Module transparency 

Transparency group compositing — replaces Splash::beginTransparencyGroup, Splash::endTransparencyGroup, and Splash::paintTransparencyGroup.

PDF transparency model (§11.3–11.4)

A transparency group is an intermediate compositing surface. The caller:

1. Calls begin_group to allocate a fresh group bitmap and push it onto the stack. All subsequent paint operations target that group.
2. Renders into the group normally (fill, stroke, image, shading, glyph calls on the group bitmap).
3. Calls paint_group (or discard_group on error) to pop the group and composite it back into the underlying bitmap.

Isolated vs. non-isolated groups

Flag	Effect
isolated = true	Group starts with a transparent background (alpha = 0).
isolated = false	Group is pre-initialised with the backdrop’s colours.

Knockout groups clear the accumulated alpha on each object; non-knockout groups accumulate.

Soft masks

When soft_mask_type != SoftMaskType::None, the group is later used as a luminosity or alpha soft mask rather than being composited directly. Call extract_soft_mask on the finished GroupBitmap to obtain the mask bytes, then store them in crate::GraphicsState::soft_mask after wrapping in AnyBitmap.

C++ equivalents

• Splash::beginTransparencyGroup
• Splash::endTransparencyGroup
• Splash::paintTransparencyGroup

Structs

GroupBitmap

A group bitmap together with its compositing metadata.

GroupParams

Parameters for one transparency group, collected before begin_group.

Enums

SoftMaskType

Whether the group’s soft-mask channel is alpha-based or luminosity-based.

Functions

begin_group

Open a new transparency group and return a group bitmap to render into.

discard_group

Discard a group without compositing it (used on error paths).

extract_soft_mask

Convert a finished group bitmap into a single-channel soft mask.

paint_group

Composite a finished group back into the parent bitmap and return the saved clip.