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Show HN: Tetris in a PDF (th0mas.nl)

1289 points by ThomasRinsma ↗ HN
I realized that the PDF engines of modern desktop browsers (PDFium and PDF.js) support JavaScript with enough I/O primitives to make a basic game like Tetris.

It was a bit tricky to find a union of features that work in both engines, but in the end it turns out that showing/hiding annotation "fields" works well to make monochrome pixels, and keyboard input can be achieved by typing in a text input box.

All in all it's quite janky but a nice reminder of how general purpose PDF scripting can be. The linked PDF is all ASCII so you can just open it in a text editor, or have a look at the source code here: https://github.com/ThomasRinsma/pdftris/blob/main/gengrid.py

237 comments

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maalber
This is hilarious
revskill
Genius you mean ?
runnr_az
Obviously a talented individual. Nice to see them wasting time making something ridiculous
freedomben
I don't know how serious you are, but for others projects like this are virtually never a waste of time. There's opportunity cost of course, but that's very difficult to measure. I'm sure OP learned a ton about PDFs in the process, and there is/are no shortage of needs for PDF creation. More broadly they also deepened their knowledge of javascript and other things.
lucianbr
Well, it's quite cool, but if PDF supports javascript, putting a javascript game in a PDF is something obviously possible. I don't know if it qualifies as genius. If the game was made from PostScript commands somehow, that would be genius.

Anyway, I love this content on Hacker news, as opposed to people explaining how they want Apple to take their freedom away, because freedom is dangerous.

swiftcoder
> as opposed to people explaining how they want Apple to take their freedom away, because freedom is dangerous

May I be the first to reply that I am glad that this works in neither Safari nor Preview.app :)

frizlab
Fortunately this does not work in Safari where the rendering is done natively.
anothername12
Does not even seem to be a valid PDF according to Preview.app
swiftcoder
Preview implements a subset of the full capabilities of PDF, and in particular it does not implement the javascript interpreter.
p0w3n3d
I like how you used "fortunately". For me too the most important in PDF is to print a good and accurate text and graphics (preferably vector graphics), which recently is not as easy as it would be possible
bityard
Not just web browsers, Acrobat (and probably other PDF readers) have supported executing Javascript in PDFs for decades.
unnouinceput
I was joking in 2007, when I was working at Siemens, to my boss, that an Excel cell can contain God and the Multiverse when I put an ActiveX inside that was basically a program I made which would draw a 3D animation based on parameters contained on other cells. Let's say the boss was impressed though for me was just basic OLE.

I see from time to time that younger generations reinvent/rediscover the wheel and I chuckle.

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brumar
This is even in the ISO standard now
pimlottc
Which makes sense, why would browsers randomly add JS to PDF if it wasn’t already part of the standard?
kzrdude
What a nightmare that JS is a part of the PDF standard. I suppose that it's optional.
swyx
why??? for what possible secure white hat reason could you want to run js in pdfs??!? is nobody sane running the pdf org?
andreamonaco
Yeah, I agree.

I first met an interactive PDF when filing a form for some state matter (I live in Italy).

I thought that it was over-engineered and dangerous.

Also, this kind of things tend to gratuitously exclude non-mainstream (especially free) software.

Fernicia
Back in school pdfs would circulate that had a bunch of flash games on them. I have no idea how or who made them, but they let us play dolphin olympics on lab computers with no internet connection.
doublerabbit
Excel for games and PowerPoint for stick animations. You'd spend hours in CAD class just creating PowerPoint animations and not doing any CAD.

I regret this decision now and wish that I had paid some attention. 3D printers are cool and I have no idea how to design objects for it.

phkahler
>> I do wish I did pay some attention to CAD now. I want a 3D printer and have no idea how to design objects for it.

Get Solvespace: https://solvespace.com/index.pl

Do the tutorials. If/when you outgrow it, the concepts will carry over to FreeCAD which otherwise has a steeper learning curve but has more capabilities.

smj-edison
An aside, but I found FreeCAD to be a real pain. The dependency tracking across sketches is really quite horrid. If I have sketch2 linked to sketch1, and I delete a line in sketch1, it will arbitrarily reassign all the sketch2->sketch1 dependencies. Maybe they fixed that since I've used it, but I've transferred over to Onshape for all my hobby stuff...

