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.
(cherry picked from commit 27e8f452b3)
Pass through mode uses the same compositing logic as REPLACE mode,
however, it's a special case of REPLACE, where the layer is already
composited against the backdrop. This allows us to take a few
shortcuts that aren't generally applicable to REPLACE mode.
Add a dedicated op class for pass through mode, derived from the
REPLACE mode op, implementing these shortcuts.
