如何使用 html canvas 和 javascript 用鼠标绘制平滑的连续线

Question

我正在尝试使用 html5 画布和纯 javascript 创建一个简单的绘图/绘画程序。我的工作正常，但是当绘制和移动鼠标太快时，线会断开，最后我会得到一行点 - 我怎样才能使它成为一条平滑的连续线？

建议将不胜感激！我对 JS 很陌生，所以代码示例非常有用，在此先感谢。

当前的JS是：

var canvas, ctx
var mouseX, mouseY, mouseDown = 0

function draw(ctx,x,y,size) {
  ctx.fillStyle = "#000000"
  ctx.beginPath()
  ctx.arc(x, y, size, 0, Math.PI*2, true)
  ctx.closePath()
  ctx.fill()
}

function clearCanvas(canvas,ctx) {
  ctx.clearRect(0, 0, canvas.width, canvas.height)
}

function onMouseDown() {
  mouseDown = 1
  draw(ctx, mouseX, mouseY, 2)
}

function onMouseUp() {
  mouseDown = 0
}

function onMouseMove(e) {
  getMousePos(e)
  if (mouseDown == 1) {
      draw(ctx, mouseX, mouseY, 2)
  }
}

function getMousePos(e) {
  if (!e)
      var e = event
  if (e.offsetX) {
      mouseX = e.offsetX
      mouseY = e.offsetY
  }
  else if (e.layerX) {
      mouseX = e.layerX
      mouseY = e.layerY
  }
 }

function init() {
    canvas = document.getElementById('sketchpad')
    ctx = canvas.getContext('2d')
    canvas.addEventListener('mousedown', onMouseDown, false)
    canvas.addEventListener('mousemove', onMouseMove, false)
    window.addEventListener('mouseup', onMouseUp, false)
}
init();

<canvas id="sketchpad" width="500" height="500"></canvas>

Answer 1

用鼠标绘制平滑曲线。

遗憾的是，如果您想忠实于艺术家的预期路线，这并不容易。

它涉及记录整个鼠标行程。描边完成后，将点数减少到细节限制（由艺术家设置），然后对剩余点应用贝塞尔平滑函数。

可以在绘制笔画时完成，但对于某些设备，如果线条变得很长，这可能会变得太多。由于在实时显示平滑线时，线条细节减少会查看所有点，因此有些人不喜欢随着线条变长而略微变化的方式。

演示

下面的代码演示了一个我发现有用的解决方案。

• 使用左键绘制平滑完成一键释放。
• 使用右键进行实时平滑绘制（蓝线）。
• 单击鼠标中键清除。

使用顶部的两个滑块设置平滑量和细节量。左键拖出一个笔划，显示原始线条。松开鼠标后，线条会被简化、平滑并添加到背景图像中。

var canvas = document.getElementById("canV"); 
var ctx = canvas.getContext("2d");


// mouse stuff
var mouse = {
    x:0,
    y:0,
    buttonLastRaw:0, // user modified value 
    buttonRaw:0,
    buttons:[1,2,4,6,5,3], // masks for setting and clearing button raw bits;
};
function mouseMove(event){
    mouse.x = event.offsetX;  mouse.y = event.offsetY; 
    if(mouse.x === undefined){ mouse.x = event.clientX;  mouse.y = event.clientY;}    
    if(event.type === "mousedown"){ mouse.buttonRaw |= mouse.buttons[event.which-1];
    }else if(event.type === "mouseup"){mouse.buttonRaw &= mouse.buttons[event.which+2];
    }else if(event.type === "mouseout"){ mouse.buttonRaw = 0; mouse.over = false;
    }else if(event.type === "mouseover"){ mouse.over = true; }
    event.preventDefault();
}
canvas.addEventListener('mousemove',mouseMove);
canvas.addEventListener('mousedown',mouseMove);
canvas.addEventListener('mouseup'  ,mouseMove); 
canvas.addEventListener('mouseout'  ,mouseMove); 
canvas.addEventListener('mouseover'  ,mouseMove); 
canvas.addEventListener("contextmenu", function(e){ e.preventDefault();}, false);