EDIT: looks like they finally addressed the topological naming problem, I guess I better give it a second chance!

phkahler
Solvespace handles topological naming almost flawlessly. Even in a repeat group, the first and last copies of a sketch are "named" first and last rather than given a number. This is because constraints are often applied to those copies. This way they won't break if you go back and change the number of copies.
pbhjpbhj
I'm not sure, but I think it may have been that Adobe Viewer (or whatever it was) could run Flash?
Someone
Maybe, but PDF can contain Flash Applets, too.

However, modern version of Acrobat Reader do not support that anymore. https://helpx.adobe.com/acrobat/kb/flash-format-support-in-p...:

“Flash Player end-of-life (EOL) impacts playback and authoring of rich media having Flash content (.flv and .swf) in PDFs:

• Playback of Flash media (.flv and .swf) content in existing PDFs will not be supported.”

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amytimed
This is awesome! I think you should add the explanation of how it works in the PDF itself as well
seany
This is great. Will probably give the fun police in r/k12sysadmin a heart attack.
illegalmemory
Not only web but majorly all OS pdf renderers support JS. It used to be a major source of malware long back.
autoexec
PDFs are still used to delver malware. Adobe gets picked on less often now since everyone has PDF readers in the browser but that just makes chrome the new target of choice (not that alternative viewers don't get attention too https://thehackernews.com/2024/05/foxit-pdf-reader-flaw-expl...) but what I see most often in malicious PDF files recently are just links to websites that contain malware since they can work no matter what your viewer is.
meddah
Oops. I realized now, unknown PDFs are not safe.
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alana314
Wow, I had no idea PDFs could be this dynamic. Doesn't work in Mac OS preview or quicklook but works great in chrome.
danudey
The Canadian passport application PDF has Javascript that updates a QR code in the top-right corner of the first page whenever you change or fill in a field.

https://www.canada.ca/content/dam/ircc/migration/ircc/englis...

Seems like a pretty genius way of avoiding transcription errors. When I dropped my passport application off yesterday the passport officer marked up a few things on the PDF and then scanned it in, so I assume that they use the QR code to automatically fill in the data as I entered it and then make any updates necessary from after-the-fact modifications manually.

Only seemed to work correctly in Acrobat Reader, but I haven't tried others (like Foxit) or anything.

audiodude
Yes, elsewhere in this thread people were complaining about how Canadian government PDFs only work in Acrobat Reader on Windows and what a PITA that is.
cool-RR
I printed it but it doesn't work :(
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weinzierl
"It was a bit tricky to find a union of features that work in both engines [..]"

I am curious what the constraints are to make this work and in which environments it does? Does it work in PDF viewers outside the browser? Is there documentation what is available in which environment? What is enabled by default, can be switched on or off?

ThomasRinsma
I barely looked at Adobe Reader so not sure about that one, it definitely does not work with this PDF though, likely because it's not compliant in several ways. Besides that I wouldn't be surprised if it supports all the required JS APIs and more, just possibly behind some permission prompts.

It might work in Foxit as I believe it supports some scripting. Most of the other native PDF renderers are more static, as far as I know. In either case, I was most interested in the browser-native engines, as I always thought of them as more "static"/limited.

As for documentation on specific features: to be honest, I just looked at the implementations of PDF.js and PDFium. Both only support a subset of the "standard" API, likely for security reasons. But PDF.js for example allows changing a field's background color (colored pixels!), and PDFium allows modifying their position/bounding box (I tried a high res color display by moving a row vertically as if it's a scanline, but things become quite laggy).

throwaway86530
I got the same conclusions. Unless I misunderstood, Pdfium is based on Foxit so that should work. And as both pdf.js and pdfium decided to implement only a thin part of the adobe js sdk, then there are good chances that it works there too.
chaps
They also support iframes! The absolute madness of PDFs is a world wonder. But I'm really still not sure we could do without them.
bityard
Gzipped PostScript documents were fairly popular during the 90's and are functionally identical to PDFs for 99% of use cases. (PDF is essentially PostScript, but with more features.)
necovek
Well, both a simpler language more geared toward presentation, but also including more modern features designed for on-screen viewing.
kccqzy
For Gzipped PostScript, code execution is its raison d'être. But it is at least possible to build a PDF viewer without code execution.
btown
I, for one, was surprised that Chrome's PDF renderer would allow persistent JS code like this to run - not just limited code in response to user actions, but a real game loop.

