Add PNG to SVG color separation tool for multi-color 3D printing

Created split_png_by_brightness.py to split rasterized logos into separate
SVG files by color for Underground Magnetics logo conversion.

Key features:
- OpenCV contour hierarchy detection for proper hole handling
- Letters with enclosed shapes (d, o, g, etc.) now render correctly
- Tight brightness thresholds to avoid antialiasing artifacts
- Automatic bounding box cropping for optimal file sizes
- Black text: brightness < 40 (236k pixels, 21 shape groups)
- Grey icon: brightness 108-118 (119k pixels, 2 shape groups)

Results:
- um_black.svg: 6.1KB, 3070x233px (was 139KB, 3613x391px)
- um_grey.svg: 728B, 413x390px (was 253KB, 3613x391px)

Files:
- split_png_by_brightness.py: Main color separation tool
- IMPROVEMENTS.md: Detailed changelog and comparison
- README_underground_magnetics.md: Usage documentation
- underground-magnetics.eps: Source logo file
- um_black.svg: Separated black text (cropped)
- um_grey.svg: Separated grey icon (cropped)
- requirements.txt: Added opencv-python dependency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

README_underground_magnetics.md

@@ -0,0 +1,66 @@
+# Underground Magnetics Logo - Color Separation
+
+Successfully split the Underground Magnetics EPS logo into separate SVG files by color for multi-color 3D printing.
+
+## Process
+
+1. **EPS to PNG Conversion**: Used ImageMagick to convert `underground-magnetics.eps` to PNG format
+   - Output: `underground-magnetics-0.png` and `underground-magnetics-1.png`
+
+2. **Color Separation**: Used `split_png_by_brightness.py` to extract separate colors
+   - Analyzed brightness values to distinguish grey icon from black text
+   - Created separate SVG files for each color region
+
+## Output Files
+
+- **underground-magnetics_grey.svg** - Grey "UM" icon (126,801 pixels, 1609 paths)
+- **underground-magnetics_black.svg** - Black "Underground Magnetics" text (237,130 pixels, 27 paths)
+
+## Usage
+
+To convert these SVG files to 3D models for multi-color printing:
+
+```bash
+# Convert grey icon to STL
+python svg_logo_to_stl.py underground-magnetics_grey.svg um_grey --width-mm 100 --base-thickness 1.5 --feature-height 2.5
+
+# Convert black text to STL
+python svg_logo_to_stl.py underground-magnetics_black.svg um_black --width-mm 100 --base-thickness 1.5 --feature-height 2.5
+```
+
+## Tools Created
+
+### split_png_by_brightness.py
+Splits PNG images into separate SVG files based on brightness/color clusters.
+
+**Usage:**
+```bash
+python split_png_by_brightness.py <input.png> [options]
+
+Options:
+  --output-dir, -o        Output directory (default: same as input)
+  --prefix, -p            Prefix for output files (default: input filename)
+  --dark-threshold INT    Brightness threshold for dark/black (default: 50)
+  --mid-threshold INT     Brightness threshold for mid/grey (default: 150)
+```
+
+**Example:**
+```bash
+python split_png_by_brightness.py underground-magnetics-0.png --prefix underground-magnetics
+```
+
+This creates separate SVG files for each color region detected in the PNG.
+
+## Color Detection
+
+The script analyzes pixel brightness to separate colors:
+- **Dark pixels** (brightness < 50): Black text → `_black.svg`
+- **Mid-range pixels** (50 ≤ brightness < 150): Grey icon → `_grey.svg`
+- **Light pixels** (brightness ≥ 150): White background (ignored)
+
+## Next Steps
+
+1. Import the separate SVG files into the 3D conversion tool
+2. Generate STL files for each color
+3. Create 3MF assembly with multiple selectable parts
+4. Import into BambuStudio for multi-color printing