Paints a transparency group: its content composited as a group, then the result composited onto the backdrop as one object.
void DrawGroup(in DisplayGroup group)
The group: whether it is isolated and knockout, its blending colour space, the alpha and blend mode its result composites with, its bounds, and its content (Replay(IRenderingBackend)). The list does not replay the content itself: the backend replays it, usually into an offscreen layer, once or more.
ISO 32000-2 §11.4 (group compositing, §11.4.4 to §11.4.6 and the summary in §11.4.8), §11.6.6 (transparency group XObjects: initial backdrop, initial blend mode and alphas, group colour space, final compositing) and §11.7.4.4 (the implicit non-isolated knockout group of a combined fill and stroke). The content's paints composite with one another into the group, over a transparent initial backdrop when the group is isolated and over the backdrop at the group otherwise (in a knockout parent, the parent's initial backdrop); in a knockout group each paint composites with the initial backdrop only and replaces, in proportion to its shape, what earlier paints of the group left. The group's colour, shape and alpha (with the backdrop's contribution removed, §11.4.4) then composite onto the backdrop with Opacity and BlendMode, through the clip in force at this call (§8.5.4) and SoftMask (§11.6.4.3).
Colour: in a group with a ColorSpace colours convert to it and blend there (subtractive spaces complemented, §11.3.4); the result converts to the parent's space with Conversion. The page group (Page) is isolated and composites onto the page's white (§11.4.7).
A backend that does not composite groups can replay the content directly (group.Replay(this)): that is exact for a
non-isolated, non-knockout group in the Normal blend mode at opacity 1 (§11.4.4 NOTE 5), and an approximation otherwise.