在 canvas 中绘制正弦波

Question

我正在尝试在 canvas 中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。

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

HTML

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

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();

我想要的输出

Answer 1

您正在增加 counter 的值太高，将其调小：

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

绘制图表的整个宽度而不是一半：

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

您需要更高的振幅：

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

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

Answer 2

你的代码太难了。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;">

Answer 3

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;">

Answer 4

使用bezierCurveTo，这只是一个例子，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;">

Answer 5

使用单循环你可以轻松

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

Answer 6

根据 bartpie 的回答：

const l = 180; // length
const heightRatio = 2;
const tf = l * 2 / 5; // two fifths of length

const ctx = document.getElementById("canvas").getContext("2d");
ctx.translate(10, 100); // just for display


ctx.beginPath();
ctx.strokeStyle ='red';

ctx.moveTo(0,0);
ctx.lineTo(l,0);
ctx.stroke();

ctx.beginPath();
ctx.strokeStyle ='#000';

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

ctx.stroke();

<canvas id="canvas" width="200" height="220">

Answer 7

老问题但普遍感兴趣。

这里的所有答案都非常有用，但是 none 的代码 注释得很好 ，所以我创建了一个片段来解释每一点发生的事情并绘制了很多为清楚起见变量。感谢大家帮助我理解，希望这对其他人有帮助。

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;">

Answer 8

function plotSine(amplitude, frequency) {
  const canvas = document.getElementById('canvas');
  if (canvas.getContext) {
    const ctx = canvas.getContext('2d');
    
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    
    ctx.beginPath();
    
    ctx.strokeStyle = '#343a40';
    ctx.lineWidth = 2;
    
    var x = 0;
    var y = 0;
    
    while (x < canvas.width) {
      y = (canvas.height / 2) + amplitude * Math.sin(x / frequency);
      ctx.lineTo(x, y);
      
      x += 1;
    }
    
    ctx.stroke();
  }
}

plotSine(40, 20);

code {
  background-color: #eee;
  border-radius: 3px;
  padding: 0 3px;
}

<canvas id="canvas" width="480" height="360">
  <p>
    Your browser does not support the <code>&lt;canvas&gt;</code> element.
  </p>
</canvas>

在 canvas 中绘制正弦波

drawing sine wave in canvas

html

canvas

sine-wave