Re: Image rendering very slow with elaborated selections (v.2.4.1)

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From: "Jose Pires" <Jose.Mario.Pires@xxxxxx>
Date: Thu, 22 Nov 2007 00:36:08 +0000

> I wonder if someone is experiencing the same
> problem: whenever I have a more complicated
> selection active (namely those resulting from a
> lot of clicks in "fuzzy select" mode), Gimp slows
> down to the point of not even displaying the menus
> or filling dialog boxes. The problem disappears
> immediately as soon as I deselect everything.

> I began by suspecting my new toy: a tablet pen
> (Wacom Bamboo One), which I acquired some days
> after installing Gimp 4.0, but the fact is that
> the problem also shows with it unplugged to the
> computer. I didn't use version 4.0 much before I
> installed the tablet.

Well, two things can slow programs down
dramatically, overwhelming CPU cycle needs, and
using so much RAM that the program and/or its data
overflows to swap memory. Keeping the MS-Windows
task manager open to the "processes" tab should
let you distinguish the cases.

The task manager tells you (at the bottom) how much
of your total real + swap memory is allocated; if it
is more than your real memory, _some_ programs are
swapped out. If it's a lot more than your real
memory, GIMP is surely one of them.

If the problem is overflow to swap, turn off any
other optional programs you have running, and see if
that gives fairly immediate help.

The task manager also tells you (beside the GIMP
task process) what percent of CPU cycles GIMP is
using. If that gets and stays high, you almost
certainly are _not_ swapping heavily; waiting for
swap makes a program idle most of the time. In that
case you're likely to have pressed your processor
to its limits "walking ants".

On the gripping hand, there can be issues separately
if you've given GIMP too little _or_ too big an
allocation for "tiles memory". Too little and tiles
are unavailable when needed because they are in swap
memory, too much and the tiles crowd the executable
code into swap memory instead.

All that said, there are some things you can do to
minimize the problem.

Select => sharpen will make the selection follow
pixel boundaries (if I understand correctly) which
will cut down on "within pixel" math, but may give
your end result of using the selection a case of the
"jaggies" you don't like.

Using the selection editing tool (red box, lower
left corner of the image window) with the foreground
color set to black and alternately using the pencil
or the eraser will let you quickly paint in needed parts
of the selection rather than fighting with fuzzy
selection to get every missed pixel, and also turns
off "walking ants" while it is in use. It usually
suffices to just use fuzzy selection to select the
"exact" edge of the area you want. It's then pretty
easy to "select by painting" the interior part with
a big pencil brush.

A complex trick that lets you do fuzzy selection
piecemeal is to create a new transparent layer over
your work layer, do some selections focused on your
work layer, then focus on the new layer and fill the
selection with white (or black, doesn't matter),
then clear the selection, go back, and select some
more bits. Repeat this cycle as much as needed until
the new layer is color-filled right where you want
selection to happen in the work layer. Now on the
layers dialog, with the new layer as focus, do
"alpha to selection", which will give you back the
selection you just built all in one piece, shift
focus to the work layer and do whatever was your
intention with that selection. Since the "fill" will
be translucent in the "fuzzy" parts of the fuzzy
selection, this gives you a smooth selectoion at the
edges when that translucency is taken into account
by "alpha to selection" deciding how _much_ selected
the translucent part is based on what alpha value it

[That was probably clear as mud, but it's the best I
know how to describe it.]


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