?

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Hi,

On Thu, 2007-04-26 at 12:38 +0200, peter@xxxxxxxxxxx wrote:
> 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.

Yes, and I said in response that we are well aware of the fact that a
lot of the algorithms used in GIMP just plain suck and that there's a
lot to do all over the place. Now what's your point in repeating that
over and over again? Please point out specific problems and, preferably,
come up with patches.

> > 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.

Artificial was the wrong term to use here. Sorry about that. What I
meant to say is that the proposed test is not well chosen since it
doesn't test the problem pointed out here. Instead it focuses on the
behavior at the image border which is a different problem.

> 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.

I couldn't reproduce your test, probably because you didn't provide all
parameters (like for example the blur radius). But I think it suffers
from the same problem. In order to blur two large areas in a way that
yields a single (almost) solid colored area, you need to use a large
blur radius. Your test image is then small compared to the blur radius
and again you are mainly looking at the border behaviour.

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

Any image with a test pattern that is smaller than the blur radius will
do. You can use the Stripes pattern or create a pattern using the Grid
or Checkerboard plug-ins.

I have reduced the rounding errors in our blur plug-ins today. The
results are a lot more stable now, even after multiple iterations.


Sven


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