Color-code resistor zigzag symbols with standard 4-band colors

Map physical resistor color bands onto the IEEE zigzag schematic symbol.
Each sub-segment is colored to match its band (digit, multiplier, tolerance),
with wire-colored entry/exit and a gap before tolerance mimicking real spacing.
Parseable values 0.01–1GΩ get color; parametric/out-of-range fall back to mono.

README.md

@@ -38,6 +38,7 @@ SpiceBook lets you write a document that says: "here's why this filter works, he
   - Click-drag zoom, scroll to pan
 - **ngspice backend** -- subprocess execution with binary `.raw` file parsing
 - **Notebook operations** -- create, open, edit, save, delete, reorder cells, run individually or all at once
+- **Color-coded resistor schematics** -- zigzag symbols colored with standard 4-band resistor color codes. A 10kΩ resistor renders Brown-Black-Orange-Gold directly on the schematic symbol, bridging the gap between physical components and abstract schematics. Falls back to standard monochrome for parametric or out-of-range values.
 - **Keyboard shortcuts** -- `Ctrl+S` save, `Shift+Enter` run cell
 - **Clean URLs** -- `/notebook/rc-lowpass-filter` via Astro dynamic routes
 - **15 example notebooks** included:
@@ -139,7 +140,9 @@ spicebook/
 │   │   ├── config.py        # Settings from environment
 │   │   ├── engine/          # SPICE engine abstraction
 │   │   │   ├── base.py      # Abstract base class
-│   │   │   └── ngspice.py   # ngspice subprocess + .raw parser
+│   │   │   ├── ngspice.py   # ngspice subprocess + .raw parser
+│   │   │   ├── schematic.py # Netlist → SVG schematic (SchemDraw)
+│   │   │   └── resistor_colors.py  # Color-coded zigzag resistors
 │   │   ├── models/          # Pydantic schemas
 │   │   │   ├── notebook.py  # Notebook, Cell, CellOutput
 │   │   │   └── simulation.py

