Concepts Of Constraints Affordances And Mapping Computer Science Essay


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Norman describes that the physical constraints limit possible operations as they are operated based on properties of physical world like size, shape etc and no special training is required to perform any action, if there are physical constraints imposed on a device. These are more effective and useful to the users if they can easily see and interpret to perform the desired function. Semantic constraints are based on the meaning of the situation to limit the number of possible actions and are dependent on the user's knowledge of the situation and of the world, which could become a powerful and important clue. Cultural constraints are based on a set of accepted cultural conventions for social situations. This knowledge is acquired by schemas and knowledge structures having general rules and information to interpret situations. Logical constraints use some logic to limit the actions in terms of Natural Mappings (left-right, up-down), Sequence (actions performed in obvious order) and Completeness (using all parts, filling all fields). A good example of building Lego motorcycle is illustrated to explain these concepts.

The author clearly explains the concepts of constraints, affordances and mapping by applying them to the design of real world objects like doors and switches. The regular designs of them confuse the users because of the lack of enough clues and visible signals that prompt for the right operation. He says that if a simple design needs instructions or pictures, it is likely a bad design. The design should effectively solve the mapping and grouping problems that occur.

Two more important principles for knowing what to do in situations are Visibility and Feedback. Relevant parts like system states and controls should be made visible using displays and/or sounds for better designs like doors must have handles to operate. If the capabilities are visible on the design, it does not need memory on how to use and if the functionalities are hidden, problems occur. The design and usability of an object is greatly improved by making the invisible parts of it clearly visible.

The instant information of any system related to the performance or the validation of the projected results can be achieved through a feedback mechanism. It is essential factor that verifies if the task is successfully completed like good visual display for visual feedback. Immediate response and obvious effect for each action signals the user on what is taking place. For instance it could be a sound that's made or could be a change in a physical state. For those things that can't be made visible, sounds serve as the best feedback. They even grab the user's attention to convey the information, which is indeed useful but deficit at times. Often silence may raise problems when sound feedback is expected. If there is no proper visibility or feedback for any interactive device, there could be chances of gulfs both in execution and evaluation between the user and the intended action. However, for a good conceptual model and for a better device, a designer should effectively incorporate all these concepts in the design.

Part 2 - Description of various concepts with examples


Constraints limits to the apparent operation of any piece of equipment. It suggests the accurate actions so that the things should fall into right place. On a daily basis constraints can be classified as physical, semantic, cultural, and logical.

Physical constraints are the physical limitations that restrict possible events. There is no particular guidance required and the value of physical constraints depends on the properties of physical world.

Semantic constraints are the limitations based on the meaning of the situation which controls the set of feasible actions. Fundamentally these constraints depend on our knowledge of the circumstances and of the physical world.

Cultural constraints come into picture when we deal with new devices and these constraints are the basis for many problems which depends on accepted cultural conventions.

Logical constraints are nothing but using logic to constrain actions like natural mappings, to follow a sequence by placing or doing things in a correct order, and also using all the given parts in order to achieve the completeness.


Positive Example:

Inserting a sim card into a mobile device is a good example for constraints, as the shape of the sim card exactly fits into the designed shape of the sim tray in the mobile phone.

In this example, constraints are clearly used to limit the physical space available to fit specific shape of sim card. It suggests the accurate action needed so that the sim card fits in the right place. This uses the physical constraints and semantic constraints where it depends on the physical limitations of the size and the knowledge of the world about cell phones. The interaction between the user and the action (inserting the sim card) in a mobile phone can be improved by the physical constraints as the shape and size present on the device are exactly similar to the shape and size of the sim card, which prevents user from making errors.

Negative Example:

Iphone is a good example where constraints are not effectively used and there are possibilities of occurring errors.

While talking to a person on iPhone, the sensitive touch feature of it either ends up the call abruptly or the mute/ speaker/ hold buttons gets clicked automatically without the user's notice, as the keypad cannot be locked while the user is on call. Even if the user presses the keypad lock on the top, the call ends up. And if a phone is handed to the other person while on call, and if the user unknowingly touches the screen, the call might get ended or any action on the screen could take place. It would be difficult for a completely naïve user to talk on an iPhone. On the other hand, the user can lock the keypad and talk on the phone without any issues, if he uses the head phones. This obstructs the interaction of the user with the iPhone to perform his/her desired task.