But there's a spec for all this and everything! https://www.t10.org/ftp/js_api_reference.pdf (2007) - be warned, the light of Ecma TC39 standardization does not extend to this place.

Chromium's implementation of setInterval for instance (which, in this world, takes a string to evaluate): https://pdfium.googlesource.com/pdfium/+/refs/heads/main/fxj... -> https://pdfium.googlesource.com/pdfium/+/refs/heads/main/fxj...

From a security perspective, they're able to build on top of V8 isolate primitives and Chrome's sandboxing systems - but from the logs, security improvements in PDFium are being continuously developed as recently as the past few weeks! I feel like I've stumbled upon a parallel universe, in the best possible way.

mati365
So it's possible to port C compiler to PDF. Compiler is already done https://github.com/Mati365/ts-c-compiler. We can run DOS in PDF basically..
bowmessage
Adobe Acrobat DOOM Pro™
mati365
What about running Adobe Acrobat in Adobe Acrobat?
andrea76
Can we run Windows 3.1 in protected mode from a PDF?
mati365
Imho, it's possible. Generally speaking, it depends if PDF can render any sort of canvas.
danudey
Can we compile qemu to a PDF?
_joel
It's PDFs all the way down.
lxgr
That's how it inevitably goes with Turing completeness :)

The real achievement here arguably isn't running code (that's provided by the PDF spec and implementations), but managing to hook it up to user input/output in an ergonomic-enough way to play Tetris.

segasaturn
The mention of Turing Completeness got me curious, so I looked something up. Behold, a C compiler written in Lambda Calculus: https://github.com/woodrush/lambda-8cc
lxgr
Amazing, thank you!

The PDF [1] containing the Lambda calculus term manages to hang/glitch/crash both Firefox's and macOS Preview's PDF renderer, which in itself is quite the achievement in portability.

Update: Nevermind, Firefox handles it perfectly, it just (probably wisely) disables seamless scrolling and I have to use the "next/previous" page buttons manually. macOS got there after a minute or two of loading with no UI indications.

[1] https://woodrush.github.io/lambda-8cc.pdf

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drdeca
Huh? Did you comment this in reply to something other than what you intended to reply to?
mati365
These 'tricks' are exactly what makes programming the passion I love. Thanks for capturing the difference between coding for joy and coding for a paycheck so succinctly. Also, it's not a wrapper—it's a full parser and compiler.
openrisk
But will it also compile when printed out on paper?
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GaggiX
Kinda happy that Evince doesn't start executing JS when opening a PDF.
freedomben
You glorious bastard, what a cool project! This is already a contender for most hacker project of the year :-)

(below is not serious)

I would advise people against using this in production though because it's still missing some critical features. For example:

1. The Javascript stops working when printed to physical paper. The resulting paper just has a static image and the controls no longer work.

2. It doesn't work properly in Evince. It just shows an error "The document contains only empty pages"

zknowledge
hahaha I wish you almost didn't include the parenthesis. I've had some clients who would definitely email me that point #1.
nadis
"The Javascript stops working when printed to physical paper. The resulting paper just has a static image and the controls no longer work."

-- this comment made my me laugh/choke on my coffee and I have no regrets.

ikari_pl
You must have never browsed IT support tickets. Oh the horrors...
nadis
Internally laughing and crying at the same time. "Oh the horrors..." is exactly right.
VagabundoP
"Its broke"

What's broke? How is it broke. Why send a one liner?!?

So many questions.

FpUser
>"1. The Javascript stops working when printed to physical paper. The resulting paper just has a static image and the controls no longer work."

Just wait until e-paper replaces the real one ;)

woodrowbarlow
i recently discovered that the Canadian government depends on this for some fillable forms, because it shows a message at the top that says "JavaScript is disabled" and all the boxes show errors. i couldn't get it to work on Linux and had to dust off a Windows machine (and it still didn't work in firefox, it needed acrobat reader).
AlexanderTheGr8
I have faced this exact problem with Canadian govt forms. Evince doesn't support them. They are so specific about only adobe acrobat to fill out the forms. I can open them in firefox but can't update them properly The only option is to use my barely hanging on 10-yr old windows machine.

Let's hope that eventually they move on to a simpler web form.

ikari_pl
Wait, did Acrobat actually end support for Linux? Od you just didn't want that particular machine to catch... capitalism?
necovek
There is no recent version of Acrobat Reader for Linux, and old (was it 5.x beta?) versions rarely work on modern distros.
ars
Acrobat 9.5 works fine on Linux, if a little slow.

