Spot the Difference

How to play spot the difference

1. A level is one painted scene shown twice, with a small number of things changed in the second picture. The twenty four levels carry five, six or seven changes, and a counter shows how many you have found against how many the level holds.
2. Tap a change in either picture. A tap counts when it lands within the changed object's reach, which is twenty six pixels or fifty five percent of the width the object is drawn at, whichever is larger, so a big thing is no harder to hit than a small one. When two unfound changes are both in reach, the nearer one is the one you get.
3. A found change is marked in both pictures at once. A thing that moved counts as one change with two homes, the one it left and the one it took, and tapping either of them finds it.
4. A tap that reaches nothing costs you nothing at all. No heart, no star, no failed level: the only thing this game ever submits is a finished picture, so exploring is free.
5. Miss six times on one pair of pictures and the game marks a change for you at no cost. That happens once per pair, and it takes the first change you have not found.
6. Find every change and the level is solved. The solve pays its stars, counts towards the level you are on, and a Next button carries you to the following pair.

The two ways to play

What to do when you are stuck

• Work the cast, not the picture. Every difference is one of the painted objects the level names, and a level names ten to twelve of them while each picture draws nine to eleven, so taking the objects one at a time and checking each against the other picture covers the whole puzzle with nothing left over.
• Count the kinds. The engine knows six: something gone, something that exists in the second picture only, something moved, something facing the other way, something a different size and something a different colour. Eleven levels use five of those kinds once each, twelve use six once each, and one level carries seven changes with size used twice. So on twenty three of the twenty four levels a kind you have already found will not turn up again, and the kinds still missing tell you what to hunt for.
• Look at colour and at outline separately. A recoloured object keeps its place, its width, its rotation and its shadow, and only its hue and saturation move, while a mirrored or resized one keeps its colour untouched. One pass that compares colours and one that compares shapes each catch what the other walks past.
• If you are sure something changed and the tap did nothing, tap again nearer the middle of the object. The tap is measured from the object's own authored centre, and for a moved object from both of its centres, with a reach of twenty six pixels or fifty five percent of the drawn width, whichever is larger. Misses are not wasted either: the sixth one on a pair earns you a free mark.

Controls

• A pointer press on either picture is the only input this mechanic binds. It adds no key handler, nothing to drag and nothing to hold down, and the press is compared against the authored marks of the changes rather than against whatever sprite the pointer is over.
• Each picture is marked as an image and announces the name of the scene and which of the two it is, so a screen reader reads picture one and picture two by name.
• Hint is the one power-up this mechanic declares, so the shell builds that button and no other. It marks the first change you have not found, costs thirty stars, spends a saved hint token before it spends stars, and hands back exactly what you spent when there is nothing left to mark.
• The Next control that appears after a solve and the Back control labelled Back to the map are both real buttons, built with an explicit button type.
• The card's own answer form is hidden for the whole round. The mechanic fills it and submits it for you the moment the last change is found, and hides it again straight away.

Questions about spot the difference

What do stars do here, and is there anything to buy?

Stars pay for hints, and there is nothing to buy: the arcade this wing sits in is an ads-only economy with no in-app purchases in it. Solving a pair for the first time pays a hundred stars for each point of its difficulty, and the levels run from one point to six, so a pair pays between a hundred and six hundred. Meeting a numbered level's target pays a chest on top, the bonus card pays fifty, and a first daily solve of the day pays the level's own value again. A hint costs thirty. The balance itself is filed under a storage key with no page in it, so it is one balance rather than one per page, and a browser holding no balance yet is started at a hundred stars.

Is there a timer, and can I fail a level?

Neither. Nothing counts down and nothing measures how long you look: the whole mechanic holds two waits, a 260 millisecond pause between your last find and the solve being submitted, and the 500 millisecond life of the mark a miss leaves behind. A tap that finds nothing is never submitted as a wrong answer. The arcade keeps five hearts that refill one every thirty minutes, and a heart is only taken when a wrong answer is graded wrong, which this mechanic never sends, so the pictures cannot cost you one.

Can I get stuck on a pair?

There are two ways out and both are inside the level. Miss six times on the pair and the game marks a change for you for free, once per pair. Hint marks one for thirty stars and can be used again for as long as changes are left. Both take the first change you have not found in the level's authored order, so between them they can walk the whole list, and Hint refunds what you spent if you press it when there is nothing left to mark.

Is anything remembered when I come back?

Yes, in this browser. The numbered level you are on, how many solves you have inside it, your hearts and the answers you have solved are filed under a key that includes this page's own address, and the play layer files its per card record under a key that includes the page too. The star balance and the daily record are filed under keys that carry no page, so those two are single records rather than per page ones. A pair you leave part way through is not saved at all: what you have found, how many times you have missed and whether the free mark was used are held in memory for the life of the card, and only a finished pair is ever written down.