Fix organic tree smoothing drift and layer mapping (#14069)

[bluetianyu]

PR OrcaSlicer#14069 — Fix organic tree smoothing drift and layer mapping

Summary

This PR fixes two defects in the organic tree support post-processing pipeline:

1. Smoothing drift: During iterative collision avoidance + Laplacian smoothing, node positions could drift without bound because only per-iteration step caps were enforced—cumulative displacement had no constraint. Over up to 100 iterations, a node could theoretically drift up to 70mm, breaking the 3D branch structure.

2. Layer mapping for raft layers: layer_idx_ceil() and layer_idx_floor() used a uniform-spacing formula that failed for all Z values falling within the raft-layer range. The formula returned raft_layers.size() (the first object-layer index) for every raft-zone Z, causing collision detection to skip raft layers entirely.

Changes

TreeSupport3D.cpp — Add vertical-structure constraints

Added limit_candidate_to_linked_layers lambda inside organic_smooth_branches_avoid_collisions(). After each collision-avoidance or Laplacian-smoothing nudge, the candidate position is now constrained by the physically reachable range from its linked upper (parent) and lower (child) nodes:

allowed_shift = max(0, neighbor_radius - current_radius) + maximum_move_distance_slow;
candidate = constrain_to_anchor(candidate, neighbor.prev_position, allowed_shift);

• The constraint is applied after both the collision-avoidance nudge and the Laplacian-smoothing nudge.
• constrain_to_anchor is a simple clamp: if the candidate exceeds allowed_shift from the anchor, it is pulled back along the same direction to the boundary; otherwise it passes through unchanged.
• The allowed_shift formula is physically meaningful: the radius difference between connected nodes represents the lateral clearance the branch geometry can accommodate, plus the configured slow-move margin.

TreeSupportCommon.hpp — Fix Z-to-layer-index mapping for raft layers

Rewrote layer_idx_ceil() and layer_idx_floor() to handle raft layers and object layers separately:

• Raft-layer zone (z < first_object_z): Uses std::lower_bound / std::upper_bound binary search on the ordered config.raft_layers array to find the exact index.
• Object-layer zone (z >= first_object_z): Uses the existing linear formula (which was correct for this zone).
• Extracted first_object_support_layer_z() helper to clarify the boundary between raft and object layers.

Before / After

Scenario	Old behavior	New behavior
layer_idx_ceil(0.35) with raft at [0.20, 0.40, 0.55]	Returns 3 (wrong—first object layer)	Returns 1 (correct—raft1)
layer_idx_floor(0.35) with raft at [0.20, 0.40, 0.55]	Returns 3 (wrong—first object layer)	Returns 0 (correct—raft0)
Node after 50+ smoothing iterations	Can drift arbitrarily far from parent/child	Constrained within allowed_shift of linked layers

Scope of impact

• Affected code: organic_smooth_branches_avoid_collisions() and layer_idx_ceil/floor()
• Affected feature: Organic tree support only (not grid / snug / default supports)
• Behavior change: In normal cases, the constraints are wide enough to not interfere with legitimate collision avoidance. The fix only prevents pathological drift and incorrect layer lookups.
• Backward compatibility: No changes to profiles, project files, or configuration.

Testing

• Verified that collision avoidance still converges (num_moved reaches zero) within the iteration limit.
• Verified layer_idx_ceil/floor return correct indices for Z values across raft layers, the raft-object boundary, and object layers.
• Manual comparison of organic tree support 3D preview before/after on models known to trigger drift.

Files changed

File	Lines
src/libslic3r/Support/TreeSupport3D.cpp	+60
src/libslic3r/Support/TreeSupportCommon.hpp	+29

Before fix：

default.mp4

After fix：

default.mp4