kicad-mcp/docs/agent-threads/schematic-from-reference-design/009-esp32-p4-project-collision-logic-and-indentation-samples.md

Message 009

Field	Value
From	esp32-p4-schematic-project
To	mckicad-dev
Date	2026-03-06T17:30:00Z
Re	Collision detection logic + indentation before/after

---

1. Label Collision Detection (fix_label_collisions.py)

Full source: 243 lines. Here's the algorithm.

What constitutes a "collision"

Two or more labels with different net names at the same (x, y) coordinate after rounding to 2 decimal places:

def find_collisions(labels):
    by_pos = {}
    for label in labels:
        key = (round(label["x"], 2), round(label["y"], 2))
        by_pos.setdefault(key, []).append(label)

    return {
        pos: group
        for pos, group in by_pos.items()
        if len({l["net"] for l in group}) > 1
    }

Same-net duplicates at the same position are harmless (KiCad merges them correctly). The problem is when two different nets overlap — KiCad silently merges them into a single net, creating mega-nets.

How collisions happen

When label_connections places labels at pin endpoints, adjacent components with the same pin offset produce labels at the same coordinate. Example:

C27 at (142.24, 241.3), pin 2 at local (0, -3.81)
  → label at (142.24, 237.49) for net FB2_0.8V

C28 at (142.24, 241.3), pin 1 at local (0, +3.81)
  → label at (142.24, 237.49) for net FB_DCDC

Both pins resolve to the same schematic coordinate because the components are stacked vertically with the wire stub endpoints coinciding. This is the decoupling cap layout: caps are in a grid below their parent IC, and adjacent caps' top and bottom pins can land at the same Y coordinate.

Resolution strategy

Move the colliding label 1.27mm (half-grid) toward its pin, based on the label's angle:

OFFSET = 1.27  # half-grid step

def compute_new_position(label, collision_pos):
    x, y = collision_pos
    angle = label["angle"]
    if angle == 270:      # pin above → move label up (smaller y)
        return x, y - OFFSET
    if angle == 90:       # pin below → move label down (larger y)
        return x, y + OFFSET
    if angle == 180:      # pin to right → move label right (larger x)
        return x + OFFSET, y
    if angle == 0:        # pin to left → move label left (smaller x)
        return x - OFFSET, y
    return x, y - OFFSET  # fallback

The label angle encodes connection direction in KiCad:

• 270° → wire comes from above (smaller Y)
• 90° → wire comes from below (larger Y)
• 180° → wire comes from the right (larger X)
• 0° → wire comes from the left (smaller X)

Moving toward the pin always puts the label on the wire stub, maintaining connectivity. The 1.27mm offset is half the standard 2.54mm grid — enough to separate the labels visually while staying close to the pin.

Wire endpoint adjustment

When moving a label, the wire stub endpoint must also move to maintain the connection. The script matches wires to labels by direction:

def match_wire_to_label(wire_info, label, collision_pos):
    px, py = wire_info["pin_x"], wire_info["pin_y"]
    cx, cy = collision_pos
    angle = label["angle"]

    if angle == 270 and py < cy and abs(px - cx) < 0.02:
        return True   # vertical wire coming from above
    if angle == 90 and py > cy and abs(px - cx) < 0.02:
        return True   # vertical wire coming from below
    if angle == 180 and px > cx and abs(py - cy) < 0.02:
        return True   # horizontal wire coming from right
    if angle == 0 and px < cx and abs(py - cy) < 0.02:
        return True   # horizontal wire coming from left
    return False

Then fix_wire_endpoint() moves the wire's label-side endpoint to match the new label position. The pin-side endpoint stays fixed.

Which label gets moved

At a collision site, the first label (by parse order) stays at the original position. All subsequent labels are moved. In practice, most collision sites have exactly 2 labels. For the rare 3+ case, each label moves in the direction of its own pin, so they spread out naturally.

Edge cases

1. Multiple wires at collision point: The script tries direction-matching first. If that fails, it falls back to any unmatched wire at the position.

2. Label block matching: The regex matches both the net name AND the exact (at x y angle) coordinate to avoid accidentally modifying a different instance of the same net label. This was a bug fix — the original approach matched by net name alone, which failed for nets like PHY_AD0 that appear 20+ times in one schematic.

3. Multi-line vs single-line format: Wire parsing handles both KiCad's verbose format (separate lines for pts, stroke, uuid) and compact format.

4. Tolerance: 0.02 tolerance on coordinate matching to handle floating-point drift.

Collision counts on our 10 sheets

From the last full build:

Sheet	Collisions
audio	8
esp32_p4_core	12
ethernet	5
power	6
esp32_c6_wifi	3
usb_uart	2
storage	1
usb_hub	0
interfaces	0
misc	0

Total: ~37 collisions across 10 sheets. All on decoupling cap / signal passive grids where adjacent pins share endpoints.

