On Thu, Jul 23, 2009 at 11:41 AM, Martin Cracauer <craca...@cons.org> wrote:

> Martin Nordholts wrote on Thu, Jul 23, 2009 at 12:40:06AM +0200:
> > On 07/23/2009 12:27 AM, Martin Cracauer wrote:
> > >Do I understand that correctly that you will move to
> > >single-floats/color?
> >
> [...]
> >
> > If you in the yet not officially released GIMP 2.7 do View -> Use GEGL,
> > then the layers will be composited using GEGL. In other words, we have
> > the layers etc ported to a GEGL graph. It is worth mentioning that it is
> > technically trivial to insert non-destructive nodes in the graph, but
> > our focus now is getting GIMP 2.8 out.
>
> I use the git version of last week.  Lost my tablet (probably due to
> some dbus API issue) but works otherwise.
>
> Let me just poke some more.
>
> And does all this survive layer copying and other changes?


GEGL graphs are completely non-destructive (of course, you can flatten part
or all of a graph  to destroy information at any time.)
The plan for GIMP here is that it will only modify the graph in the course
of usual operations, which will enable the option of fully non-destructive
workflow.
In this system, we simply have to decide the way to present this ability to
insert arbitrary nodes at arbitrary positions to the user.
The idea I regard as most sensible here is simply treating nodes like a
container -- that is, the input being made up of 1 or more node outputs
composited together. With this idea, you would be attaching an effect to a
group of layers (and effects

There are issues with the above (primarily, I oversimplified -- we need to
deal with nodes that simply produce, like eg. checkerboard, constant color,
as well as filtering nodes), but that's a reasonable overall way to think
about it for now.
IMO GIMP is heading towards providing a fairly thin abstraction layer over
the abilities of GEGL graphs, which in general is good news for anyone
concerned about possible leaky abstraction ("does all this survive layer
copying and other changes") -- thick abstraction layers tend to be much
leakier (for example, Photoshop's "Adjustment Layers" idea -- they are only
layers in an absurdly broad sense (so broad as to be nearly meaningless), so
they tend to disobey common-sense rules followed by other types of layer.)


In short:

the capacity for inserting arbitrary nodes is available in GEGL, but GIMP
does not currently adulterate the constructed layer graph in any way; When
some ability to control the graph in a finer way (like I described) is
implemented, we ought to have moved on to the next-generation file format,
which should support it in a straightforward way

Even shorter:
A completely theoretical yes, since such functionality is currently
available in GEGL, not leveraged by GIMP or presented to the user yet.


>
> Let's use an example: I like to use the levels tools with a
> non-destructive adjustment first and although 2.6/2.7 allow me to take
> the levels I found right to curves I usually don't do this.  I prefer
> to commit the level change, then duplicate the layer and mess with the
> curves in the new layer.  This, of course, causes me losses from
> interpolation with the 8-bit channels, where it would not if I would
> edit levels and curves in the same moment without committing levels
> first and start over with curves.  Does current 2.7 carry floating
> point layers through all of this?


There are no floating point layers yet. During the composition, the layer
pixels are automatically converted to floating point, and converted back
after composition. to display the finished projection. Thus, the difference
in the resulting image is minimal.

During the application of a color tool, a similar thing happens: pixels are
converted to float, the change is applied, and pixels are converted back.
Eventually, using a color tool will just modify the image graph rather than
directly writing pixels; at this time,
floating point values will be preserved through that operation (and
presumably most subsequently operations -- obviously if there is an explicit
'convert to 8bit' operation in use,  floating point values are not going to
last beyond that.


>
> I just tried this and I get the same teeth in the histogram in 2.7 no
> matter whether I asked it to use GEGL or not, but I'm not sure I
> activate it the right way.  This was just on a layer originating from
> a JPEG.
>
> Does 2.7 as is support storing and reloading the floating point format
> in *.xcf files?

Again, floating point currently has nothing to do with the normal
representation of image in the GIMP, yet.
This should answer your question.
I believe we are planning to move to a new native image format for 3.0,
which will address such problems as: metadata support being bolted on rather
than a standard part of the file format, more sophisticated ICC support,
support of higher bitdepths, support of different color models (LAB, YCbCr
etc)...

David
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