// Line simplification based on
// the Ramer–Douglas–Peucker algorithm
// referance https://en.wikipedia.org/wiki/Ramer%E2%80%93Douglas%E2%80%93Peucker_algorithm
// points are and array of arrays consisting of [[x,y],[x,y],...,[x,y]]
// length is in pixels and is the square of the actual distance.
// returns array of points of the same form as the input argument points.
var simplifyLineRDP = function(points, length) {
    var simplify = function(start, end) { // recursize simplifies points from start to end
        var maxDist, index, i, xx , yy, dx, dy, ddx, ddy, p1, p2, p, t, dist, dist1;
        p1 = points[start];
        p2 = points[end];   
        xx = p1[0];
        yy = p1[1];
        ddx = p2[0] - xx;
        ddy = p2[1] - yy;
        dist1 = (ddx * ddx + ddy * ddy);
        maxDist = length;
        for (var i = start + 1; i < end; i++) {
            p = points[i];
            if (ddx !== 0 || ddy !== 0) {
                t = ((p[0] - xx) * ddx + (p[1] - yy) * ddy) / dist1;
                if (t > 1) {
                    dx = p[0] - p2[0];
                    dy = p[1] - p2[1];
                } else 
                if (t > 0) {
                    dx = p[0] - (xx + ddx * t);
                    dy = p[1] - (yy + ddy * t);
                } else {
                    dx = p[0] - xx;
                    dy = p[1] - yy;
                }
            }else{
                dx = p[0] - xx;
                dy = p[1] - yy;
            }
            dist = dx * dx + dy * dy 
            if (dist > maxDist) {
                index = i;
                maxDist = dist;
            }
        }

        if (maxDist > length) { // continue simplification while maxDist > length
            if (index - start > 1){
                simplify(start, index);
            }
            newLine.push(points[index]);
            if (end - index > 1){
                simplify(index, end);
            }
        }
    }    
    var end = points.length - 1;
    var newLine = [points[0]];
    simplify(0, end);
    newLine.push(points[end]);
    return newLine;
}



// This is my own smoothing method 
// It creates a set of bezier control points either 2nd order or third order 
// bezier curves.
// points: list of points
// cornerThres: when to smooth corners and represents the angle between to lines. 
//     When the angle is smaller than the cornerThres then smooth.
// match: if true then the control points will be balanced.
// Function will make a copy of the points

var smoothLine = function(points,cornerThres,match){  // adds bezier control points at points if lines have angle less than thres
    var  p1, p2, p3, dist1, dist2, x, y, endP, len, angle, i, newPoints, aLen, closed, bal, cont1, nx1, nx2, ny1, ny2, np;
    function dot(x, y, xx, yy) {  // get do product
        // dist1,dist2,nx1,nx2,ny1,ny2 are the length and  normals and used outside function
        // normalise both vectors
        dist1 = Math.sqrt(x * x + y * y); // get length
        if (dist1  > 0) {  // normalise
            nx1 = x / dist1 ;
            ny1 = y / dist1 ;
        }else {
            nx1 = 1;  // need to have something so this will do as good as anything
            ny1 = 0;
        }
        dist2  = Math.sqrt(xx * xx + yy * yy);
        if (dist2  > 0) {
            nx2 = xx / dist2;
            ny2 = yy / dist2;
        }else {
            nx2 = 1;
            ny2 = 0;
        }
       return Math.acos(nx1 * nx2 + ny1 * ny2 ); // dot product
    }
    newPoints = []; // array for new points
    aLen = points.length;
    if(aLen <= 2){  // nothing to if line too short
        for(i = 0; i < aLen; i ++){  // ensure that the points are copied          
            newPoints.push([points[i][0],points[i][1]]);
        }
        return newPoints;
    }
    p1 = points[0];
    endP =points[aLen-1];
    i = 0;  // start from second poitn if line not closed
    closed = false;
    len = Math.hypot(p1[0]- endP[0], p1[1]-endP[1]);
    if(len < Math.SQRT2){  // end points are the same. Join them in coordinate space
        endP =  p1;
        i = 0;             // start from first point if line closed
        p1 = points[aLen-2];
        closed = true;
    }       
    newPoints.push([points[i][0],points[i][1]])
    for(; i < aLen-1; i++){
        p2 = points[i];
        p3 = points[i + 1];
        angle = Math.abs(dot(p2[0] - p1[0], p2[1] - p1[1], p3[0] - p2[0], p3[1] - p2[1]));
        if(dist1 !== 0){  // dist1 and dist2 come from dot function
            if( angle < cornerThres*3.14){ // bend it if angle between lines is small
                  if(match){
                      dist1 = Math.min(dist1,dist2);
                      dist2 = dist1;
                  }
                  // use the two normalized vectors along the lines to create the tangent vector
                  x = (nx1 + nx2) / 2;  
                  y = (ny1 + ny2) / 2;
                  len = Math.sqrt(x * x + y * y);  // normalise the tangent
                  if(len === 0){
                      newPoints.push([p2[0],p2[1]]);                                  
                  }else{
                      x /= len;
                      y /= len;
                      if(newPoints.length > 0){
                          var np = newPoints[newPoints.length-1];
                          np.push(p2[0]-x*dist1*0.25);
                          np.push(p2[1]-y*dist1*0.25);
                      }
                      newPoints.push([  // create the new point with the new bezier control points.
                            p2[0],
                            p2[1],
                            p2[0]+x*dist2*0.25,
                            p2[1]+y*dist2*0.25
                      ]);
                  }
            }else{
                newPoints.push([p2[0],p2[1]]);            
            }
        }
        p1 = p2;
    }  
    if(closed){ // if closed then copy first point to last.
        p1 = [];
        for(i = 0; i < newPoints[0].length; i++){
            p1.push(newPoints[0][i]);
        }
        newPoints.push(p1);
    }else{
        newPoints.push([points[points.length-1][0],points[points.length-1][1]]);      
    }
    return newPoints;    
}

