by google

google / knusperli

A deblocking JPEG decoder

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The goal of Knusperli is to reduce blocking artifacts in decoded JPEG images, by interpreting quantized DCT coefficients in the image data as an interval, rather than a fixed value, and choosing the value from that interval that minimizes discontinuities at block boundaries.

Left: a traditional JPEG decoder (Imagemagick 6.9.7-4). Right: Knusperli.

Lena JPEG, zoomed Lena Knusperli, zoomed

Lena JPEG Lena Knusperli


Knusperli builds with Bazel:

CC=gcc bazel build :knusperli
bazel-bin/knusperli input.jpg output.png


A JPEG encoder quantizes DCT coefficients by rounding coefficients to the nearest multiple of the elements of the quantization matrix. For every coefficient, there is an interval of values that would round to the same multiple. A traditional decoder uses the center of this interval to reconstruct the image. Knusperli instead chooses the value in the interval that reduces discontinuities at block boundaries. The coefficients that Knusperli uses, would have rounded to the same values that are stored in the JPEG image.


This is not an officially supported Google product.

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