More than 100 billion images and videos already carry an invisible Google watermark, along with the equivalent of 60,000 years of audio. Almost nobody who made them knows it is there.
That watermark is called SynthID, and since May 2026 it has stopped being a Google-only project. If your marketing team generates a product image, if a supplier sends you a photo, if an advertisement appears using your shop front, an invisible marker may now travel with that file.
This guide explains what SynthID is, how it works, what it can prove, and the three things it cannot prove.
What is SynthID?
SynthID is a digital watermarking technology built by Google DeepMind that embeds an imperceptible signal into AI-generated images, video, audio and text at the moment the file is created. The signal is invisible to a human, survives ordinary editing, and can be read back by a paired detector to show the content was machine-generated.
The key word is at the moment of creation. SynthID is not a logo stamped onto a finished picture. It is woven into the pixels while the model is drawing them, which is why cropping the corner off does not remove it.
Google introduced the technology three years ago and has now applied it, by its own count, to over 100 billion images and videos.
How does SynthID actually work?
SynthID makes tiny, patterned adjustments to the pixels of an image, the frames of a video or the waveform of audio while the file is being generated. A matching Google detector model looks for that pattern and reports whether it is present. Humans cannot see or hear the change, and the quality of the output is not affected.
A useful comparison: a paper watermark in a banknote is visible when you hold it to the light. SynthID is closer to a pattern woven into the paper fibres themselves, spread across the whole sheet rather than sitting in one corner.
That distribution is what gives it durability. According to Google and independent testing write-ups, a SynthID signal generally survives:
--- JPEG compression, including repeated re-saving
--- Resizing and moderate cropping
--- Colour and brightness adjustment
--- Screenshots
It gets weaker under aggressive re-compression, heavy cropping, or a screenshot of a screenshot. It is durable, not indestructible.
Which AI tools add a SynthID watermark in 2026?
As of the May 2026 announcement, SynthID is no longer confined to Google products. Google, OpenAI, NVIDIA, Kakao and ElevenLabs have all committed to embedding the watermark in their AI-generated output, which covers a large share of the images and audio a small business is likely to encounter.
Google set this out in its own post, Making it easier to understand how content was created and edited, published 19 May 2026.
Two developments matter more than the list of names.
Watermarking is being paired with Content Credentials.
SynthID says a file was made by AI. C2PA Content Credentials is a separate industry standard that records how a file was created and edited, with or without AI. Google now attaches both, and Meta has said it will begin labelling camera-captured media with Content Credentials on Instagram.
Cameras are being marked too.
Pixel 10 was the first phone to attach Content Credentials to photos taken in its native camera app, and Google is extending this to video on Pixel 8, 9 and 10. Proving that something was genuinely photographed is becoming as important as proving something was generated.
How can you check whether an image was made with AI?
The practical route for most people in 2026 is the Gemini app. Upload the image, video or audio file and ask a plain question such as "Is this made with AI?". Google reports the feature has already been used 50 million times globally. Verification is also rolling out into Google Search, through Lens, AI Mode and Circle to Search, and into Chrome.
There are three things worth knowing before you rely on the answer.
--- A positive result is strong evidence. If a SynthID signal is found, the file was almost certainly generated by a participating AI tool.
--- A negative result proves very little, for reasons covered in the next section.
--- Detection currently runs on Google infrastructure. The image watermark and its detector are not open source, so a third party cannot build an independent SynthID checker. Google has open-sourced only its text watermarking method.
What can SynthID not tell you?
SynthID answers exactly one question: was this file produced by a participating AI generator. It does not identify which tool made it, who prompted it, when it was made, or whether it has been edited since. Absence of a watermark is not evidence that a file is authentic.
Three limits deserve to be written down, because most coverage skips them.
1. It is information-poor by design.
A hit tells you "AI made this" and nothing else. It will not tell you that the invoice photo came from a competitor, or that the product shot was generated by your own agency rather than photographed as promised.
2. It only marks content from participating generators.
An image made with a tool that does not embed SynthID returns no signal at all, even though it is entirely synthetic. Anyone assuming "no watermark equals real photo" has drawn the wrong conclusion.