// creates a drawable image
var createImage = function(w,h){
    var image = document.createElement("canvas");
    image.width = w;
    image.height =h; 
    image.ctx = image.getContext("2d"); 
    return image;
}  

// draws the smoothed line with bezier control points.
var drawSmoothedLine = function(line){
    var i,p;
    ctx.beginPath()
    ctx.moveTo(line[0][0],line[0][1])
    for(i = 0; i < line.length-1; i++){
       p = line[i];
       p1 = line[i+1]
       if(p.length === 2){ // linear 
            ctx.lineTo(p[0],p[1])
       }else
       if(p.length === 4){ // bezier 2nd order
           ctx.quadraticCurveTo(p[2],p[3],p1[0],p1[1]);
       }else{              // bezier 3rd order
           ctx.bezierCurveTo(p[2],p[3],p[4],p[5],p1[0],p1[1]);
       }
    }
    if(p.length === 2){
        ctx.lineTo(p1[0],p1[1])
    }
    ctx.stroke();
}

// smoothing settings
var liveSmooth;
var lineSmooth = {};
lineSmooth.lengthMin = 8;  // square of the pixel length
lineSmooth.angle = 0.8;      // angle threshold
lineSmooth.match = false;  // not working.
// back buffer to save the canvas allowing the new line to be erased
var backBuffer = createImage(canvas.width,canvas.height);
var currentLine = [];
mouse.lastButtonRaw = 0;  // add mouse last incase not there
ctx.lineWidth = 3;
ctx.lineJoin = "round";
ctx.lineCap = "round";
ctx.strokeStyle = "black";
ctx.clearRect(0,0,canvas.width,canvas.height);
var drawing = false;  // if drawing
var input = false;  // if menu input
var smoothIt = false;  // flag to allow feedback that smoothing is happening as it takes some time.
function draw(){
    // if not drawing test for menu interaction and draw the menus
    if(!drawing){      
        if(mouse.x < 203 && mouse.y < 24){
            if(mouse.y < 13){
                if(mouse.buttonRaw === 1){
                    ctx.clearRect(3,3,200,10);
                    lineSmooth.angle = (mouse.x-3)/200;
                    input = true;
                }
            }else
            if(mouse.buttonRaw === 1){
                ctx.clearRect(3,14,200,10);
                lineSmooth.lengthMin = (mouse.x-3)/10;
                input = true;
            }
                
            canvas.style.cursor = "pointer";
        }else{
            canvas.style.cursor = "crosshair";
            
