On screen, a group (or object) with opacity, a blend mode, an opacity mask or
knockout whose content blends is drawn offscreen over a copy of its backdrop
and mixed back in by its opacity, mask and clip (a group blend mode then
applies to the backdrop-free result, as PDF defines it). Isolate Blending keeps
the content starting from transparency. Clip groups no longer isolate their
children (blending content is clipped per drawing), and non-isolated knockout
groups composite their elements against the art below the group. Groups
without blending inside still render as plain layers.
In PDF, such groups are written as non-isolated transparency groups (a
constant-alpha soft mask instead of the writer's isolated opacity group),
knockout groups included.
doc: Node::blends_through / children_blend.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Fill and stroke layers (and type characters' fill and stroke) carry an
overprint flag. object.setOverprint {fill?, stroke?, item?, ids?} sets it
(groups: their contents; type: its characters too; one undo step, none when
nothing changes) and attributes.info reads it back (null where mixed).
Overprint Preview and Separations Preview draw overprinting fills and strokes
with Multiply. The old Document.unknown["overprintBlack"] id list migrates to
the new flags on load; Overprint Black now sets the flags.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Knockout Group is now Knockout {Neutral, Off, On} (old files' true loads as
On, false as Neutral). In a knockout group each child erases what the
children below it drew where it paints, then adds itself, so it composites
against the group's backdrop; neutral plain groups pass their children
through to the enclosing knockout group. transparency.set / object.setProps
take knockout "on"|"off"|"neutral"|bool; transparency.info reports the
state. PDF and SVG write knockout groups as nested soft masks with the same
look (PDF warns); the Transparency panel has a tri-state checkbox.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
