A Glossary of Video Codec Terms (H.264, HEVC, AV1, VP9, ProRes)

Video compression has its own vocabulary. This glossary is the reference we wish existed the first time we opened FFmpeg. Pair with Zro7 Compress Video for hands-on experiments.

Container vs codec

• Container (MP4, MKV, MOV, WebM): the box holding video, audio, subtitles.
• Codec: the algorithm that compresses each track (H.264 video, AAC audio).
• A .mp4 usually contains H.264 or HEVC video; a .webm usually contains VP9 or AV1.

The major video codecs

• H.264 / AVC: 2003. Ubiquitous. Every phone/browser plays it. Baseline for compatibility.
• H.265 / HEVC: 2013. ~50% smaller than H.264 at same quality. Patent-encumbered — Firefox skipped it for years.
• AV1: 2018. Royalty-free. ~30% smaller than HEVC. Slow to encode; hardware decoding now widespread.
• VP9: 2013. Google's HEVC competitor. YouTube's default before AV1.
• ProRes: Apple's editing codec. Huge files, near-lossless — for cutting, not delivery.
• DNxHD/HR: Avid's equivalent to ProRes.

Quality controls

• Bitrate (kbps/Mbps): data per second. Higher = bigger + usually better.
• CBR (constant bitrate): predictable size, wasteful.
• VBR (variable bitrate): allocates bits to complex scenes.
• CRF (Constant Rate Factor): quality target, 0-51 for H.264/HEVC; ~23 is default, ~18 is visually lossless.
• Two-pass: encode once to analyze, again to allocate bits — best size/quality.

Frames

• I-frame (keyframe): fully self-contained. Seek points.
• P-frame: predicted from previous frames.
• B-frame: predicted from both directions.
• GOP (Group of Pictures): distance between keyframes. Short GOP = better seeking, bigger file.

Color

• Chroma subsampling: 4:4:4 = full color, 4:2:2 = broadcast, 4:2:0 = consumer/web default.
• Bit depth: 8-bit standard, 10-bit for HDR/HEVC.
• Color space: BT.709 (HD), BT.2020 (4K/HDR), sRGB (web).
• HDR: HDR10 (static), HDR10+ (dynamic), Dolby Vision (proprietary dynamic).

Audio in video

• AAC: default for MP4, 128-256 kbps typical.
• Opus: default for WebM/MKV, better quality per bit.
• AC3/E-AC3: Dolby, common in Blu-ray rips.
• PCM: uncompressed, used in editing.

FFmpeg concepts

• -c:v libx264: video codec choice.
• -preset: speed/size tradeoff — ultrafast … veryslow. Slower = smaller.
• -crf 23: quality target.
• -c:a copy: passthrough, no audio re-encode.
• Remux: change container without re-encoding — instant, lossless.

Updated January 2, 2027 · Zro7 editorial team.