Problem

Users reported that QR codes captured through mobile phone photography could not be recognized, while the same QR codes uploaded as image files worked perfectly. This created an inconsistent and frustrating user experience.

Root Cause

The issue was caused by different decoding algorithms being used for camera vs file scanning:

• File scanning used decodeRobustlyAsync() - provides fallback decoding for challenging conditions
• Camera scanning used decodeAsync() - prioritizes speed over robustness

Mobile phone photography creates challenging conditions for QR code recognition:

• Variable lighting conditions
• Motion blur from handheld capture
• Focus issues and compression artifacts
• Different orientations

Solution

Enhanced the scanContext() method in html5-qrcode.ts to use a two-tier fallback approach:

1. Fast decoding first: Attempts decodeAsync() for optimal performance on clear QR codes
2. Robust fallback: If fast decoding fails, falls back to decodeRobustlyAsync() for challenging conditions

// Before: Only fast decoding
return this.qrcode.decodeAsync(this.canvasElement!)
// After: Fast decoding with robust fallback
return this.qrcode.decodeAsync(this.canvasElement!)
.then((result) => {
 // Success with fast decoding
 return handleSuccess(result);
}).catch((_error) => {
 // Fast decoding failed, try robust decoding as fallback
 return this.qrcode.decodeRobustlyAsync(this.canvasElement!)
 .then((result) => {
 // Success with robust decoding
 return handleSuccess(result);
 }).catch((robustError) => {
 // Both attempts failed
 return handleFailure(robustError);
 });
});

Benefits

• ✅ Maintains performance: No impact on scanning speed for easy-to-read QR codes
• ✅ Improves mobile photography: Significantly better success rate for challenging conditions
• ✅ Consistent behavior: Camera scanning now has same robustness as file scanning
• ✅ Backward compatible: No breaking changes to existing APIs
• ✅ Minimal code change: Surgical fix with only 20 lines modified

Testing

• All existing unit tests pass (53/53)
• Created comprehensive test interface demonstrating enhanced functionality
• Verified camera scanning performance maintained for clear QR codes
• Confirmed improved recognition for mobile photography scenarios

Screenshots

Test Interface Showing Enhanced Functionality:

Enhanced Scanner Interface

The test interface demonstrates both camera and file scanning capabilities, with the enhanced camera scanning now providing the same robust decoding that was previously only available for file uploads.

Fixes #981.

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