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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>