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Alpha value - Transparency

When we want to determine the alpha value of a p5.js element, we define a fourth value for the color.

The smaller the alpha value the more transparent the element. 

Example 

Here is an example code of two ellipses with different alpha values :

function setup() {
  // create canvas
  createCanvas(800,500);
  // set background color
  background('#B2B2B2');
}
function draw () {
  //this ellipse will have a low transparency value
  fill (175,68,39,50);
  ellipse(width/2,height/2,100);
  // this ellipse will have a high transparency value
  fill (124,156,39,5);
  ellipse(450,height/2,100);
}

Result of the above code:

Exercise

  1. Open your Visual Studio editor and the p5yourName folder.
  2. Open the file ex812.js in your editor and save it as ex833.js
  3. Open the file ex812.html in your editor and save it as ex833.html
  4. In the ex833.html file, update the link to ex833.js  from exersice812.js
  5. Go to the index.html file and create, under Module 8, a link to the ex833.html  file with the title "Alpha value".

Modify the ex833.jsfile to create an ellipse which will move horizontally from the left to the right end of the canvas and will change its color and alpha value according to the direction it has. You can see here an example.

Answer:

// create a variable tp control the direction of the ellipse
let control = false;
// make ellipse starting from the left end of the canvas
let i=30;
function setup() {
  // create canvas
  createCanvas(800,500);
  // set background color
  background('#31bc33');
}
function draw () {
  // when ellipse is in the left edge, control would be/become false
  if (i==30) {
    control = false;
  }
  // if ellipse is inside the canvas width (we substract 30 which is the radius
  // of our ellipse) and control is false
  if (i<(width-30) && control==false){
    // state fill and alpha of the ellipse
    fill(146,87,67,40);
    //state stroke color and transparency
    stroke(234,65,88,5);
    i++
    // increase its x position (ellipse moves in the right direction)
    ellipse(i,height/2,60);
  }
  else {
    // else control will change to true
    control = true;
    //change the color and fill of the ellipse
    // state fill and alpha of the ellipse
    fill(56,57,67,100);
    //state stroke color and transparency
    stroke(5,165,188,50);
    // decrease its x position (ellipse moves in the left direction)
    i--;
    ellipse(i,height/2,60);
  }
}

Do a Git commit with the message "Alpha value".

  • See more about the alpha() function
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