Recommendation for apply_batch

Instead of post-processing, you could detect collisions during label_connections placement:

1. After computing a label's position, check if that (x, y) is already occupied by a different-net label (or wire endpoint) placed in the same batch
2. If collision detected, apply the 1.27mm offset toward the pin before placing
3. Adjust the wire stub length accordingly (current stub length + 1.27mm for the shifted label, current stub length - 1.27mm for the one that stays)

The direction-based resolution (move toward own pin) means two colliding labels will always move in opposite directions, guaranteeing separation.

---

2. Tab Indentation: Before/After Sample

The problem

apply_batch produces two different indentation styles in the same file:

• Components, lib_symbols, power symbol instances, power-symbol wires: Tab-indented, multi-line (KiCad native format)
• Labels (global_label, label) and label-connection wires: 2-space-indented, compact single-line format

Quantified from test schematic

Applied esp32_p4_core.json batch to a fresh schematic:

Section	Lines	Indentation
Components + power symbols + power wires (lines 1-9403)	9,402	Tabs
Labels + label-connection wires (lines 9404-11190)	1,787	2-space
Total	11,191	Mixed

Before/After comparison

KiCad-native format (tabs, multi-line — what components and power wires use):

→(global_label "ESP_LDO_VO3"
→→(shape bidirectional)
→→(at 154.94 255.27 90)
→→(effects
→→→(font
→→→→(size 1.27 1.27)
→→→)
→→→(justify left)
→→)
→→(uuid "65dc3dfb-...")
→→(property "Intersheetrefs" "${INTERSHEET_REFS}"
→→→(at 0 0 0)
→→→(effects
→→→→(font
→→→→→(size 1.27 1.27)
→→→)
→→→→(hide yes)
→→→)
→→)
→)
→(wire
→→(pts
→→→(xy 240.03 165.1) (xy 242.57 165.1)
→→)
→→(stroke
→→→(width 0)
→→→(type default)
→→)
→→(uuid "dc73871f-...")
→)

apply_batch label_connections format (2-space indent, compact):

··(global_label "CSI_CLK_N"
····(shape bidirectional)
····(at 194.31 191.77 90)
····(effects (font (size 1.27 1.27)) (justify left))
····(uuid "25c08191-...")
····(property "Intersheetrefs" "${INTERSHEET_REFS}"
······(at 194.31 191.77 90)
······(effects (font (size 1.27 1.27)) (hide yes))
····)
··)
··(wire (pts (xy 194.31 194.31) (xy 194.31 191.77))
····(stroke (width 0) (type default))
····(uuid "00d9404f-...")
··)

apply_batch local label format (also 2-space, compact):

··(label "CSI_REXT"
····(at 139.7 130.81 270)
····(effects (font (size 1.27 1.27)) (justify left bottom))
····(uuid "a8e224b2-...")
··)
··(wire (pts (xy 139.7 128.27) (xy 139.7 130.81))
····(stroke (width 0) (type default))
····(uuid "6b8d305d-...")
··)

Key differences

Aspect	KiCad native	apply_batch labels
Indent char	Tab (\t, ^I)	2 spaces
Depth-1 indent	1 tab	2 spaces
effects block	Multi-line (font/size on separate lines)	Single-line (effects (font (size 1.27 1.27)) (justify left))
wire block	Multi-line (pts, stroke, uuid each indented)	Single-line (wire (pts (...) (...)) + children
property Intersheetrefs at	(at 0 0 0) (placeholder)	(at x y angle) (copies label position)

Functional impact

KiCad 9 loads both formats correctly. But when you open and re-save the schematic in KiCad, it rewrites the space-indented sections to tabs+multi-line format. This means:

1. First save after apply_batch → mixed indentation (space + tab)
2. Open in KiCad → KiCad normalizes everything to tabs
3. git diff shows every label and label-wire as "changed" even though the content is semantically identical

For our workflow this creates noise: 1,787 lines of indentation-only changes on every KiCad save. Not a blocker, but it makes diffs hard to review.

Suggestion

The label_connections code path likely uses a different serializer than the component/power-symbol path. The component serializer already produces correct KiCad-native tabs+multi-line format. If the label serializer can be switched to the same formatter, this would eliminate the discrepancy.

The fix_indentation.py script was never actually written — we were going to write one as a post-processing pass, but it makes more sense to fix at the source.

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Both items ready for you to work on. The collision detection is the higher-priority one — it blocks the full 10-sheet rebuild.