An open-source film algorithm hit the App Store and earned 1.4 stars in two weeks: what SpektraLab teaches you is to check the license first

SpektraLab is a photo-editing app that simulates film development based on spectral physics. It started as an open-source Python project by an Italian developer. A student in Guangzhou rebuilt it as a native Mac app that runs about 100x faster. It's free and open source, and it took off in Xiaohongshu's photography circles in October. 26 Later, the student released a paid iPhone version to capture that traffic, and also took heat from people who said he was profiting off open-source work. Below: why it took off, how much the mobile version is actually selling, and whether others can copy it.

"Took me forever, but the mobile version is finally out"

Around October 2026 of 27, a Xiaohongshu user "Diet Coke" posted: "Took me forever, but the SpektraLab mobile version is finally out," with 229 likes. That's the app's creator, Qiu Hanze, known on GitHub as JamesQiu2005. The day before, SpektraLab's iPhone / iPad version launched on the App Store, free to download, with a Plus monthly subscription at ¥9.9, annual at ¥98, and a permanent version at ¥268.

Two days later the mood turned. On October 29, 30, a Xiaohongshu user "Valkyrie" posted "SpektraLab copies an open-source project and violates the open-source license?" (446 likes, 124 comments), and "Lychee" posted "Free open-source film emulation gets 'wrapped' and launched as a paid app?" (250 likes). On the upstream project's GitHub, someone opened issue #62 on October 28, titled "GPL licensing clarification: SpektraLab iOS release and corresponding source."

By November 6, the China App Store had 51 ratings in total, averaging 1.37 stars (public iTunes API, 11-06). Since October 31 the review page has been almost entirely one-star: "Thought it was spektrafilm, turns out it's a knockoff charging money," "The open-source project explicitly forbids commercial use," "Still dare to sell 268?" One review's title is just six words: "Another 何同学."

What it is

The store page's opening line is: "Remember the feeling of waiting in a darkroom for a photo to slowly appear?" What it does is expose your photo onto a "measured roll of film," develop it, then enlarge it onto a measured sheet of photo paper, computing the light from 380 to 780 nanometers across 81 wavelengths one by one. The store page says what you get "isn't a filter layer, but a photo that has been developed." It includes 24 film stocks and 9 paper types, with controls for enlarger filtration, exposure, and contrast. On iPhone you choose your film roll before pressing the shutter.

The algorithm isn't theirs. In February 2025, Italian developer Andrea Volpato open-sourced spektrafilm (originally named agx-emulsion) on GitHub. Written in Python, it simulates the "negative → paper → scan" chain spectrally. The code is GPL-3.0, and the film profile files are CC BY-SA 4.0. The results are excellent, but it only runs on the command line, and a 45 megapixel image takes about 30 seconds.

Qiu turned it into a native app: SwiftUI plus C++ / Metal, bringing the same image down to 1.13 seconds (the number on his portfolio page), with a full set of "darkroom" interactions. The free open-source macOS version came first, and the paid iPhone version followed.

Why it took off

The results are the advertising. Phrases like "not a filter, it's development" and "81 wavelengths" travel well in Chinese photography circles, and one before-and-after comparison is enough for a Xiaohongshu post. The praise says the same thing: "Tried it briefly, convenient, and the color and grain are both really good" (China App Store, 11-04).

A free, open-source desktop version gave bloggers the nerve to promote it. On September 19 the author himself posted "Spektralab: a 'physically real' film emulation app" (733 likes). On October 11, Weibo photography blogger EASONZZ wrote "My friend... completely open-source and free" (168 reposts). What really drove volume were tutorials by others: on October 31, "Zhifei Post-Production Class" posted "Is SpektraLab's film simulation good? Windows version available for download!" with 1,122 likes and 1,618 saves, currently the hottest post. On November 4, "Orange Classmate" posted "Will we still need color grading in the future?" with 1,061 likes. GitHub stars rose from 218 in October 19 to 1,440 in November 6. The two Mac versions were downloaded 9,548 and 12,923 times from late October to early November, and the Windows beta 7,984 times (GitHub Releases).

The mobile version launched mid-hype. People had been asking for an iOS version in GitHub issues for a while. It launched in October 26, just ahead of the "Zhifei" tutorial.

Even the upstream project got pulled along. spektrafilm went from 1,151 stars in October 28 to 1,966 stars in November 6, gaining 209 stars in one day on November 3, the same week as the Xiaohongshu peak.

How much volume the mobile version really has

Not much, and the rating count can't be read as a download count here.

China store ratings, Oct 30 5, Nov 2 30, Nov 3 reset to zero (inferred: the ratings were reset with the new version), Nov 6 back up to 46 to 51. Most of the new ratings are bad reviews, driven by the controversy, not by users. The US store has only 7 ratings, averaging 1.29 stars, which also looks like controversy spillover.

