Curves on HTML canvas can be drawn using arcs, but drawing a complex diagram using arcs is quite a tedious task. In the given circumstance, Bezier curve will be very useful in providing more flexibility in drawing curves. Bezier curves on HTML canvas are drawn using a start point, one or more control point/points and an endpoint. Example: In the case of drawing a landscape, real-world objects, irregular shapes etc. Bezier Curves can be drawn in two ways:
- Quadratic Bezier Curve
- Cubic Bezier Curve
Quadratic Bezier Curve: This curve is controlled by one control point. Syntax:
moveTo(start_pnt_X, start_pnt_Y); context.quadraticCurveTo(cntrl_pnt_X, cntrl_pnt_Y, end_pnt_X, end_pnt_y);
Example: This example create a curve using quadratic bezier curve.
html
<!DOCTYPE html> <html> <head> <title> Quadratic Bezier Curve </title> </head> <body> <h1>Quadratic Bezier Curve</h1> <canvas id="CanvasOfGeeks" width="400" height="200" style="border:solid 4px green"> <script> var c = document.getElementById("CanvasOfGeeks"); var context = c.getContext("2d"); var start_pnt_X = 50; var start_pnt_Y = 150; var cntrl_pnt_X = 200; var cntrl_pnt_Y = 30; var end_pnt_X = 350; var end_pnt_Y = 150; /* Start new path */ context.beginPath(); context.lineWidth=3; context.strokeText( ".", cntrl_pnt_X, cntrl_pnt_Y); /* Starting point of the curve */ context.moveTo(start_pnt_X, start_pnt_Y); context.quadraticCurveTo(cntrl_pnt_X, cntrl_pnt_Y, end_pnt_X, end_pnt_Y); /* drawing line on the canvas */ context.stroke(); </script> </body> </html> |
Output: 
Explanation:
- Pre-requisite: HTML Canvas Basics
- First Line: Reference for canvas object is stored in variable ‘c’ using DOM concept. Second Line: Without having drawing context of canvas nothing can be drawn on it.
var c = document.getElementById("CanvasOfGeeks");
var context = c.getContext("2d");
- One can change the width of line by overriding the value of “lineWidth” attribute of context object.
context.lineWidth=3;
- For putting a dot over the coordinate of control point.You can see the dot in the figure shown above.
context.strokeText( ".", cntrl_pnt_X, cntrl_pnt_Y);
- This function is used to draw a curve from the start point mentioned in function.
context.quadraticCurveTo(cntrl_pnt_X, cntrl_pnt_Y, end_pnt_X, end_pnt_Y);
- This function is used to move the context.
context.moveTo(start_pnt_X, start_pnt_Y);
Note: Please keep the control point within the canvas boundary. Cubic Bezier Curve: This curve is controlled by two control points. Syntax:
moveTo(start_pnt_X, start_pnt_Y);
context.bezierCurveTo(cntrl_pnt_1_X, cntrl_pnt_1_Y, cntrl_pnt_2_X,
cntrl_pnt_2_Y, end_pnt_X, end_pnt_y);
Example: This example create a curve using cubic bezier curve.
html
<!DOCTYPE html> <html> <head> <title> Cubic Bezier Curve </title> </head> <body> <h1>Cubic Bezier Curve</h1> <canvas id="CanvasOfGeeks" width="400" height="200" style="border:solid 4px green"> <script> var c = document.getElementById("CanvasOfGeeks"); var context = c.getContext("2d"); var start_pnt_X = 50; var start_pnt_Y = 100; var cntrl_pnt_1_X = 150; var cntrl_pnt_1_Y = 30; var cntrl_pnt_2_X = 250; var cntrl_pnt_2_Y = 170; var end_pnt_X = 350; var end_pnt_Y = 150; /* Start a new Path */ context.beginPath(); context.lineWidth=3; /* Representing first control point */ context.strokeText( ".", cntrl_pnt_1_X, cntrl_pnt_1_Y); /* Representing second control point */ context.strokeText( ".", cntrl_pnt_2_X, cntrl_pnt_2_Y); /* Starting point of the curve */ context.moveTo(start_pnt_X, start_pnt_Y); context.bezierCurveTo(cntrl_pnt_1_X, cntrl_pnt_1_Y, cntrl_pnt_2_X, cntrl_pnt_2_Y, end_pnt_X, end_pnt_Y); /* Drawing line on the canvas */ context.stroke(); </script> </body> </html> |
Output: 
Example: This example draw a fish using Bezier Curve. Input:
html
<!DOCTYPE html> <html> <head> <title> Drawing a fish using Bezier Curve </title> </head> <body> <canvas id="CanvasOfGeeks" width="400" height="200" style="border:solid 4px green"> <script> var c = document.getElementById("CanvasOfGeeks"); var context = c.getContext("2d"); /* Start a new Path */ context.beginPath(); context.lineWidth=3; /* Upper curve of the fish, from mouth to tail */ context.moveTo(60, 120); context.bezierCurveTo(90, 30, 200, 130, 310, 55); /* Lower curve of the fish, from mouth to tail */ context.moveTo(60, 120); context.bezierCurveTo(90, 170, 200, 110, 310, 160); /* Upper half of tail */ context.moveTo(310, 55); context.quadraticCurveTo(320, 80, 280, 110); /* lower half of tail */ context.moveTo(310, 160); context.quadraticCurveTo(320, 120, 280, 110); /* Eye of the fish */ context.moveTo(100, 100); context.arc(100, 100, 5, 0, 2*Math.PI); /* Mouth of the fish */ context.moveTo(60, 120); context.lineTo(80, 120); context.stroke(); </script> </body> </html> |
Output: 
