artifact-detect

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