Why a photo of a cat on a screen is not a cat

A week into testing, someone sent me a screen recording. They had pointed their phone at a laptop showing a photograph of a ginger cat, tapped scan, and been told no. Their message was one line: why won't it take this, it's obviously a cat.

They were right. It was obviously a cat. It was also obviously a laptop.

That refusal is the most argued-about decision in the whole game, and it is the one I am least willing to move on. So here is the full reasoning, including the part where being strict costs us something real.

What a screen looks like to a detector

To your eye, a cat on a wall and a cat on a display are both just a cat. To a camera and the software reading its frames, they are not remotely the same object. A display is a flat, self-lit sheet of glass with a repeating grid of subpixels behind it. A cat is a lumpy three-dimensional animal covered in fur that scatters light in a hundred directions at once.

That difference shows up in a handful of ways:

• Moiré. Your camera's sensor grid and the display's pixel grid interfere with each other and produce faint ripples and colour bands across areas that should be smooth. Fur does not ripple like that.
• Even backlighting. A screen emits its own light, uniformly, across its whole surface. Daylight and street light arrive from somewhere, so one side of a real cat's face is brighter than the other, whiskers throw thin shadows, and the thin skin of the ears glows where light passes through it.
• Flatness. Everything on a screen sits at exactly one distance from your lens. A real scene does not. The paving in front of the cat and the fence behind it are at different distances and behave differently as the lens finds focus.
• Reflections. Glass hands you back the room. A window behind you, a ceiling light, the outline of the person holding the phone.
• Edges. A bezel. A browser tab. A rounded corner, a status bar, a thumb resting at the side of the frame.
• Banding. Phone shutters read the sensor line by line, and panels redraw line by line, so the two slide past each other and leave faint horizontal bands that shift between frames.
• Second-hand colour. An image on a screen has already been through a camera, a compressor and a panel. Blacks lift, gradients tread on each other, and the wet-fur highlights that an overcast sky gives you never come back.

Printed photos have their own giveaways: paper texture, a sheen where the light hits, the dot pattern of the print itself. None of these signals is proof on its own. Any of them can turn up in an honest photograph. The judgement is made across several of them together, which is also exactly why it is possible to be wrong in both directions.

A collection is only worth what it cost you

The strictness is not really about cheating, in the way people mean when they say cheating. It is about what a card is.

Every card in your album is a cat cut out of a photo you took while standing in front of that cat. That is the whole product. Take the standing-in-front-of-it part away and the album becomes a folder of pictures off the internet with a rarity label stuck on the corner. In most collecting games the scarce thing is a number a developer chose. Here the scarce thing is your Tuesday afternoon and the fact that you took the long way home.

So Everyday, Uncommon, Rare and Legendary only mean anything if the input is honest. Liveness is not a security feature bolted onto the side of the game. It is the game. A map that fills up with cats nobody ever met is not a map of anything.

Being strict costs real cats

Here is the honest bill. A detector tuned to refuse screens will sometimes refuse a genuine cat, because the conditions that make a real animal look flat and evenly lit are ordinary outdoor conditions. This is what tends to trip it, and what usually fixes it.

I would rather refuse fifty honest cats than accept one screenshot, and I say that knowing precisely how irritating those fifty are. A refused cat costs you a retake, or at worst one cat you did not get. An accepted screenshot costs everyone the meaning of the album, permanently, and quietly, and there is no way to take it back afterwards.

One rule outranks getting the shot. Never make the cat's day worse. Do not corner it, do not follow it, do not block the gap it would leave through, do not reach for it, and never rattle food to draw a cat across a road or onto a driveway. Do not step onto someone's property for a card. Crouch, stay still, blink slowly, and let the cat decide. A cat that walks off has given you its answer, and the ones you leave alone are the ones that are still there next week.

Why there is no gallery import

This is the most requested feature, by a distance, and the answer is not going to change. The gallery cannot tell me the thing I need to know.

• A screenshot saved to your camera roll and a photo you took are the same kind of file. Nothing in the format separates them.
• Half of what the check relies on only exists at the moment of capture: several frames rather than one, how the lens hunts for focus, how the scene sits at different distances. A saved image is one flattened frame with all of that thrown away.
• File metadata is not evidence. A date and a location in a header can be edited in seconds, and messaging apps strip them out anyway.
• The catch is something you do while you are there. You hold out a snack can, the cat creeps towards it, and you let go the instant its nose touches. That is a moment, not a file operation.
• Because there is no import, the app has no reason to go through your camera roll at all. It needs the camera while you are using it, and nothing else.

What an honest card feels like later

I opened the album a few weeks ago to chase a layout bug and stopped looking at the layout. There was the tabby that sits on the low wall outside the chip shop, caught on an evening when it had just stopped raining and everything was orange. There was a thin black cat from a car park in a town I was only in for one afternoon. I remember the hill. I remember that I nearly did not bother.

That is not sentimentality about a picture. It is the difference between a collection and a download folder. And it only holds together because the rule underneath it never bends. Paying changes nothing about your odds or your rarities. Timing, walking and a confident scan are the only things that decide a catch, and the yarn ball a catch leaves behind unwinds as you walk, roughly two thousand steps for a small one and eight thousand for a great big one, measured as GPS distance while the app is open. If the photograph at the start of that chain can be faked, none of the rest of it is worth anything.

If it refuses a cat you are actually standing in front of

Then that one is on me, not on you. Change the light rather than the distance, get lower without looming, and give it one more go. If it still says no, leave the cat in peace and tell us what the scene was like. I would much rather keep fixing the detector than loosen it, because a detector that lets a laptop through has not been made more generous. It has just been turned off.

PoCats is free on iPhone and Android. There is no advertising of any kind in it, no loot boxes, and an optional membership called PoCats Club that does nothing whatsoever to your odds. The one thing it will not do, ever, is take a photograph of a photograph and stamp it "Purrfect!".

Questions people ask

Free tools for your next cat encounter

Make a keepsake with the cat card maker, using a photo you own. For your next walk, the body language decoder reads a cat's tail, ears and eyes back to you, and the street cat name generator names the one that has been sitting on your wall all summer.

Where this fits in PoCats

PoCats is a cat spotting game: you photograph a real cat you meet, it is checked as a living animal, and it becomes a collectible card cut from your own picture. The catch itself is a two-second timing moment you can try in your browser, and the whole loop is explained here.