VarDCT vs Modular in JPEG XL


VarDCT and Modular are both “modes” used for encoding in JPEG XL.

  • VarDCT – is designed for lossy encoding. It’s based on a variable block size Discrete Cosine Transform (DCT).
  • Modular – is designed for lossless and near-lossless encoding.


Both VarDCT and Modular can perform lossy compression. Each has its strengths, which are:

  • VarDCT – natural or complex images, like photographs.
  • Modular – images with sharp edges and flat areas, like digital art, text, screenshots, etc.

JPEG XL uses VarDCT for lossy and Modular for lossless. If you want to use Modular for lossy encoding, you need to specify it.


  • VarDCT – higher effort results in higher quality, but the file size may not be smaller.
  • Modular – higher effort always results in lower file size. The quality stays almost the same.

XL Converter

You can try out both of these modes in XL Converter.

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