        }
        if(mouse.buttonRaw === 0 && input){
            input = false;
            mouse.lastButtonRaw = 0;
        }
        ctx.lineWidth = 0.5;
        ctx.fillStyle = "red";
        ctx.clearRect(3,3,200,10);
        ctx.clearRect(3,14,200,10);
        ctx.fillRect(3,3,lineSmooth.angle*200,10);
        ctx.fillRect(3,14,lineSmooth.lengthMin*10,10);

        ctx.textAlign = "left";
        ctx.textBaseline = "top";
        ctx.fillStyle = "#000"
        ctx.strokeRect(3,3,200,10);
        ctx.fillText("Smooth "+(lineSmooth.angle * (180 / Math.PI)).toFixed(0)+"deg",5,2)
        ctx.strokeRect(3,14,200,10);
        ctx.fillText("Detail "+lineSmooth.lengthMin.toFixed(0) + "pixels",5,13);

    }else{
        canvas.style.cursor = "crosshair"; 
    }
    if(!input){
         ctx.lineWidth = 3;
        if(mouse.buttonRaw === 4 && mouse.lastButtonRaw === 0){
            currentLine = [];
            drawing  = true;

            backBuffer.ctx.clearRect(0,0,canvas.width,canvas.height);
            backBuffer.ctx.drawImage(canvas,0,0);
            currentLine.push([mouse.x,mouse.y])
        }else
        if(mouse.buttonRaw === 4){
            var lp = currentLine[currentLine.length-1]; // get last point
            // dont record point if no movement
            if(mouse.x !== lp[0] || mouse.y !== lp[1] ){
                currentLine.push([mouse.x,mouse.y]);
                ctx.beginPath();
                ctx.moveTo(lp[0],lp[1])
                ctx.lineTo(mouse.x,mouse.y);
                ctx.stroke();
                liveSmooth = smoothLine(
                    simplifyLineRDP(
                        currentLine,
                        lineSmooth.lengthMin
                    ),
                    lineSmooth.angle,
                    lineSmooth.match
                );
                ctx.clearRect(0,0,canvas.width,canvas.height);
               ctx.drawImage(backBuffer,0,0);
                ctx.strokeStyle = "Blue";
                drawSmoothedLine(liveSmooth );
                ctx.strokeStyle = "black";
            }
        }else
        if(mouse.buttonRaw === 0 && mouse.lastButtonRaw === 4){
            ctx.textAlign = "center"
            ctx.fillStyle = "red"
            ctx.fillText("Smoothing...",canvas.width/2,canvas.height/5);
            smoothIt = true;
        }else
        if(smoothIt){
            smoothIt = false;
            
            var newLine = smoothLine(
                simplifyLineRDP(
                    currentLine,
                    lineSmooth.lengthMin
                ),
                lineSmooth.angle,
                lineSmooth.match
            );
            ctx.clearRect(0,0,canvas.width,canvas.height);
            ctx.drawImage(backBuffer,0,0);
            drawSmoothedLine(newLine);
            drawing  = false;
            
        }


        if(mouse.buttonRaw === 1 && mouse.lastButtonRaw === 0){
            currentLine = [];
            drawing  = true;

            backBuffer.ctx.clearRect(0,0,canvas.width,canvas.height);
            backBuffer.ctx.drawImage(canvas,0,0);
            currentLine.push([mouse.x,mouse.y])
        }else
        if(mouse.buttonRaw === 1){
            var lp = currentLine[currentLine.length-1]; // get last point
            // dont record point if no movement
            if(mouse.x !== lp[0] || mouse.y !== lp[1] ){
                currentLine.push([mouse.x,mouse.y]);
                ctx.beginPath();
                ctx.moveTo(lp[0],lp[1])
                ctx.lineTo(mouse.x,mouse.y);
                ctx.stroke();
            }
        }else
        if(mouse.buttonRaw === 0 && mouse.lastButtonRaw === 1){
            ctx.textAlign = "center"
            ctx.fillStyle = "red"
            ctx.fillText("Smoothing...",canvas.width/2,canvas.height/5);
            smoothIt = true;
        }else
        if(smoothIt){
            smoothIt = false;
            
            var newLine = smoothLine(
                simplifyLineRDP(
                    currentLine,
                    lineSmooth.lengthMin
                ),
                lineSmooth.angle,
                lineSmooth.match
            );
            ctx.clearRect(0,0,canvas.width,canvas.height);
            ctx.drawImage(backBuffer,0,0);
            drawSmoothedLine(newLine);
            drawing  = false;
            
        }
    }
    // middle button clear
    if(mouse.buttonRaw === 2){
        ctx.clearRect(0,0,canvas.width,canvas.height);
    }
    mouse.lastButtonRaw = mouse.buttonRaw;

    requestAnimationFrame(draw);

