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Guetzli is a perceptual JPEG encoder developed by Google that specializes in compression of high-end image quality, albeit at very low speed.

Guetzli is known to be very resource intensive, requiring several orders of magnitude more processing time than other JPEG encoders, and a lot of memory. Guetzli supports only the top of JPEG's quality range (quantizer settings 84–100) and supports only sequential (non-"progressive") encoding. Guetzli is more effective with bigger files.

Guetzli uses methods to optimize compression efficiency that target mainly the quantization step. It constructs custom quantization tables for each file, decides on color subsampling, and quantizes some adjacent DCT coefficients to zero, balancing benefits in the run-length encoding of coefficients and preservation of perceived image fidelity. Zeroing the right coefficients is the most effective tool in Guetzli, which is used as a makeshift means of spatially adaptive quantization. The optimizations are guided by Butteraugli.

It is implemented as a command-line tool in C++.



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Panos Alevropoulos

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23 July 2021

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