codec
/ˈkoʊdɛk/ nounA method, implemented in software or hardware, for encoding and decoding digital video — compressing image data for storage or transmission and reconstructing it for playback.
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Technology An algorithm, and the software or hardware that implements it, which compresses a stream of digital images into a smaller encoded form and decompresses it for display; the term names both halves of the process.
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Technology By extension, a named compression standard or format used to identify how video data has been encoded, such as H.264 or ProRes. A clip may be described as encoded with the H.265 codec at a given bitrate.
What a codec does
A codec (from coder-decoder) is the method by which digital video is compressed for storage or transmission and reconstructed for viewing. Raw footage read from an image sensor produces very large quantities of data: at high resolution and frame rate, an uncompressed stream can exceed the practical limits of recording media and networks. A codec reduces this volume by discarding redundancy in the image data, encoding the result into a compact bitstream, and defining how that bitstream is decoded back into displayable frames.
Lossy and lossless compression
Compression is either lossless or lossy. Lossless compression allows the original data to be reconstructed exactly, but achieves only modest reductions in size. Lossy compression discards information judged least perceptible — often fine detail and color differences to which human vision is less sensitive — achieving far smaller files at the cost of some fidelity. Most delivery codecs, including H.264 and H.265, are lossy; the degree of loss is governed by the chosen bitrate or quality setting. A codec’s bit depth also bears on how much of a frame’s dynamic range is preserved, since low bit depth or aggressive quantization can coarsen tonal gradations and introduce visible banding.
Intra-frame and inter-frame coding
Codecs differ in whether they compress each frame independently or exploit similarities between successive frames. Intra-frame (or all-intra) coding compresses every frame on its own, as a self-contained image; ProRes and DNxHD work this way, which eases editing because any frame can be decoded in isolation. Inter-frame coding stores some frames in full and represents others as the differences from neighboring frames, using motion estimation to predict how picture content shifts over time. This yields substantially higher compression, as in H.264 and H.265, but decoding a given frame may require reference to others.
Codec versus container
A codec is distinct from a container (or wrapper) format, though the two are often confused. The codec defines how the audio and video data are encoded; the container — such as MP4, MOV, or MKV — is the file structure that holds those encoded streams together with metadata, timing information, and multiple tracks. A single container format can carry data compressed with any of several codecs, so the file extension alone does not identify the codec in use.
See also
- image sensor— The electronic component that captures a digital image, converting light into electrical charge at a grid of photosites — the digital counterpart to photographic emulsion.
- resolution— The amount of fine spatial detail an image can render — how closely two points can lie and still be distinguished. In digital capture it is stated as a pixel count; in film it is measured as a limit of resolving power.
- raw image— Minimally processed data read directly from an image sensor, recorded before demosaicing and rendering so that white balance, exposure, and color can be reinterpreted later with greater latitude.
- dynamic range— The span of light intensities a recording medium can register between its noise floor in the shadows and its point of saturation in the highlights, conventionally measured in photographic stops.
Sources
- ITU-T Recommendation H.264, Advanced Video Coding for Generic Audiovisual Services
- ITU-T Recommendation H.265, High Efficiency Video Coding
- Richardson, “The H.264 Advanced Video Compression Standard”
Last revised: September 18, 2026