An interactive example of the canvas quadraticCurveTo() function

Written by Richard Heyes, on 29th January 2013

An interactive example of the HTML5 canvas quadraticCurveTo() function. This will help you to understand the function by showing you exactly what it's doing and the resulting code.

[No canvas support]

This is a demonstration of how to use the quadraticCurveTo() function of the canvas 2D API. A quadratic curve is a type of curve which is available to you via the function built into the canvas API. The example here should help you to understand them because the control points (the coordinates that you give to the quadraticCurveTo() function) are illustrated and you can drag them around with your mouse - showing the effect on the curve. The resulting code is shown below.

The output

This is the code that can be used to achieve the curve (the points are color coded to match the output above):

The arguments to the function

The quadraticCurveTo() function takes four arguments - ie two sets of X/Y coordinates. The curve is actually made up of three points - but the first point is the current point in your path. If you're starting a new path with the quadraticCurveTo() function then you need to use the moveTo function to move to the start point.

• Start point
  This is the moveTo() call in the example above. The quadratic curve starts from this point.
  
  
• Control point
  This is the first set of coordinates that you give to the quadraticCurveTo() function. This control point dictates the shape of the curve.
  
  
• End point
  This is the second set of coordinates given to the quadraticCurveTo() function. This is where the curve ends.