[Gimp-developer] preview window does not work

[gg at catking.net]

On Wed, 07 Mar 2007 08:53:34 +0100, Sven Neumann <sven at gimp.org> wrote:

>> 3) Ok, this is the serious one: The whole blurred region is shifted 1
>> pixel down. To check this, create a new white image with a single
>> black pixel in the middle, and blur it with radius=1. Go back and
>> forth and you will see the 3x3 gray square shifting down.
> Again, how is that a problem? This example code is meant to illustrate
> some concepts of GIMP plug-in development, in particular how to add a
> preview to your plug-in window. It is not meant to perform any useful
> image manipulation.


PS.

Blur off the menu does not seem to reproduce this. So either its a buggy  
example as Svan suggests or a drift in the plugin interface.

A quick check on rotation shows 1px drift in  x and y. Create a 20x20 and  
a 21x21 canvas put a pencil tool (11 circle) spot and rotate 90 degrees  
about the centre of the dot. Preview gets it right but it moves when  
committed. Also odd effects around borders

The offsets seem to be independant of interp method.

A non-exhaustive check on on scaling seems not to show this offset but  
like I said I found I had to add fiddle factors to achieve this. I added  
some comments to code to attempt to explain what was happening but it was  
a bit hand wavy. My belief is that I am applying a correction to an offset  
coming from outside the scaling code.


gg.