In GimpOperationLayerMode and GimpOperationReplace, make sure we
don't return a NULL output buffer, or forward a NULL input buffer,
but rather create an appropriate empty buffer in this case. This
avoids wrong results when the layer-mode op's output is connected
to the aux input of a subsequent op, as a result of the op behaving
differently with a NULL aux buffer (in particular, this can happen
when a drawable filter's output bounding box is smaller than the
drawable.)
When the result of compositing has an alpha value of 0, the
corresponding color value is not mathematically defined.
Currently, all out layer modes opt to preserve the destination's
color value in this case. However, REPLACE mode is different
enough to warrant a different behavior:
Unlike the other layer modes, when the compositing opacity
approaches 0 or 1, the output color value approaches the
destination or source color values, respectively, regardless of the
output alpha value. When the opacity doesn't approach 0 or 1, the
output color value generally doesn't approach a limit as the output
alpha value approaches 0, however, when both the destination and
source alpha values are equal, the output color value is always a
simple linear interpolation between the destination and source
color values, according to the opacity. In other words, this means
that it's reasonable to simply use the above linear interpolation
for the output color value, whenever the output alpha value is 0.
Since filters are commonly combined with the input using REPALCE
mode with full opacity, this has the effect that filters may now
modify the color values of fully-transparent pixels. This is
generally desirable, IMO, especially for point filters. Indeed,
painting with REPLACE mode (i.e., with tools that use
gimp_paint_core_replace()) behaved excatly as described above, and
had this property, before we switched gimp_paint_core_replace() to
use the common compositing code; this created a discrepancy between
painting and applying filters, which is now gone.
A side effect of this change is that we can now turn gimp:replace
into a NOP when the opacity is 100% and there's no mask, which
avoids the compositing step when applying filters. We could
previously only apply this optimization to PASS_THROUGH mode, which
is a subclass of REPLACE mode.
Note that the discussion above concerns the UNION composite mode,
which is the only mode we currently use REPLACE in. We modify the
rest of the composite modes to match the new behavior:
CLIP_TO_BACKDROP always preserves the color values of the
destionation, CLIP_TO_LAYER always preserves the color values of
the source, and INTERSECTION always produces fully-zeroed pixels.
Our composite modes don't correspond directly to the Porter-Duff
operators after which they're named, and these names aren't too
descriptive anyway.
Rename the composite modes as follows:
Source Over => Union
Source Atop => Clip to Backdrop
Destination Atop => Clip to Layer
Source In => Intersection
Update relevant code, including UI text, enumerator names, function
names, and action names.
Commit 3635cf04ab moved the special
handling of bottom-layer compositing to GimpOperationLayerMode.
This required giving the op more control over the process()
function of its subclasses. As a temporary workaround, the commit
bypassed the subclasses entirely, using "gimp:layer-mode" for all
modes. This is the reckoning :)
Add a process() virtual function to GimpOperationLayerMode, which
its subclasses should override instead of
GeglOperationPointComposer3's process() functions. Reinstate the
subclasses (by returning the correct op in
gimp_layer_mode_get_oepration()), and have them override this
function.
Improve the way gimp_operation_layer_mode_process() dispatches to
the actual process function, to slightly lower its overhead and
fix some thread-safety issues.
Remove the "function" field of the layer-mode info array, and have
gimp_layer_mode_get_function() return the
GimpOperationLayerMode::process() function of the corresponding
op's class (caching the result, to keep it cheap.) This reduces
redundancy, allows us to make the ops' process() functions private,
and simplifies SSE dispatching (only used by NORMAL mode,
currently.)
Move the blend and composite functions of the non-specialized
layer modes to gimpoperationlayermode-{blend,composite}.[hc],
respectively, to improve code organization.
Move the SSE2 composite functions to a separate file, so that they
can be built as part of libapplayermodes_sse2, allowing
libapplayermodes to be built without SSE2 compiler flags. This
allows building GIMP with SSE acceleration enabled, while running
the resulting binary on a target with no SSE accelration.
Add a "blend_function" field to the layer-mode info array, and use
it to specify the blend function for the non-specialized modes.
This replaces the separate switch() statement that we used
previously.
Remove the "affected_region" field of the layer-mode info array.
We don't need it anymore, since we can go back to using
GimpOperationLayerMode's virtual get_affected_region() function.
Last but not least, a bunch of code cleanups and consistency
adjustments.
... so that we can use it for other functions that involve
compositing regions (which we do in the next commit).
Rename gimp_operation_layer_mode_get_affect_mask() and
friends to _get_affected_region().
Stuff like passing "input" directly if "aux"'s opacity is 0, etc.
Used to be partly handled by normal mode, even though it applies
to other modes too.
Adjust the logic for the new compositing modes.
Add a GimpLayerModeAffectMask enum, and a corresponding
get_affect_mask() function to GimpOperationLayerMode, which
specifies which of the op's inputs, if any, are affected by the
mode, apart from the overlapping regions. Most modes affect only
the overlapping regions, but dissolve and replace also affect the
rest of the input. This information is used for determining if
the optimizations are applicable.
It is used both for blending and compositing, the repeated use of the word
BLEND in code made the logic involving both blending and compositing hard to
read.
Implement a common utility function gimp_blend_composite that uses utility
functions for implementing layer modes, with separate (possibly SIMD) optimized
loops for blending and compositing, with configured linear TRC, perceptual
gamma TRC or even using CIE Lab as the space.
