Html 在画布中绘制正弦波

drawing sine wave in canvas

提问by BRDroid

I am trying to draw a simple sine wave in a canvas but i am not getting it right. this is my desired output as in the picture.

我试图在画布上绘制一个简单的正弦波，但我没有做对。这是我想要的输出，如图所示。

What I have got so far is http://jsfiddle.net/RaoBurugula/gmhg61s6/4/

到目前为止我得到的是http://jsfiddle.net/RaoBurugula/gmhg61s6/4/

HTML

HTML

 <canvas id="myCanvas" width="360" height="360" style="border:1px solid #d3d3d3;">

JS

JS

 var c = document.getElementById("myCanvas");
 var ctx = c.getContext("2d");
 var i;
 for(i=0; i<360; i+= 20){
    ctx.moveTo(i+5,180);
    ctx.lineTo(i,180);

 }
 ctx.stroke();
 var counter = 0, x=0,y=180;

 //100 iterations
 var increase = 90/180*Math.PI ;
 for(i=0; i<=180; i+=10){

 ctx.moveTo(x,y);
 x = i;
y=  180 - Math.sin(counter);
counter += increase;

ctx.lineTo(x,y);
alert( " x : " + x + " y : " + y) ;
}
ctx.stroke();

My desired output

我想要的输出

采纳答案by Guffa

You are increasing counterwith a value that it too high, make it smaller:

您正在增加counter一个太高的值，让它变小：

var increase = 90/180*Math.PI / 9;

Draw the whole width of the diagram instead of half:

绘制图表的整个宽度而不是一半：

for(i=0; i<=360; i+=10){

You need a higher amplitude:

您需要更高的振幅：

y =  180 - Math.sin(counter) * 120;

Demo: http://jsfiddle.net/Guffa/gmhg61s6/5/

演示：http: //jsfiddle.net/Guffa/gmhg61s6/5/

回答by Sanxofon

Old question but of very universal interest.

老问题，但非常普遍的兴趣。

All answers here are very usefull but none has well commented code, so I've created a snippet that explains what is happening at every point and draws many variables for clarity. Thanks everyone for helping me understand, I hope this helps some one else.

这里的所有答案都非常有用，但没有一个有很好的代码注释，所以我创建了一个片段来解释每个点发生的事情并为了清晰起见绘制了许多变量。感谢大家帮助我理解，我希望这对其他人有所帮助。

var c = document.getElementById("canvas"); // Grab canvas object
var ctx = c.getContext("2d"); // Define canvas context
var w=c.width; // Canvas width => Frequency is relative to this
var h=c.height/2; // Canvas height over two => Amplitude: Volume
var f=1; // How many cycles per canvas width => Frequency: Tone & Speed
// Calculates y position from x
function calcSineY(x) {
 // This is the meat (unles you are vegan)
  // Note that:
  // h is the amplitude of the wave
  // x is the current x value we get every time interval
  // 2 * PI is the length of one cycle (full circumference)
  // f/w is the frequency fraction
 return h - h * Math.sin( x * 2 * Math.PI * (f/w) );
}
function drawSine(x){
 ctx.clearRect(0, 0, w, h*2);
  //draw x axis
  ctx.beginPath(); // Draw a new path
  ctx.strokeStyle = "green"; // Pick a color
  ctx.moveTo(0,h); // Where to start drawing
  ctx.lineTo(w,h); // Where to draw to
  ctx.stroke(); // Draw
  
  // draw horizontal line of current amplitude
  ctx.beginPath(); // Draw a new path
  ctx.moveTo(0,h); // Where to start drawing
  ctx.strokeStyle = "gray"; // Pick a color
  for(var i=0;i<x;i++){ // Loop from left side to current x
    var y = calcSineY(x); // Calculate y value from x
   ctx.moveTo(i,y); // Where to start drawing
    ctx.lineTo(x,y); // Where to draw to
  }
  ctx.stroke(); // Draw
  
  // draw amplitude bar at current point
  ctx.beginPath(); // Draw a new path
  ctx.strokeStyle = "red"; // Pick a color
  for(var i=0;i<x;i++){ // Loop from left side to current x
    var y = calcSineY(x); // Calculate y value from x
   ctx.moveTo(x,h); // Where to start drawing
    ctx.lineTo(x,y); // Where to draw to
  }
  ctx.stroke(); // Draw
  
  // draw area below y
  ctx.beginPath(); // Draw a new path
  ctx.strokeStyle = "orange"; // Pick a color
  for(var i=0;i<x;i++){ // Loop from left side to current x
    if(i/3==Math.round(i/3)) { // Draw only one line each 3 pixels
      var y = calcSineY(i); // Calculate y value from x
      ctx.moveTo(i,h); // Where to start drawing
      ctx.lineTo(i,y); // Where to draw to
    }
  }
  ctx.stroke(); // Draw
  
