Numerous factors play a role in this variability. Prior to comparing codec video-quality, it is important to understand that every codec can give a varying degree of quality for a given set of frames within a video sequence.
Each encoder implements the specification according to its own algorithms and parameters, which means that the compressed output of different codecs will vary, resulting in variations in quality and efficiency between them. The decoder component of a codec that also conforms to the specification recognises each of the mechanisms used, and thus interprets the compressed stream to render it back into raw video for display (although this will not be identical to the raw video input unless the compression was lossless). So long as the encoder component of the codec adheres to the specification it can choose any combination of these methods to apply different parts of the content. But quality/size ratio of output produced by different implementations of the same specification can also vary.Įach compression specification defines various mechanisms by which raw video (in essence, a sequence of full-resolution uncompressed digital images) can be reduced in size, from simple bit compression (like Lempel-Ziv-Welch) to psycho-visual and motion summarization, and how the output is stored as a bit stream. A codec is not a format, and there may be multiple codecs that implement the same compression specification – for example, MPEG-1 codecs typically do not achieve quality/size ratio comparable to codecs that implement the more modern H.264 specification. The quality the codec can achieve is heavily based on the compression format the codec uses.
Supported interfaces (VfW, DirectShow, etc.).Type of license (commercial, free, open source).General software characteristics – for example:.Performance characteristics such as compression/decompression speed, supported profiles/options, supported resolutions, supported rate control strategies, etc.Video quality comparisons can be subjective or objective. Commonly video quality is considered the main characteristic of codec comparisons. Video quality per bitrate (or range of bitrates).The following characteristics are compared in video codecs comparisons:
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