artifact-detect

What this skill does

# Video Artifact Detection

Identify and diagnose specific quality issues in video files. Focus on visual inspection techniques and artifact recognition, not fixing the issues.

## Artifact Categories

### 1. Compression Artifacts

**Blocking (macroblocking)**
- Square or rectangular blocks visible in motion or gradients
- Caused by: Low bitrate encoding, high quantization
- Where to look: Fast motion scenes, areas with subtle color gradients
- Severity indicator: More visible = worse quality

**Banding**
- Visible steps/bands in color gradients instead of smooth transitions
- Caused by: Low bit depth, aggressive quantization, source issues
- Where to look: Sky gradients, dark shadows, smooth surfaces
- Note: May be source issue, not encoding issue

**Mosquito noise**
- Fuzzy, shifting noise around sharp edges
- Caused by: Lossy compression struggling with hard edges
- Where to look: Text overlays, high-contrast edges, animation line art

**DCT ringing**
- Wavy patterns near sharp edges
- Caused by: JPEG-based compression (H.264/H.265 use DCT)
- Where to look: Around text, logos, sharp transitions

### 2. Post-Processing Damage

**Haloing/Ringing from sharpening**
- Bright or dark outlines around edges
- Caused by: Oversharpening filters (unsharp mask, warp sharp)
- Where to look: Around character outlines, text, any hard edges
- Key sign: Consistent glow on one side of lines throughout video

**Line warping**
- Straight lines appear wavy or distorted
- Caused by: Aggressive sharpening algorithms
- Where to look: Architecture, text, grid patterns

**Oversharpened texture**
- Unnatural crispness, "crunchy" appearance
- Caused by: Excessive sharpening combined with compression
- Comparison: Should look natural, not like HDR photo filter

### 3. Upscaling Artifacts

**AI hallucinations**
- Invented details that weren't in source
- Caused by: AI upscaling (Real-ESRGAN, Topaz, etc.)
- Where to look: Fine textures (hair, fabric), text, repeating patterns
- Key sign: Details that seem too sharp or don't match surrounding areas

**Interpolation blur**
- Soft, smeared appearance on edges
- Caused by: Bilinear/bicubic upscaling
- Where to look: Diagonal lines, fine details
- Comparison: Native resolution looks sharper

**Stairstepping (aliasing)**
- Jagged edges on diagonal lines
- Caused by: Poor upscaling algorithm, no anti-aliasing
- Where to look: Diagonal lines, curved edges

### 4. Color Issues

**Wrong color matrix symptoms**
- Overall color shift (too green, too magenta)
- Caused by: BT.601/BT.709 mismatch
- Key sign: Colors look "off" but evenly across entire frame
- Verification: Compare to known-good source

**Chroma shift**
- Color misaligned with luminance
- Caused by: Wrong chroma location, processing error
- Where to look: Hard edges - look for color fringing on one side
- Key sign: Consistent colored glow on same side of all edges

**Double range compression**
- Crushed blacks, blown highlights
- Caused by: Limited range treated as full (or vice versa) multiple times
- Where to look: Dark shadows (crushed to pure black), bright areas (clipped)

### 5. Frame Rate Issues

**Judder/stutter**
- Uneven motion, frames "jumping"
- Caused by: Frame rate mismatch, bad pulldown
- Where to look: Panning shots, steady motion

**Duplicate frames**
- Same frame appears twice (or more)
- Caused by: Frame rate conversion errors
- Detection: Step frame-by-frame, compare adjacent frames

**Telecine combing**
- Horizontal lines across moving objects
- Caused by: Improper deinterlacing of telecined content
- Where to look: Horizontal edges during motion
- Note: Not all combing = interlaced. May be telecined (3:2 pulldown)

## Visual Inspection Checklist

When examining a video for artifacts, inspect these areas in order:

### High-Priority Areas

1. **Dark gradients and shadows** - Reveals banding, blocking, crushed blacks
2. **Strong colors (especially dark reds)** - Shows compression stress, color issues
3. **Around sharp edges** - Reveals haloing, ringing, mosquito noise, chroma shift
4. **On-screen text and logos** - Shows sharpness issues, compression artifacts clearly
5. **Image borders** - Often reveals encoding problems first

### Motion-Specific Checks

6. **Panning shots** - Reveals judder, duplicate frames, motion blur issues
7. **Fast action scenes** - Shows blocking, motion compensation failures
8. **Static grain/texture areas** - Reveals whether grain was filtered or preserved

### Comparison Technique

To spot artifacts effectively:
- View at 100% zoom (1:1 pixel mapping)
- Compare against known-good source if available
- Use frame-by-frame stepping for temporal artifacts
- Toggle between source and processed version rapidly

## Reporting Findings

When reporting detected artifacts, include:

1. **Artifact type** - Name from categories above
2. **Severity** - Subtle, moderate, severe
3. **Location** - Where in frame, what type of content affected
4. **Likely cause** - Based on artifact characteristics
5. **Fixability** - Can be fixed (retag, re-encode from source) or permanent damage

## Tools for Detection

- **mpv**: Best playback, press `i` for technical info, frame step with `.` and `,`
- **vs-preview**: Frame-by-frame comparison with VapourSynth
- **SlowPics**: For sharing comparison screenshots (not imgsli - converts to JPEG)

## Additional Resources

For detailed artifact catalogs with visual examples:
- **`${CLAUDE_PLUGIN_ROOT}/references/artifacts.md`** - Comprehensive artifact guide
- **`${CLAUDE_PLUGIN_ROOT}/references/color-space.md`** - Color issue diagnosis