3. It does not stop the harm, it only labels it.
A watermark on a fraudulent image does not prevent the fraud. It gives you a way to check afterwards, if you think to check at all.
Why does this matter to a Hong Kong small business?
Two forces are converging in August 2026. Verification is arriving inside tools your staff already use, and the European Union's AI Act transparency rules, which require AI-generated images, audio, video and text to be marked in a machine-readable form, apply from this month. Even a Hong Kong company with no EU customers will feel the standard settle in through the platforms it uses.
Three situations where this becomes a practical matter rather than a technical one.
The supplier photo.
A Sheung Wan trading company receives product images from a mainland supplier before placing a first order. The photographs look immaculate. A verification check that returns an AI signal does not end the conversation, but it changes the question from "how does this product look" to "has anyone actually seen this product".
The agency deliverable.
A Causeway Bay retailer pays for a campaign shoot. If half the delivered images carry an AI watermark, the business has bought generated pictures at photography prices. This is now checkable in about fifteen seconds.
Your own marketing.
If your team uses a mainstream tool to create promotional images, those files now travel with an invisible marker and, increasingly, a visible label on the platforms where you post them. This is not a problem, but it is a decision. Decide in advance whether you are comfortable telling customers an image is AI-assisted, because the platforms are moving towards telling them for you.
The related risk, familiar to any owner who has read about AI voice cloning fraud, is that provenance tools are a check on incoming content, not a substitute for a phone call to a person you know.
There is also a quieter internal version of this problem. Staff generate images with whichever free tool they found, on personal accounts, without telling anyone. That is the same pattern described in shadow AI, and it means a company can be publishing watermarked content it never decided to publish.
The fix is not a policy document. It is one line in whatever passes for your marketing process: name the tools people are allowed to use, and name the person who checks images that arrive from outside the company.
Common misconceptions about AI watermarking
Four beliefs circulate widely and none of them holds up.
"If I remove the visible Gemini watermark, the image is clean."
The visible corner label and the invisible SynthID signal are two different things. Removing one leaves the other untouched. Paid tiers of some tools drop the visible mark while the embedded watermark stays.
"A screenshot strips the watermark."
Screenshots are one of the transformations SynthID is specifically designed to survive. Repeated heavy re-compression weakens it, a single screenshot generally does not.
"No watermark means the photo is real."
This is the most costly error. Many generators do not participate at all. A clean result narrows nothing.
"This is only relevant to media companies."
The verification tools are shipping into Search, Chrome and the Gemini app, which is to say into the software your customers and staff use daily. Access is no longer the barrier.
Frequently asked questions
Does SynthID work on text?
Yes, and Google has open-sourced its text watermarking method. Text is the weakest case in practice, because short passages carry less signal and paraphrasing degrades it quickly.
Can I check an image without a Google account?
Verification currently runs through Google surfaces such as the Gemini app, Search and Chrome, so in practice you are inside Google's tools. There is no independent SynthID reader for images.
If I generate marketing images, am I legally required to disclose it?
Hong Kong has no general labelling law for AI-generated marketing images. The EU AI Act transparency requirements apply from August 2026 and reach businesses serving EU users. Platform rules are often stricter than the law, so check the terms of the channel you publish on. This is general information, not legal advice.
Does the watermark affect image quality?
No. Google states that adding the watermark does not affect quality, accuracy or generation speed, and the adjustment is imperceptible to a human viewer.
What should a small business actually do this month?
One thing. Run three images you received from an outside party through a verification check, so the team learns the tool exists before there is a dispute that depends on it.
The bottom line
SynthID is a useful new instrument with a narrow reading. It can tell you an image was made by a machine. It cannot tell you who, when, why, or whether the version in front of you has been altered since.
Treat it the way you treat a receipt. Evidence of one specific fact, not proof of the whole story.
The businesses that come out ahead will not be the ones that check every file. They will be the ones that decided in advance which files are worth checking, and who is responsible for checking them.
We understand AI. UD stands with you.
Reviewed by the UD AI team, Hong Kong.
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