The more reliable signal is the charts: it has only charted in the China, Hong Kong and Macau Photo & Video free category charts, with a best rank of #72 to #121, and never on any overall or grossing chart (11-06). The bar to reach the top 100 in the China Photo free category is not high; that rank is roughly several thousand downloads a day (estimate, unverified). How much money it made is not visible from outside; one one-star review reads "Don't pay, the Plus membership never actually activated."

The author has responded. In the Nov 5 release, version 1.2.0 moved film edges, HEIF, all color spaces and bit depths, and the viewing light table from Plus to free; the Nov 6 version 1.2.1 only said "stability improvements." The upstream issue was still open as of Nov 6, the only reply came from a third-party user, and the original author Volpato has not made a public statement.

Where the problem lies

The GPL itself allows charging money; the problem is that charging requires releasing the source code. The person asking in issue #62 compared the C++ code of the SpektraLab desktop version with the Python code of spektrafilm line by line in several places, and pointed out that SpektraLab's own license notice says the rendering engine is GPL-3.0. Whether the iPhone version has public corresponding source, or obtained separate permission from the original author, cannot be seen. Also, whether GPL code can be put on the App Store has been disputed for a long time: in 2011 VLC was pulled from the App Store for exactly this reason.

Users don't care about these details; they only see "free stuff being sold for 268." The portfolio page of Qiu Hanze says "indie developer," and this was also dug up and pasted into the issue. An app with a 1.4 rating will suffer store conversion drag for a long time.

Who's following

Plenty, and all free.

This table shows one thing: SpektraLab's core selling point, the spectral physics model, is open source upstream, and anyone can use it. Its real contribution is performance and interaction, and within two weeks both had been replicated in free versions. In China, the phrase "physical film development" has already been used up by it and its controversy.

If it were you

Bottom line: Don't copy SpektraLab. What's worth learning is its path: turning an open-source algorithm with stunning results, which previously only ran on the command line, into an app ordinary people can use. The precondition is to settle the license first, and ideally to enter a market that hasn't yet been burned by this controversy.

Good fit for: engineering-minded devs who can write Metal / Core Image or GPU code and squeeze a Python prototype down to real-time; people with a vertical community they already live in (photography, audio, astronomy, 3D scanning), who know what that crowd wants and where they learn.

Who it doesn't suit: Anyone wanting to build yet another spektrafilm phone version for the Chinese-speaking market (there are already several free versions, and users' aversion to paying is ready-made); anyone who doesn't plan to open source and doesn't want to negotiate with the original author (closed-source GPL code on the store is the root of this 1.4-star backlash); and anyone who thinks the algorithm alone can be a moat (the upstream is open, so the only moat left is experience).

How to do it: Two paths. One: still do film emulation, but switch markets. The English-speaking world is discussing spektrafilm (How-To Geek, r/colorists, the pixls.us forum), and the name SpektraLab returns zero results on X. First approach Volpato for a commercial license or revenue share; the app is open source, and you only charge for convenience (full-size RAW export, batch processing, tethered shooting), while core film development stays free. This path bets on two things: whether the original author is willing to negotiate, and whether SpektraLab will go global compliantly first (inferred). Two: switch algorithms. Find on GitHub an image or audio project under an MIT or Apache license, with good results and only a Python implementation, and rebuild it as a native app using SpektraLab's engineering approach.

How to acquire users: Follow the first half of its playbook: free desktop version, have vertical creators write tutorials, one before-and-after image per post; launch the phone version at mid-peak heat. The difference: the first post should clearly state "whose algorithm it is, what license it uses, and that the original author knows," addressing the point most likely to get you flamed up front.

Timing: If you take the film-emulation-going-global route, you need a usable version before year-end. SpektraLab and those free ports could enter the English-speaking market at any time, and this upstream wave of attention will pass.

Signs you've misjudged: ① Volpato publicly endorses SpektraLab, or grants it dual licensing, the controversy subsides, and ratings climb back above 4 stars. That would show that "build first, get the license later" can work, and the Chinese-market window is bigger than you think. ② SpektraLab still charts in China's Photo paid category during the controversy, showing heavy users will pay for something that works well, and free ports can't take that away. ③ The reverse: Apple pulls it after complaints, or a free version like Layerlight breaks into the top 50 of the Photo free chart. That would mean this category no longer makes money on phones, and even switching markets should be approached with caution.

Data and sources

• App Store: China store product page, review page (reviews 10-31 to 11-04), version history, in-app purchase prices; public iTunes API for the China and US stores, 11-06.
• Ratings and charts: daily rating counts, China / Hong Kong / Macau Photo category free chart ranks, as of 11-06.
• GitHub: JamesQiu2005/SpektraLab (star count, Releases download counts, 11-06), andreavolpato/spektrafilm (README license notes, issue #62).
• Author portfolio page jamesqiu2005.github.io/work/spektralab.html; official site spektralab.app.
• RedNote post titles, likes and publish dates, checked on 11-06; Weibo posts by EASONZZ (reposted via Sina).
• Daily download numbers are estimated from free category chart ranks; anything marked "estimate" or "inferred" is an estimate or inference.