Kindle Comic Converter uses 2D DFT and dithering to sharpen eInk manga

A converter built around the screen, not the file

A Show HN post that reached Hacker News's front page on September 30 spotlights Kindle Comic Converter (KCC), an open-source utility that reformats comics and manga for eInk devices. According to the project's GitHub page, it supports Kindle, Kobo, ReMarkable and other readers, and its pitch is unusual for a converter: the goal is not merely smaller files but visibly better pages, achieved by processing each image for the exact panel it will be shown on.

What KCC does to the pixels

The GitHub page lays out several techniques:

• Native-resolution downscaling. The project identifies DRM-free PDFs from Humble Bundle and Fanatical as the best sources, often 12000 pixels or better on one axis, while even the 10-inch Kindle Scribe is only 2480. Resampling to the device's real dimensions lets a 600 MB volume land around 100 MB, and the project says it also improves page-turn speed, battery life and general responsiveness on readers with little memory or storage.
• Black-level correction. Manga from the Kindle Store frequently renders with gray, faded blacks; KCC restores them to black.
• Frequency-domain cleanup. On Kaleido 3 color eInk panels KCC removes the rainbow-pattern artifact using a two-dimensional discrete Fourier transform, avoiding the blur a conventional filter would introduce.
• Dithering. On jailbroken Kindles running KOreader, KCC can emit dithered 4-bit PNG or WEBP instead of 8-bit JPEG. The project describes this as halving file size with no visible loss, since eInk screens are 4-bit to begin with.
• Layout handling. Pages can be shown edge to edge without the margins most conversion methods add, and KCC semi-automatically detects two-page spreads from pre-split pages, displaying combined spreads rotated or upright.

Practical details and caveats

KCC runs on Windows, macOS and Linux, accepts folders of JPG/PNG images, CBZ or EPUB archives and PDFs, and outputs MOBI/AZW3, EPUB, KEPUB, CBZ, PDF or image folders. KindleGen and 7-Zip are optional add-ons that unlock features and speed up conversions.

The project is frank about rough edges. MOBI support on the 2025 Kindle Scribe is limited and can produce blank pages, so PDF output is recommended there. Mac users with 2022-or-newer Kindles may need Amazon's USB file transfer app or OpenMTP. The FAQ also warns against post-processing KCC output in Calibre: its handling of fixed-layout EPUB/MOBI is weak enough that even metadata edits can break formatting and page numbers.

KCC stresses it has no connection to Amazon's Kindle Comic Creator, a publisher-facing tool involving heavy manual work, and credits volunteer maintainers: founder Ciro Mattia Gonano (2012–2014), Paweł Jastrzębski (2013–2019) and Alex Xu (2023–present).

Why it matters

EInk readers are an odd target for image processing: 4-bit depth, slow refresh and tight memory budgets. Most converters ignore this and produce generic output, spending bits on detail the panel cannot display. KCC instead filters in the frequency domain, dithers to the panel's actual bit depth and resamples to its exact pixel grid — a hardware-aware pipeline more typical of professional display work, applied here to manga on consumer devices. The payoff is concrete: deeper blacks, no rainbow texture on color eInk, and files small enough to behave well on underpowered hardware. It is also a case study in how jailbreaking and alternative readers like KOreader extend locked-down hardware, with 4-bit image formats and potentially twice-as-fast CBZ conversion among the benefits the project lists.