Thierry Tracol (thierry.tracol@xxxxxxxxxxxxxx) wrote:
> With Gimp for Windows - version 1.2.3, I do the following operations :
> 1. Start with a base image
> 2. Make an elliptical selection (I have also the problem with other
> kinds of selection).
> 3. Copy it, clear it.
> 4. Put the floating selection into a new layer L1
> 5. In layer L1, go to step 2 throw step 4 and create layer L2
> 6. ....
Although I have problems to follow your steps (after step 3 there is
no floating selection) I guess I know what your problem is: It is
inherent to the way most image manipulation programs handle layers
and not a bug in the GIMP.
> In theory, the base image should be split into the different layers.
> With all the eye icons on, we should see the original base image.
Nope, this is not true and ignores, that there is the concept of
partially selected pixels - where "partially" means a percentage
and not a specific part of a pixel.
The problem is the same as in the following procedure:
1) Create new (white) image and select an (antialiased) ellipse
2) fill the ellipse with black
3) fill the ellipse with white.
Step 3 is the opposite of step 2 - so the image should be completely
white. But if you disable the selection you see that there is a
faint gray border around the former ellipse. The reason for this is,
that some of the pixels at the border have been partially selected.
Lets look at a pixel that is selected 50%.
In step 1 it is white. After filling it with black it is
50% black (since it was not fully selected). Now we fill it with
white again: it becomes 25% black ((1 - 0.5) * original color + 0.5 *
new color). So we have a slight gray pixel there and it is perfectly
reasonable...
The same can happen when you place two partially transparent pixels
on top of each other in two layers: 50% opacity in the first and 50%
opacity in the second layer does not sum up to 100% opacity, it is
around 75% opacity (the math is basically the same). So if you
cut out an ellipse and place it in a second layer on the very same
place you will get a slightly transparent border around the ellipse.
There is no way to fix this except filling the area in the lower
layer with a background color.
Hope this helps.
Bye,
Simon
--
Simon.Budig@xxxxxxxxxxx http://www.home.unix-ag.org/simon/