In order to constrain the touch feature while on call, the iPhone should have some feature of locking the keypad which doesn't end up the call.


Correct parts must be visible and they must convey the correct message to indicate what parts operate and how, so that the user can interact easily with the device. It indicates the mapping between intended actions and actual operations.

It is very obvious that when the facilities are visible, anyone can know how to implement things without the need of any sort of memory. In other words if the right parts which also conveys correct information to operate a machine are visible, a user can easily operate it without any extra memory or additional guidance. Here comes the concept of natural signals which is the common understanding of objects and their observable use and natural design that takes advantage of these signals.


Positive Example:

All the correct parts for the operation of car before start are clearly visible on the key and they convey the correct information using pictures to indicate the respective operation. Both the above car remotes are good examples for visibility, as they easily make the user understand which button is for which control/operation. The lock symbol or 'LOCK' label easily lets him understand that it is used to lock the doors of the car and similarly the remaining buttons also convey its intended operation to the user. Also the user needn't use his memory to memorize the functions on it. It maps the planned action and the actual operation of unlock/lock/opening trunk.

Negative Example:

(Front view of toy)


This is one of the examples for a child's toy. The toy says things like "put the red block in my hand" or "put the block with the number 8 in my hand" (see photo) and gives appropriate encouragement or response (a feedback) when the child does it.

Generally we bring it home, open the toy and start to play with it as it appears or suggests. But, when we took a block out of the little backpack on the toy's back and placed it in the toy's hand, we could not find the response of the toy. We would definitely think that it is broken or a damaged piece.

After some study, we figured out the small little tag near the toy's tail, which was very small and said to pull it for the toy to act in a play mode. This function of the toy will be ignored as it not visible and the interaction of the users with the device will be hindered due to lack of visibility.

Example 2:

In this example, the correct parts of the above refrigerator are not placed or visible at the correct location of the operation. It has handles on both the sides and lacks visibility as it does not convey the user about which side to use or to open the door. It seems confusing to the user. Whenever the user wants to open it, he has to recollect from his memory when he last used it on how to operate it. This design hinders its observable use and natural design that can be prone to make errors. It would be more visible and good design if the handles are placed on the front of the doors on one side.


Feedback mechanism is the immediate and obvious effect of the information to the user about the result of a process or an activity. It also gives a clear picture to the user like what action has been done and what result was accomplished. It may be in terms of sound or visual feedback. It is also a prominent concept in control system engineering and information theory. Feedback has many interesting properties that can be developed in designing various systems and it always behaves in a counterintuitive manner.


Positive Example:

The rice cooker gives good feedback to a user by switching on the 'warm' light present on it, when the rice/vegetables gets cooked and by switching on the 'cook' light when the rice is getting cooked. It gives an immediate response using a either a light signal or sound signal as feedback, making sound of 'click' when the button is pressed automatically when the job is finished. This clarifies that the operation has been completed. The user need not manually lift up the lid to check if the rice is cooked. The LEDs present on it automatically tells the user about its function. This is a good example of feedback that improves user's interaction with the functionality of the device.

Example 2:

Another good example of feedback is the smoke detector. This is a kind of feedback in the form of sound that a person can hear immediately once the device responds. When there is a fire accident or smoke in a room or house or a building, the alarm of the detector rings by itself and also a red light switches on automatically that intimates the fire station. This clearly gives immediate and obvious effect through sound signal intimating the user to respond quickly.

Negative Example:

Even now, this old iron box is being used in many rural places, which is run by charcoal. It does not have any light or sound indicator when the job is finished. Or in other words, there is no feedback signal from this device that tells the user about the completed job. One should understand its mechanism and be able to judge if the product is heated enough. There is no obvious result of this process that indicates the iron box is over heated. A sound or light signal is expected in this situation and this silence could cause dangerous fire problems. Due to this poor feedback, the user will not be able to perform his action correctly and the desired task of the user is not fulfilled, if this leads to some unexpected situations that hinder interaction.

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