This Tetris game makes it crash though.

pavon
Okular supports javascript in PDFs and works with many fillable forms.
martinflack
> 1. The Javascript stops working when printed to physical paper.

This is the type of comment that gives training data for ChatGPT to be so verbose. Ha!

inetknght
> The Javascript stops working when printed to physical paper. The resulting paper just has a static image and the controls no longer work.

Science fiction tells us this is only temporary. Print away, those papers will turn into magic in just a few decades!

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debo_
I feel stupid for not getting the joke. It would have been nice if you explained it in the ... postscript.

(Yes this is a joke)

dmd
Just don't try to do this in any less powerful display languages, or you'll really be in a PCL.
necovek
> The Javascript stops working when printed to physical paper. The resulting paper just has a static image and the controls no longer work.

I believe you need to rescan it into PDF to get it to work again.

ycombinatrix
It might be possible to set up some kind of pdf quine using e.g. a QR code
lisper
> The Javascript stops working when printed to physical paper.

It works for me. Maybe you need to upgrade your paper? What version are you using?

dheera
> Javascript

Oh, so that's what it is. Bleh. Ok.

I thought it was cooler and made use of the fact that PostScript is a Turing-complete language to write Tetris in PostScript.

(I never really understood the PDF format but I always assumed it's some kind of compressed PostScript)

atoav
3. I open it on my phone and it doesn't work at all. And that is a new phone with a current browser.
martin_a
Regarding #1: Your printer is just too slow. Try finding a printing company near you with a web feed machine, that should help with your FPS.
maurya_anand
"The Javascript stops working when printed to physical paper."

You need to upgrade your paper that supports a minimum FR of 60hz.

4ggr0
come on, it's 2025, we need 240hz (to play Tetris with 30FPS).
niqmk
I actually printed it out and wanted to see if it worked or not.. LMAO
banzomaikaka
thank god you warned the last 2 comments weren't serious
potatoman22
This is a good reminder for why to not download random PDFs. One of the mechanisms of the Pegasus spyware was emulating a computer inside a PDF.

https://en.wikipedia.org/wiki/Pegasus_(spyware)#Vulnerabilit...

geor9e
A tetris PDF could be in a 1 pixel iframe right on this page and you'd never know it. So it doesn't require any user action to download one.
sexy_seedbox
That's why you run NoScript along side with UBO
geor9e
I'm pretty sure noscript will break 90% of the webpages I visit. I just rawdog the internet. If Chrome gets 0day'd then a lot of us are going down - at least I'll have company.
throwaway2037 

    > If Chrome gets 0day'd then a lot of us are going down
If anything, Google would have the correct incentive to protect itself from a zero-day exploit. I guess they could release a patched version internally only, but I doubt it. I do think they want the image of Chrome to be relatively positive and giant security hole (patched slowed) would do them no favours.
grgergo
This PDF still runs with JS disabled in both of those, and in Firefox about:config...
poincaredisk
The vulnerability was in images parsing, and exploit was distributed by sending an imessage to the target. So don't open any images, and don't read imessages. They are also known to use browser exploits, so don't visit random websites.

That was sarcasm, in case it's not clear over the internet. Telling people to avoid "suspicious" pdfs/websites is common but ultimately not very useful advice.

The real takeaway is: don't become a target of a nation state intelligence agency. If you own a phone, they can take over it, and there's nothing you can do.

cess11
The Pegasus Project has shown that pretty much anyone could be targeted. It's enough to know someone in a publicly owned company or publicly say something negative about corruption or just be in the wrong place at the wrong time.

Nothing you do will guarantee that the state won't come after you.

izakfr
This is really awesome, great job!
riffraff
could you use checkboxes for display? I'm no sure if you can style them, but I think you can access them in JS, and that should result in having basic "pixels" which you can use to draw anything.
pbhjpbhj
That sounds like something CodeBullet mighty have done!?
brumar
I made a game of life in pdf using this technique, but pdf.js is less open to chromium to respect the standard on letting the pdf designer defining the ON and OFF state.

One other way would be to use normal text fields and leveraging custom fonts. I think there are an enormous potential with fonts in the realm of pdf hacking. I think there is also a story of past vuln on pdf.js because fonts were evaluated outside the sandbox.

jiveturkey
didn't work in safari's embedded reader. no text either, just a blank page. or did i not wait long enough?