This commit will make all transform tools run on a link layer cumulate
their transform matrix on top of the previous transform steps. It means
that as long as you don't edit the pixels another way (e.g. with a paint
tool), all your transformations will apply as a single transformation.
For instance it means that applying several transform tool steps on a
monitored link layer will be less destructive than applying the exact
same transformations on the exact same "normal" layer contents.
Even scaling an image to 1x1 then back to a big size will work very
fine!
Note nevertheless the following limitations in current implementation:
* The link layer with transformations will still show as a standard link
layer. Nothing says it has transformation applied on it right now.
* To drop transformations applied on a link layer, you have to discard
the link info, then monitor the link again. A specific action in the
contextual menu could be worth it.
* This should work with all transform tools (scale, rotation, unified,
perspective, 3D, even Warp…) but it won't work for the Flip tool, nor
will it work for the Transform actions. I will need to implement
GimpItem's rotate(), flip() and resize() methods next.
* The layer mask would still be destructively transformed (I have not
made any tests with layer masks yet, but this should be done next
too).
* I think that the "scaled-only" property is now meaningless. It is now
being replaced by the presence of the GimpMatrix3. Nevertheless I have
still not removed this property.
* The load/save code has not been redone yet to include all these
changes.
The kind of caveats we'd have to know about (and which are not planned
for change, because it's just how it is):
* Any intermediate interpolation methods are dropped when cumulating
transform steps. Only the last interpolation is stored. This is
because anyway the interpolation is only there as the best algorithm
where we visually see the less quality loss. Applying several
transformations as a single matrix will always be visually better than
applying several matrices (whatever the intermediate interpolation
methods chosen).
* This only works with the "Adjust" clipping method (basically no
clipping) because 2 transform steps with clipping won't produce the
same result as the multiplied matrix with clipping. It means that
applying a transform with clipping will downgrade your link layer to
being a normal layer.
The only issue I have with this is how to best convey that clipping is
a major setting setting here, which disables our less-destructive
abilities. Right now, people will just have to "know" it.
* Vector link layers in particular will have 2 levels of
non/less-destructivity transforms. In particular any scaling (both
through "Scale Image", "Scale Layers" and transform tools — since I
added code to detect a matrix doing only scaling and optionally
translation) done **first** will be completely non-destructive since
we will simply reload the original vector source at the right
dimensions. Any other kind of transforms will be appended through the
cumulative matrix, as raster link layers. This also includes scaling
done **after** other transforms, since we cannot easily move the
scaling first (matrix multiplication is not commutative). This second
level of scaling will therefore be *less* destructive, but still
destructive.
It is possible eventually to improve the whole thing if we add some
day the ability to request loading a vector image with a transform
matrix (it will then be up to each vector format plug-in to support
this feature or not).
Note: it could be argued that this whole implementation could in fact be
moved over to base layers, since it would allow also less-destructivity
when applying multiple transformations in a row. We would only merge
results once we edit pixels more directly.
But I think that it's not a bad idea to experiment with this new code in
the link layer. Eventually I may likely move this to the parent
GimpLayer if no blocking issues are found.
Instead of keeping a "modified" property in GimpLinkLayer, we just check
if the link is being monitored. It's also a better wording because we
may "discard the link information" without actually modifying the layer
pixels.
Also I now actually shut down the file monitoring process. This can be a
bit expensive so when we don't need it, let's really free the
GFileMonitor.
This commit also fixes scaling of link layer (which got broken along the
way) and improves the undo code.
Note: I'll probably want to modify the XCF flags, but let's do this in
the end, depending on further changes too.
These are layers who content depends on another source (right now only
an external image). This can be useful when for instance working at
several people on a single artwork, hence being able to load new
versions of an image without even touching anything in the XCF (for
instance, say you draw an animated character while someone else is
taking care of the background).
Similarly to what we do for text layers, once you start modifying the
contents, it turns into a "normal" layer. The link information is still
available though, so it is possible to revert to the monitoring state
with the menu item "Monitor Linked Image" which appear when a link layer
became a normal layer.
This is not finale as I'm still experimenting. In particular, I have not
implemented XCF saving/loading yet for this new layer type.
