JPEG Sawmill is a web application that splits up progressive JPEGs into their individual scans. The image resulting from each scan can be viewed, as well as the differences between adjacent images in the series. Progressive display can also be simulated in slow motion, as if the image is being downloaded over a slow connection.

The JPEG image standard supports both sequential and progressive encoding modes. Sequential JPEGs are encoded in a single "scan" from top left to bottom right. Progressive JPEGs reconstruct the image in a series of scans, where each subsequent scan improves image quality.

Progressive mode is useful for websites, as the initial scans quickly provide an acceptable placeholder while the rest is downloaded. Also, progressive JPEGs usually reduce file size because the rearranged image data is more compressible.

JPEG progressive mode is highly customisable¹; the settings can be tuned for specific use cases or even optimised per-image.

Images are split into their constituent scans using the same general approach as "JPEG Scan Killer". First, the markers at the end of each scan are located using the jpeg_inspector library. Then new images are created, containing all the data before each marker with an "End of Image" marker appended.

The jpeg_inspector library was designed for use as a WebAssembly module. It avoids using file IO, so emscripten's virtual file system isn't needed. Instead the API just lets the calling JavaScript get the offset of the end of the next scan. This cuts the resulting .wasm size down to 8kB for a release build ².

On the JavaScript side, the new images for each scan are made using subarrays of the full JPEG buffer. These are views onto the same underlying data. To display the images, createObjectUrl is used to allow them to be referenced from the DOM. This means the scan images don't need encoding as base64 data URLs.

This design means the memory required for processing is roughly twice the file size³⁴.

• Frederic Kayser for showing how to split a progressive JPEG into individual scans.

  • JSK -JPEG Scan Killer- progressive JPEG explained in slowmo
  • Source code (jsk.c)

• Karl Thibault for showing how to use Emscripten to embed JSK in a web app

  • https://github.com/Notuom/progressive-jpeg-scans

• The Emscripten authors for Emscripten

  • https://github.com/emscripten-core/emscripten

• Sebastiano Merlino for liblittletest

  • https://github.com/etr/liblittletest