robbyt/format-explain

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