backend/src/spicebook/engine/resistor_colors.py

@@ -0,0 +1,163 @@
+"""Resistor color code bands mapped onto zigzag schematic symbols.
+
+Maps standard 4-band resistor color codes directly onto the IEEE zigzag
+(zigzag) symbol segments. Each zigzag sub-segment is colored to match
+its corresponding band, teaching the color code while reading schematics.
+
+Band-to-segment mapping (4-band):
+    Seg 0 (entry half-rise):  wire color (default)
+    Seg 1 (peak-to-valley):   1st significant digit
+    Seg 2 (valley-to-peak):   2nd significant digit
+    Seg 3 (peak-to-valley):   multiplier
+    Seg 4 (valley-to-peak):   gap — wire color (mimics physical spacing)
+    Seg 5 (peak-to-valley):   tolerance
+    Seg 6 (exit half-rise):   wire color (default)
+"""
+
+import re
+
+from schemdraw.elements.twoterm import Element2Term
+from schemdraw.segments import Segment
+
+# ── SPICE Value Parser ──────────────────────────────────────
+
+_SPICE_SUFFIXES = {
+    "T": 1e12,
+    "G": 1e9,
+    "MEG": 1e6,
+    "K": 1e3,
+    "M": 1e6,  # For resistors, bare M = Mega (milliohm resistors are vanishingly rare)
+    "U": 1e-6,
+    "N": 1e-9,
+    "P": 1e-12,
+    "F": 1e-15,
+}
+
+_SUFFIX_RE = re.compile(
+    r"^([0-9]*\.?[0-9]+)\s*(MEG|[TGKMUNPF])?$",
+    re.IGNORECASE,
+)
+
+
+def parse_spice_value(value_str: str) -> float | None:
+    """Convert a SPICE value string to numeric ohms.
+
+    Handles: "10k", "4.7MEG", "100", "2.2K", "0.1", "10k ic=0"
+    Returns None for parametric expressions like "{R_val}".
+    """
+    if not value_str:
+        return None
+
+    token = value_str.strip().split()[0]
+
+    if token.startswith("{"):
+        return None
+
+    # Try bare numeric first
+    try:
+        return float(token)
+    except ValueError:
+        pass
+
+    m = _SUFFIX_RE.match(token)
+    if m:
+        number = float(m.group(1))
+        suffix = (m.group(2) or "").upper()
+        multiplier = _SPICE_SUFFIXES.get(suffix, 1.0)
+        return number * multiplier
+
+    return None
+
+
+# ── Hex Colors Tuned for Mims Background (#f8faf6) ─────────
+
+BAND_HEX: dict[str, str] = {
+    "black": "#1a1a1a",
+    "brown": "#8B4513",
+    "red": "#CC0000",
+    "orange": "#E67300",
+    "yellow": "#B8960B",  # Dark goldenrod — visible on off-white
+    "green": "#006400",
+    "blue": "#0000CC",
+    "violet": "#7B00B3",
+    "gray": "#666666",
+    "white": "#D0D0D0",  # Light gray — visible on off-white
+    "gold": "#B8860B",
+    "silver": "#808080",
+    "pink": "#FF69B4",
+}
+
+
+# ── Zigzag Geometry (matches SchemDraw 0.22 ResistorIEEE) ───
+
+_RW = 1 / 6  # reswidth
+_RH = 0.25  # resheight
+
+_ZIGZAG_POINTS = [
+    (0, 0),
+    (0.5 * _RW, _RH),
+    (1.5 * _RW, -_RH),
+    (2.5 * _RW, _RH),
+    (3.5 * _RW, -_RH),
+    (4.5 * _RW, _RH),
+    (5.5 * _RW, -_RH),
+    (6 * _RW, 0),
+]
+
+
+# ── Custom Element ──────────────────────────────────────────
+
+
+class ColorCodedResistor(Element2Term):
+    """IEEE zigzag resistor with segments colored as standard color code bands.
+
+    Uses an overlay strategy: segments[0] is the full 8-point zigzag path
+    (needed by Element2Term._place() for lead extension), then individual
+    2-point colored segments are appended at a higher zorder so they render
+    on top, hiding the base path under the band colors.
+    """
+
+    def __init__(self, value_ohms: float, tolerance_pct: float = 5, **kwargs):
+        super().__init__(**kwargs)
+
+        # segments[0]: full zigzag — required for Element2Term lead geometry
+        self.segments.append(Segment(_ZIGZAG_POINTS))
+
+        # Get 4-band color names from SchemDraw's existing logic
+        from schemdraw.pictorial.pictorial import resistor_colors
+
+        band_names = resistor_colors(value_ohms, tolerance_pct)
+
+        # Map band colors to the 7 zigzag sub-segments
+        hex_colors = _map_4band(band_names)
+
+        # Add colored overlay segments at higher zorder
+        for i, hex_color in enumerate(hex_colors):
+            if hex_color is not None:
+                self.segments.append(
+                    Segment(
+                        [_ZIGZAG_POINTS[i], _ZIGZAG_POINTS[i + 1]],
+                        color=hex_color,
+                        zorder=3,
+                    )
+                )
+
+
+def _map_4band(
+    band_names: tuple[str, str, str, str | None],
+) -> list[str | None]:
+    """Map 4-band color names to 7 zigzag sub-segment slots.
+
+    Returns a list of 7 hex color strings (or None for wire-colored segments).
+    The gap at seg 4 mimics the physical spacing before the tolerance band.
+    """
+    c1, c2, c3, ct = band_names
+    return [
+        None,  # seg 0: entry half-rise (wire)
+        BAND_HEX.get(c1),  # seg 1: 1st significant digit
+        BAND_HEX.get(c2),  # seg 2: 2nd significant digit
+        BAND_HEX.get(c3),  # seg 3: multiplier
+        None,  # seg 4: gap (wire — mimics physical spacing)
+        BAND_HEX.get(ct) if ct else None,  # seg 5: tolerance
+        None,  # seg 6: exit half-rise (wire)
+    ]

backend/src/spicebook/engine/schematic.py

@@ -579,6 +579,11 @@ def _get_element(comp: SpiceComponent, models: dict[str, str]):
     prefix = comp.prefix
 
     if prefix == "R":
+        from spicebook.engine.resistor_colors import ColorCodedResistor, parse_spice_value
+
+        ohms = parse_spice_value(comp.value)
+        if ohms is not None and 0.01 <= ohms <= 1e9:
+            return ColorCodedResistor(ohms)
         return elm.Resistor()
     elif prefix == "C":
         return elm.Capacitor()

frontend/public/llms.txt

@@ -246,6 +246,17 @@ No request body. Cell must be type `spice`.
 }
 ```
 
+**Color-coded resistors**: Resistors with parseable values between 0.01 and 1e9 ohms render as IEEE zigzag symbols with segments colored according to the standard 4-band resistor color code. A 10k resistor shows Brown-Black-Orange-Gold bands directly on the zigzag. The entry/exit half-segments and the gap before the tolerance band use the default wire color, mimicking the physical spacing that indicates reading direction. Parametric values (e.g. `{R_val}`) and out-of-range resistances fall back to a standard monochrome zigzag.
+
+Band-to-segment mapping (7 zigzag sub-segments):
+- Seg 0 (entry): wire color
+- Seg 1: 1st significant digit
+- Seg 2: 2nd significant digit
+- Seg 3: multiplier
+- Seg 4: gap (wire color, mimics physical spacing)
+- Seg 5: tolerance
+- Seg 6 (exit): wire color
+
 ---
 
 ### Waveforms