Vision Framework Diagnostics

Systematic troubleshooting for Vision framework issues: subjects not detected, missing landmarks, low confidence, performance problems, coordinate mismatches, text recognition failures, barcode detection issues, and document scanning problems.

Overview

Core Principle: When Vision doesn't work, the problem is usually:

1. Environment (lighting, occlusion, edge of frame) - 40%
2. Confidence threshold (ignoring low confidence data) - 30%
3. Threading (blocking main thread causes frozen UI) - 15%
4. Coordinates (mixing lower-left and top-left origins) - 10%
5. API availability (using iOS 17+ APIs on older devices) - 5%

Always check environment and confidence BEFORE debugging code.

Red Flags

Symptoms that indicate Vision-specific issues:

Symptom	Likely Cause
Subject not detected at all	Edge of frame, poor lighting, very small subject
Hand landmarks intermittently nil	Hand near edge, parallel to camera, glove/occlusion
Body pose skipped frames	Person bent over, upside down, flowing clothing
UI freezes during processing	Running Vision on main thread
Overlays in wrong position	Coordinate conversion (lower-left vs top-left)
Crash on older devices	Using iOS 17+ APIs without @available check
Person segmentation misses people	>4 people in scene (instance mask limit)
Low FPS in camera feed	maximumHandCount too high, not dropping frames
Text not recognized at all	Blurry image, stylized font, wrong recognition level
Text misread (wrong characters)	Language correction disabled, missing custom words
Barcode not detected	Wrong symbology, code too small, glare/reflection
DataScanner shows blank screen	Camera access denied, device not supported
Document edges not detected	Low contrast, non-rectangular, glare
Real-time scanning too slow	Processing every frame, region too large

Mandatory First Steps

Before investigating code, run these diagnostics:

Step 1: Verify Detection with Diagnostic Code

let request = VNGenerateForegroundInstanceMaskRequest()  // Or hand/body pose
let handler = VNImageRequestHandler(cgImage: testImage)

do {
    try handler.perform([request])

    if let results = request.results {
        print("✅ Request succeeded")
        print("Result count: \(results.count)")

        if let observation = results.first as? VNInstanceMaskObservation {
            print("All instances: \(observation.allInstances)")
            print("Instance count: \(observation.allInstances.count)")
        }
    } else {
        print("⚠️ Request succeeded but no results")
    }
} catch {
    print("❌ Request failed: \(error)")
}

Expected output:

• ✅ Request succeeded, instance count > 0 → Detection working
• ⚠️ Request succeeded, instance count = 0 → Nothing detected (see Decision Tree)
• ❌ Request failed → API availability issue

Step 2: Check Confidence Scores

// For hand/body pose
if let observation = request.results?.first as? VNHumanHandPoseObservation {
    let allPoints = try observation.recognizedPoints(.all)

    for (key, point) in allPoints {
        print("\(key): confidence \(point.confidence)")

        if point.confidence < 0.3 {
            print("  ⚠️ LOW CONFIDENCE - unreliable")
        }
    }
}

Expected output:

• Most landmarks > 0.5 confidence → Good detection
• Many landmarks < 0.3 → Poor lighting, occlusion, or edge of frame

Step 3: Verify Threading

print("🧵 Thread: \(Thread.current)")

if Thread.isMainThread {
    print("❌ Running on MAIN THREAD - will block UI!")
} else {
    print("✅ Running on background thread")
}

Expected output:

• ✅ Background thread → Correct
• ❌ Main thread → Move to DispatchQueue.global()

Decision Tree

Vision not working as expected?
│
├─ No results returned?
│  ├─ Check Step 1 output
│  │  ├─ "Request failed" → See Pattern 1a (API availability)
│  │  ├─ "No results" → See Pattern 1b (nothing detected)
│  │  └─ Results but count = 0 → See Pattern 1c (edge of frame)
│
├─ Landmarks have nil/low confidence?
│  ├─ Hand pose → See Pattern 2 (hand detection issues)
│  ├─ Body pose → See Pattern 3 (body detection issues)
│  └─ Face detection → See Pattern 4 (face detection issues)
│
├─ UI freezing/slow?
│  ├─ Check Step 3 (threading)
│  │  ├─ Main thread → See Pattern 5a (move to background)
│  │  └─ Background thread → See Pattern 5b (performance tuning)
│
├─ Overlays in wrong position?
│  └─ See Pattern 6 (coordinate conversion)
│
├─ Person segmentation missing people?
│  └─ See Pattern 7 (crowded scenes)
│
├─ VisionKit not working?
│  └─ See Pattern 8 (VisionKit specific)
│
├─ Text recognition issues?
│  ├─ No text detected → See Pattern 9a (image quality)
│  ├─ Wrong characters → See Pattern 9b (language/correction)
│  └─ Too slow → See Pattern 9c (recognition level)
│
├─ Barcode detection issues?
│  ├─ Barcode not detected → See Pattern 10a (symbology/size)
│  └─ Wrong payload → See Pattern 10b (barcode quality)
│
├─ DataScannerViewController issues?
│  ├─ Blank screen → See Pattern 11a (availability check)
│  └─ Items not detected → See Pattern 11b (data types)
│
└─ Document scanning issues?
   ├─ Edges not detected → See Pattern 12a (contrast/shape)
   └─ Perspective wrong → See Pattern 12b (corner points)

Diagnostic Patterns

Pattern 1a: Request Failed (API Availability)

Symptom: try handler.perform([request]) throws error

Common errors:

"VNGenerateForegroundInstanceMaskRequest is only available on iOS 17.0 or newer"
"VNDetectHumanBodyPose3DRequest is only available on iOS 17.0 or newer"

Root cause: Using iOS 17+ APIs on older deployment target

Fix:

if #available(iOS 17.0, *) {
    let request = VNGenerateForegroundInstanceMaskRequest()
    // ...
} else {
    // Fallback for iOS 14-16
    let request = VNGeneratePersonSegmentationRequest()
    // ...
}

Prevention: Check API availability in axiom-vision-ref before implementing

Time to fix: 10 min

Pattern 1b: No Results (Nothing Detected)

Symptom: request.results == nil or results.isEmpty

Diagnostic:

// 1. Save debug image to Photos
UIImageWriteToSavedPhotosAlbum(debugImage, nil, nil, nil)

// 2. Inspect visually
// - Is subject too small? (< 10% of image)
// - Is subject blurry?
// - Poor contrast with background?

Common causes:

• Subject too small (resize or crop closer)
• Subject too blurry (increase lighting, stabilize camera)
• Low contrast (subject same color as background)

Fix:

// Crop image to focus on region of interest
let croppedImage = cropImage(sourceImage, to: regionOfInterest)
let handler = VNImageRequestHandler(cgImage: croppedImage)

Time to fix: 30 min

Pattern 1c: Edge of Frame Issues

Symptom: Subject detected intermittently as object moves across frame

Root cause: Partial occlusion when subject touches image edges

Diagnostic:

// Check if subject is near edges
if let observation = results.first as? VNInstanceMaskObservation {
    let mask = try observation.createScaledMask(
        for: observation.allInstances,
        croppedToInstancesContent: true
    )

    let bounds = calculateMaskBounds(mask)

    if bounds.minX < 0.1 || bounds.maxX > 0.9 ||
       bounds.minY < 0.1 || bounds.maxY > 0.9 {
        print("⚠️ Subject too close to edge")
    }
}

Fix:

// Add padding to capture area
let paddedRect