  // draw sin curve point to point until x
  ctx.beginPath(); // Draw a new path
  ctx.strokeStyle = "black"; // Pick a color
  for(var i=0;i<x;i++){ // Loop from left side to current x
    var y = calcSineY(i); // Calculate y value from x
    ctx.lineTo(i,y); // Where to draw to
  }
  ctx.stroke(); // Draw
}
// Define initial value of x positiom (leftmost side of cnanvas)
var x=0;
// Start time interval
var interval = setInterval(function(){
 drawSine(x); // Call draww function every cycle
 x++; // Increment x by 1
 if(x>w){
     x=0; // x cannot be more than canvas with, so back to 0
        f++; // increment frequency for demonstration
    }
},10); // Loop every 10 milliseconds

<canvas id="canvas" width="320" height="120" style="border:1px solid #999;">

回答by quantummind

Your code is unnecessarily difficult. Try it so simple:

您的代码不必要地困难。试试这么简单：

    var c = document.getElementById("myCanvas"); // get the canvas object to draw onto
    var ctx = c.getContext("2d"); // will use simpe 2D context on the canvas
    
    for(x=0; x<360; x += 20) { // 360 steps for entire sine period
        ctx.moveTo(x+10,180);  // for dashed line, go to start of next dash
        ctx.lineTo(x,180);  // then draw the short line
    }
    ctx.moveTo(0,180);  // back to the left before drawing the sine
    
    for(x=0; x<=360; x+=1) { // 360 steps (degrees) for entire sine period
        y = 180.0 - Math.sin(x*Math.PI/180)*120; // calculate y flipped horizontally, converting from DEG to RADIAN
        ctx.lineTo(x,y); // draw the point
    }
    ctx.stroke(); // strokes the drawing to the canvas

<canvas id="myCanvas" width="360" height="360" style="border:1px solid #d3d3d3;">

回答by csabaD

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
var i;
var amplitude = 90;
var width = c.width;
var height = c.height / 2;
var step = 1;
var frequency = 4;
ctx.moveTo(0, height);
ctx.lineTo(width, height);
ctx.stroke();
ctx.beginPath();
ctx.moveTo(0, height);
var c = width / Math.PI / (frequency * 2);

for (i = 0; i < width; i += step) {
  var x = amplitude * Math.sin(i / c);
  ctx.lineTo(i, height + x);
}

ctx.strokeStyle = '#0096FF';
ctx.stroke();

<canvas id="myCanvas" width="600" height="300" style="border:1px solid #d3d3d3;">

回答by bartpie

Use bezierCurveTo, this is only an example you should adjust parameters to get nice sinusoid

使用bezierCurveTo，这只是一个示例，您应该调整参数以获得漂亮的正弦曲线

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.moveTo(50,50);
ctx.bezierCurveTo(120,-100,200,250,250,50);
ctx.bezierCurveTo(300,-100,350,250,430,50);
ctx.lineWidth = 5;
ctx.strokeStyle = '#003300';
ctx.stroke();

<canvas id="myCanvas" width="550" height="360" style="border:1px solid #d3d3d3;">

回答by Birbal Singh

With Single loop you can easily

使用单循环，您可以轻松

var c = document.getElementById("myCanvas");
      var ctx = c.getContext("2d");
     var y=180;
      //draw axis
      ctx.beginPath();
      ctx.strokeStyle = "red";
      ctx.moveTo(0,  y/2);
      ctx.lineTo(y,  y/2);
      ctx.stroke();
      // draw sin wave
      ctx.beginPath();
      ctx.moveTo(x,y);
      ctx.strokeStyle = "black";
      for(var x=0; x<=180; x+=1){
          ctx.lineTo(x,((y/2)  + y/2  * Math.sin( ((x+-y/2)/90) * Math.PI)));
          ctx.stroke();
          }

<canvas id="myCanvas" width="360" height="360" style="border:1px solid #d3d3d3;">

https://i.stack.imgur.com/9oEN0.png

https://i.stack.imgur.com/9oEN0.png

回答by EliSherer

Based on bartpie's answer:

基于 bartpie 的回答：

const l = 100; // length
const tf = l * 2 / 5; // two fifths of length

const ctx = document.getElementById("canvas").getContext("2d");
ctx.translate(20, 40); // just for display

ctx.moveTo(0,0);
ctx.bezierCurveTo(tf,-(l-tf),l-tf,l-tf,l,0);

ctx.lineWidth = 2;
ctx.stroke();

<canvas id="canvas" width="200" height="100">