An Introduction To Programming With Processing

Friction simulation

Friction is the force that resists the relative motion of something else (like an object, a gas, a liquid, etc), and is the effect that we will be attempting to simulate (in a simplified form) in this sketch. By moving the mouse up and down within the Display Window the image should try to follow the mouse and slow down as it approaches the mouse, this will create the impression of friction. We'll start by creating three new variables. Add the following statements at the top of the sketch, below the PImage statement:
float yPos;       /*declaration related to image Y position*/
float difY;         /*declaration related to difference between mouse and image's Y position*/
int frict = 30;    /*initialization, friction. high values make image move slower*/
The float keyword is used for declaring floating point variables. Floating point values are numbers that have a fractional part, that is to say they are numbers with a decimal value such as 1.0, -2.9865, 10000.423, 0.0000001 and so on. Floating point numbers will tend to require more memory for storage so if you can get away with representing the number data as type int and not compromising the integrity of your program you should consider doing so. We are using the float datatype in this program because we will be using division in one of our expressions. This is important because true division will always return a number with a decimal value, if you were to use an int instead of a float or double depending on the programming language you are using, you risk having datatype conversion errors or truncated integers returned. As a result when you use division in your expressions always consider whether representing that data with an int is a good idea or not. The three variables yPos, difY and frict are going to be used to represent three different values of which only one will remain constant throughout the duration that the program is running. The only variable that will remain constant is the frict variable. As a result the frict variable has been initialized through a declaration and assignment statement outside of both setup() and draw() structures. This value will control how fast or slow we would like the image to move towards the mouse. Creating a variable such as this within the global variable scope, will give us access to the variable within both setup() and draw() thereby allowing us to tweak the value associated with the variable and have it update throughout the program. This will save us some unnecessary work in the long run. When the sketch is complete setting the frict value high will cause the image to appear to have more resistance when moving towards the position of the mouse ultimately making the image move slower, and setting this value low (but not to zero, because division by zero is not permitted in mathematics, as is the case in programming) will cause the image to appear to have no resistance and move almost immediately to the Y position of the mouse. The variable yPos will be used to store the Y position of the image and also update the Y position of the image, and the variable difY will be used to store the difference between the mouse's Y position and the center of the image. Both of these variables will need to be updated regularly for each of the cycles of the draw() function, this is because each time the user moves the mouse the position of the image must change as a result of difY now being less or more depending on whether the mouse is closer or further from the images center.

Variable naming conventions

Before proceeding, it's worth noting a convention used for naming the variables in this program. There are several rules (depending on what language you are programming in) that determine what a valid variable name is. Generally, you'll find that many popular higher level languages share these similarities regarding valid variable declaration:

  • 1. Variable names can only contain alphanumeric characters and underscores (that is “_”, “a” to “z”, “A” to “Z” and/or “0” to “9”). Spaces cannot be used in variable names.
  • 2. Variable names cannot start with numbers.
  • 3. Variable names should be descriptive but not verbose, for example “yPos” as opposed to “Y_Position_Of_The_Image”.
  • 4. Use consistent casing. Variable names generally do not start with an upper-case character. This convention is reserved for Classes. In the previous example I've used a mixture of both upper-case and lower-case characters to name my variables such as yPos which is intended to be an abbreviation for “y Position” or difY which is an abbreviation for “the difference in Y positions”. Some programmers might use different conventions including underscores because to them this might look more readable y_pos or dif_y. Either of these conventions is perfectly valid. But you should determine which convention you are using, and stick with it throughout your program. Changing from one naming convention to another can often make a program confusing and less readable.
  • 5. Your variable names should be self-documenting, that is to say they should be descriptive about their purpose without having to include additional comments explaining their purpose. This point is with the exception of intentionally, explicitly documented code, which is generally a convention used for teaching programming.
  • 6. Variables that are typed in all upper-case characters are often referred to as constants. Their value is not supposed to change. This is simply a naming convention and there is nothing stopping you from changing the value of a constant in many higher level languages, although this is not recommended. So beware when using this naming convention as it could lead to confusion if not used properly.