}

draw();

.canC { width:1000px;  height:500px; border:1px black solid;}

<canvas class="canC" id="canV" width=1000 height=500></canvas>

Answer 2

好问题！我推荐你一个网站https://developer.mozilla.org/zh-CN/docs/Web/API/Canvas_API 来了解更多的canvas API。

我认为使用lineTo 比使用arc 更好。所以我希望这段代码对你有所帮助。

var canvas, ctx;
var mouseDown = 0, lastX, lastY;



function draw(ctx,x,y) {
  ctx.beginPath();
  ctx.moveTo(lastX,lastY);
  ctx.lineTo(x,y);
  ctx.closePath();
  ctx.stroke();
}

function clearCanvas(canvas,ctx) {
  ctx.clearRect(0, 0, canvas.width, canvas.height)
}

function onMouseDown(e) {
  var xy = getMousePos(e);
  lastX = xy.mouseX;
  lastY = xy.mouseY;
  mouseDown = 1;
}

function onMouseUp() {
  mouseDown = 0
}

function onMouseMove(e) {
  if (mouseDown == 1) {
      var xy = getMousePos(e);
      draw(ctx, xy.mouseX, xy.mouseY);
      lastX = xy.mouseX, lastY = xy.mouseY;
  }
}

function getMousePos(e) {
    var o = {};
  if (!e)
      var e = event
  if (e.offsetX) {
      o.mouseX = e.offsetX
      o.mouseY = e.offsetY
  }
  else if (e.layerX) {
      o.mouseX = e.layerX
      o.mouseY = e.layerY
  }
  return o;
 }

function init() {
    canvas = document.getElementById('sketchpad')
    ctx = canvas.getContext('2d')
    canvas.addEventListener('mousedown', onMouseDown, false)
    canvas.addEventListener('mousemove', onMouseMove, false)
    canvas.addEventListener('mouseup', onMouseUp, false)
}
init();

Answer 3

您可以保存最后一个位置并在最后一个点和实际点之间画一条线。

if (lastX && lastY && (x !== lastX || y !== lastY)) {
    ctx.fillStyle = "#000000";
    ctx.lineWidth = 2 * size;
    ctx.beginPath();
    ctx.moveTo(lastX, lastY);
    ctx.lineTo(x, y);
    ctx.stroke();
    // ...
   lastX = x;
   lastY = y;
}

在 mouseup 事件中将两个变量设置为零。

var canvas, ctx
var mouseX, mouseY, mouseDown = 0,
    lastX, lastY;

function draw(ctx,x,y,size) {
    if (lastX && lastY && (x !== lastX || y !== lastY)) {
        ctx.fillStyle = "#000000";
        ctx.lineWidth = 2 * size;
        ctx.beginPath();
        ctx.moveTo(lastX, lastY);
        ctx.lineTo(x, y);
        ctx.stroke();
    }
  ctx.fillStyle = "#000000";
  ctx.beginPath();
  ctx.arc(x, y, size, 0, Math.PI*2, true);
  ctx.closePath();
  ctx.fill();
  lastX = x;
  lastY = y;
}

function clearCanvas(canvas,ctx) {
  ctx.clearRect(0, 0, canvas.width, canvas.height)
}

function onMouseDown() {
  mouseDown = 1
  draw(ctx, mouseX, mouseY, 2)
}

function onMouseUp() {
  mouseDown = 0;
  lastX = 0;
  lastY = 0;
}

function onMouseMove(e) {
  getMousePos(e)
  if (mouseDown == 1) {
      draw(ctx, mouseX, mouseY, 2)
  }
}

function getMousePos(e) {
  if (!e)
      var e = event
  if (e.offsetX) {
      mouseX = e.offsetX
      mouseY = e.offsetY
  }
  else if (e.layerX) {
      mouseX = e.layerX
      mouseY = e.layerY
  }
 }

function init() {
    canvas = document.getElementById('sketchpad')
    ctx = canvas.getContext('2d')
    canvas.addEventListener('mousedown', onMouseDown, false)
    canvas.addEventListener('mousemove', onMouseMove, false)
    window.addEventListener('mouseup', onMouseUp, false)
}

init();

<canvas id="sketchpad" width="600" height="300"></canvas>