A processing program is known as a sketch. A sketch contains 1 or more commands. Commands are instructions for the computer to execute.  A command is followed by round brackets ‘()’ and ends with a semi colon ‘;’ . The above sketch contains 3 commands

1. createCanvas()  - tells the computer to set the size of the canvas
2. background() - tells the computer to fill canvas with a color
3. rect()  -  tells the computer to draw a rectangle

Commands may have to be passed extra values between the round brackets known as parameters separated by a comma

For example the createCanvas() command is passed 2 parameters : the width – 600  and height – 400 of the canvas.

Note:  Commands have to be grouped inside the setup() function between the curly braces ‘{‘ and ‘}’.  For now we will code only inside the setup function. Delete the draw() function inserted by the P5JS editor.

Type the above code (sketch 1) in your Processing editor.

Run the code by clicking play.   Alternately you can also press Ctrl + Enter  on the keyboard

You can stop the sketch by clicking Stop

Go ahead change the values of the parameters  and see what happens. Change the width and height of the canvas by passing different values to the createCanvas() command

Note : Processing is a case sensitive language. This means capital ‘A ‘ (upper case) is not the same as small ‘a’ (lower case).

Comments

Your code can also contain comments. Comments are preceded with 2 slashes. In the sketch below, lines 2, 4 and 6 are comments. Comments are ignored by the computer when the sketch is executed by the computer. Programmers use comments to describe their code.

                        Sketch 1

Login to P5JS

If you want to save your sketches you need to Login to P5JS. Click the Log In link at the Top Right and then click login with google to sign in to P5JS

Rename your sketch

Click on the pencil next to the name of the sketch to rename the sketch. Go ahead rename your sketch to ‘day01_sk01’

Saving your Sketch

To save your sketch open the ‘File’ menu and click ‘Save’. Alternately you can press Ctrl + S  on the keyboard.

Opening your saved sketch

You can open your sketch by clicking  ‘Open’ from the File menu.

Making a copy of your sketch

You can make a copy of your saved sketch by clicking ‘Duplicate’ from the File menu.

The processing coordinate system

The canvas is made of tiny squares (invisible to the naked eye) called pixels . Every pixel has a horizontal position (on the X axis) and a vertical position (on the Y axis). See figure below. We can refer to a point on the canvas by specifying the X value and the Y value of the pixel.  Below is a magnified image of a canvas that is 20 pixels wide and 20 pixels tall.

Drawing a line

We can draw a line with the line()  command. A line is between 2 points (see figure below).

1. The point where the line starts – point A

2. The point where the line ends – point B

 So we have 1 set of X and Y values for point A and another set of X and Y values for point B

Therefore we have to pass 4 parameters to the line() command:-     line(x1,y1,x2,y2)

x1,y1 specifies the location of point A and x2,y2 specifies the location of point B

Go ahead type the below code in the processing editor and run the sketch.

                        Sketch 2 draw a line

Create a sketch that will draw 2 lines as shown in the picture below..

Line 1 – from top left corner to the centre of the canvas

Line 2 – from the top right corner to the centre of the canvas

Hint:

How to draw line 1 ? Find the position (x,y) of the start point of the line (the top left corner) – 0,0. Find the position of the end point of the line (the centre of the canvas).  The horizontal (x) centre is at the halfway point which is half the width of the canvas i.e. 600 / 2 = 300. The vertical centre (y) will be half the height i.e. 400/2 = 200.

Therefore location of the start point is 0,0 and the end point is 300,200

Draw line 1 by typing the following command

line(0,0,300,200);

In the same way find the start point and end point of line 2 and use the line command to draw line 2.

                        Sketch draw lines

Drawing a Rectangle

We can draw a rectangle from with the rect()  command which requires 4 parameters :-   rect(x,y,width, height);

x and y specifies the position of the top left corner of the rectangle (See figure below) because processing draws rectangles from their top left corner

                        Sketch 3 draw rectangle

You will notice that the top left corner of the rectangle is in the centre (x=300,y=200) of the canvas.

The rectMode(CENTER)  command can be used to tell the computer to change the drawing mode for rectangles to the centre of the rectangle. Then x and y will specify the centre of the rectangle. See figure below.

If you run the sketch below, the rectangle will appear exactly in the centre of the canvas.

                        Sketch 4 draw rectangle in the centre of the canvas

Note : The ‘M’ in rectMode () is upper case (Capital). Also ‘CENTER’ is upper case.

Drawing circles

We can use the ellipse()  command to draw circles. The ellipse()  command takes the same parameters as the rect()  command :- ellipse(x,y,width,height);

Note the default drawing mode for circles is centre. This means the x and y values will specify the centre of the circle.

Go ahead try out the ellipse command.

                        Sketch 5

Create a sketch to display a cart on the canvas as shown in the figure below

Hint: The left wheel (circle) is 60 pixels left of the centre

The right wheel (circle) is 60 pixels right of the centre

The width of the 2 wheels is 50 pixels

The body of the cart (rectangle)  is 25 pixels above the centre

The width of the cart body is 200 pixels and the height is 20 pixels

                        Sketch 6 draw cart

See what will happen if we draw the body before the wheels

Question

In the above example - What will be the X, Y  coordinates of the bottom left corner of the rectangle which is the cart body? What will be the  X, Y coordinates of the top right corner of the rectangle? See figure :-

Hint: The left and right of the rectangle are half the width away from the rectangle’s centre

The top and bottom are half the height away from the rectangle’s centre

Ans : rectangle’s width = 200 , half of 200 = 100

rectangle’s height = 20,  half the height = 10

Centre of rectangle = 300,175

Bottom Left corner = 300-100, 175+10  = 200,185

Top Right corner = 300+100,175-10 = 400,165

Practice assignments

1. Sketch an alien as shown in the figure below.

Hint:

Draw a rectangle for the body

Draw a big circle for the face

Draw 2 small circles for the eyes

Draw 2 lines for the leg

Hint: draw the rectangle from the centre (use the rectMode() command to set the drawing mode for rectangles)

To know where to draw the face you will have to calculate the position of the top of the rectangle which forms the body

To know where to draw the left leg you will have to calculate the position of the bottom left corner of the rectangle

To know where to draw the right leg you will have to calculate the position of the bottom right corner of the rectangle

Recap

• createCanvas (width, height);    // sets the width and height of the canvas (first command in a sketch)
• X coordinate value specifies the horizontal position on the canvas
• Y coordinate value specifies the vertical position on the canvas
• line (x1, y1, x2, y2);   // draws a line
• rect (x, y, width, height);   // draws a rectangle
• rectMode (CENTER);   // sets the drawing mode for rectangles to centre
• ellipse (x, y, width, height);   // draws a circle