format-explain

What this skill does

# Video Format Explanations

Provide clear, educational explanations of video technical concepts. Route to detailed references for in-depth information.

## Core Concepts

### Container vs Codec

**Container formats** (mkv, mp4, webm, avi, mov):
- Package that holds video, audio, subtitles, metadata
- Like a ZIP file for media streams
- Changing container = remuxing (fast, no quality loss)
- Different containers support different features (mkv supports more subtitle formats than mp4)

**Video codecs** (H.264/AVC, H.265/HEVC, VP9, AV1):
- Algorithm that compresses/decompresses video data
- The actual encoding that determines quality and file size
- Changing codec = reencoding (slow, causes quality loss)

**Key insight**: A file's extension (.mkv, .mp4) tells you the container, not the codec. An mkv and mp4 can contain identical H.264 video.

### Codec vs Encoder

**Codec/Format** (H.264, H.265): The specification/standard for how video is encoded
**Encoder** (x264, x265, NVENC): Software that creates video in that format

Different encoders produce different quality at same bitrate:
- x264/x265: Best quality, slow (CPU-based)
- NVENC/QuickSync: Fast, larger files for same quality (GPU-based)
- Apple VideoToolbox: Middle ground on macOS

### Resolution

**What it is**: Pixel dimensions (1920x1080, 3840x2160)
**What it's not**: Quality

Resolution indicates the pixel grid size, not visual fidelity. A badly encoded 4K video can look worse than a well-encoded 1080p video.

**Native vs upscaled**: Many "4K" releases are upscaled from lower resolution masters. The file has 4K pixels but no more detail than the source.

### Frame Rate

**Common rates**:
- 23.976 fps (24000/1001): Film, most streaming content
- 24 fps: True film rate (rare in digital distribution)
- 29.97 fps (30000/1001): NTSC video
- 25 fps: PAL video
- 59.94/60 fps: High frame rate, gaming content

**23.976 vs 24**: Nearly identical to viewers. The fractional rate exists for NTSC compatibility reasons from analog TV era.

**CFR vs VFR**:
- CFR (Constant Frame Rate): Every frame has same duration
- VFR (Variable Frame Rate): Frame durations vary (common in screen recordings, phone videos)

### Bit Depth

**8-bit**: Standard, 256 levels per color channel, 16.7 million colors
**10-bit**: HDR standard, 1024 levels per channel, 1 billion+ colors
**12-bit**: Professional/mastering, rarely seen in distribution

**10-bit for 8-bit content**: Encoders sometimes use 10-bit encoding for 8-bit sources because it improves compression efficiency (fewer banding artifacts).

### Bitrate

**What it measures**: Data per second (Mbps, kbps)
**What it indicates**: How much data the encoder used, not necessarily quality

Higher bitrate ≠ better quality. An inefficient encode at 20 Mbps can look worse than an efficient encode at 8 Mbps.

**VBR vs CBR**:
- VBR (Variable): Allocates bits where needed, better quality
- CBR (Constant): Fixed rate, required for some streaming, wastes bits on simple scenes

### CRF (Constant Rate Factor)

Quality-based encoding setting used by x264/x265:
- Lower CRF = higher quality, larger file
- Higher CRF = lower quality, smaller file
- Range: 0 (lossless) to 51 (worst)
- Typical: 18-23 for high quality, 23-28 for smaller files

CRF automatically adjusts bitrate based on scene complexity.

## Color Space Concepts

### Color Matrix (BT.601 vs BT.709)

Defines how YCbCr values convert to/from RGB:
- **BT.601**: Legacy standard for SD content (DVD era)
- **BT.709**: Modern standard for HD content (Blu-ray, streaming)

Wrong matrix = incorrect colors. HD content played as BT.601 will have shifted hues.

### Color Range (Limited vs Full)

**Limited range** (16-235): Standard for video, leaves headroom
**Full range** (0-255): Uses entire value range

Most video content is Limited range. Playing Limited as Full = washed out. Playing Full as Limited = crushed blacks/clipped whites.

### Chroma Subsampling

**4:4:4**: Full color resolution (rare, large files)
**4:2:2**: Half horizontal color resolution (professional)
**4:2:0**: Quarter color resolution (standard for distribution)

Human eyes are less sensitive to color detail than brightness, so 4:2:0 looks nearly identical to 4:4:4 for most content.

### HDR vs SDR

**SDR** (Standard Dynamic Range): Traditional brightness levels, ~100 nits peak
**HDR** (High Dynamic Range): Extended brightness, 1000+ nits peak, wider color gamut

HDR requires:
- 10-bit color depth (minimum)
- HDR metadata (PQ or HLG transfer function)
- Compatible display

Not all "HDR" releases are legitimate. Some are inverse tonemapped from SDR sources.

## Quick Answers

| Question | Short Answer |
|----------|--------------|
| "Is mkv better than mp4?" | Different containers, neither is "better" - mkv supports more features |
| "Should I reencode to H.265?" | Only if you need smaller files and accept quality loss |
| "Is 4K always better than 1080p?" | No - depends on source quality and encoding |
| "Why is my file so large?" | Check codec (NVENC vs x264), bitrate settings, or CRF value |
| "What's the best format?" | Depends on use case. H.264/mkv for compatibility, H.265 for size |

## Additional Resources

For detailed explanations:
- **`${CLAUDE_PLUGIN_ROOT}/references/quality-myths.md`** - Common misconceptions debunked
- **`${CLAUDE_PLUGIN_ROOT}/references/color-space.md`** - Full color space details
- **`${CLAUDE_PLUGIN_ROOT}/references/encoding-commands.md`** - Practical command examples