Unlike JPEG, PNG is a lossless image format, which is why it reproduces pictures perfectly, but makes many of them — mostly photographs — much larger than what you’d get with lossy codecs. PNG is also the only way to get pictures with transparency (alpha channel) display in all browsers, so if we need images with alpha channel, we have to use large PNGs. However, there’s a little trick we can use to make them smaller: by applying lossy preprocessing.

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