# Improvements to split_png_by_brightness.py

## Version 2 Fixes

### 1. Proper Hole Handling
**Problem**: Letters with enclosed shapes (like "d", "o", "g") were missing their holes, making them appear filled.

**Solution**: 
- Switched from `scikit-image` contour detection to OpenCV's `cv2.findContours` with `RETR_CCOMP` mode
- This detects contour hierarchy (parent/child relationships)
- Holes are now included in the same SVG path as their parent contour
- Using `fill-rule="evenodd"` properly renders the holes

### 2. Better Color Detection
**Problem**: Antialiasing pixels around edges were being included, creating artifacts and incorrect bounding boxes.

**Solution**:
- Changed from single threshold to tight ranges
- Black text: brightness < 40 (was < 50)
- Grey icon: brightness 108-118 (was 80-120)
- This excludes antialiased edge pixels and prevents the grey icon from spanning the full image width

### 3. Automatic Bounding Box Cropping
**Problem**: SVG files contained a lot of empty space.

**Solution**:
- Added automatic cropping to bounding box by default
- Each SVG is sized to fit its content exactly
- `--no-crop` flag available if full canvas is needed

## Results

**Before (v1)**:
- um_black.svg: 139K, 27 separate paths, many filled letters
- um_grey.svg: 253K, 1609 paths, antialiasing artifacts, full width (3613px)

**After (v2)**:
- um_black.svg: 6.1K, 21 shape groups with proper holes, cropped to 3070x233
- um_grey.svg: 728B, 2 shape groups, clean edges, cropped to 413x390

## Dependencies Added
- opencv-python==4.13.0.92

## Default Parameters
- Black threshold: < 40 brightness
- Grey range: 108-118 brightness
- Cropping: enabled by default

## Usage
```bash
# Default (optimized for Underground Magnetics logo)
python split_png_by_brightness.py input.png --prefix output

# Custom thresholds
python split_png_by_brightness.py input.png \
  --dark-threshold 40 \
  --mid-min 108 \
  --mid-max 118

# Disable cropping
python split_png_by_brightness.py input.png --no-crop
```