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Thread: terminology: the line between lossy and lossless

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    Question terminology: the line between lossy and lossless

    Its clear that being able to restore only an approximation of the original file would be lossy, and being able to restore an exactly identical copy of the original file is lossless.

    What is the correct terminology for a compressor that restores an exact logical reproduction of the original file's data, even though that is not byte-identical?

    For example, if you know the use the file is being put to, and you know that the decompression is exactly equivalent?

    An example might be a compressor for 3D model meshes that lists the vertices in the restored file in a different order from the input, whilst restoring correctly the exact positions of those vertices and all the edges between them.

    Another example might be an XML compressor that removes redundant whitespace within tags. And so on.

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    Well in the case of images i call them "pixel perfect" or "pixel exact". IE when you optimise a PNG file for size, or remove metadata from a JPEG just leaving the image data.
    Dunno about other file types.

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    I use the terms bit-lossless and content-lossless. Content-losslessness is quite flexible and depends on a context, you define an equivalence relation and claim that compressor preserves partitions that it creates. In case of images, I find content losslessness to be more or the same as pixel-lossless; depending on context preservation of EXIF and other metadata may or may not be required. I.e. for photos I'd say that content-lossless storage would need them and for web images it wouldn't.

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