Pngquant works in premultiplied alpha color space to give less weight to transparent colors. To improve color further, histogram is adjusted in a process similar to gradient descent (Median Cut is repeated many times with more weight on poorly represented colors).įinally, colors are corrected using Voronoi iteration (K-means), which guarantees locally optimal palette. Histogram is built with addition of a basic perception model, which gives less weight to noisy areas of the image.

Instead of splitting boxes with largest volume or number of colors, boxes are selected to minimize variance from their median value. Pngquant uses modified version of Median Cut quantization algorithm and additional techniques to mitigate deficiencies of Median Cut. PNGQuant Algorithmĭetails of how this algorithm works have been copied below for convenience but can be found towards the bottom of the webpage for the library at

Specifically we use the pngquant quantisation library which creates efficient 8-bit PNG files with an alpha channel. It uses a technique called “ Color quantization”, which basically means reducing the number of colours used in an image. Compressed images are fully standards-compliant and are supported by all current web browsers and operating systems.

Free png image compression software