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[英]Can I detect if the mouse is currently moving inside a canvas draw function?
[英]How can I draw arrows on a canvas with mouse
最近我开始玩画布元素。 现在我可以用鼠标在画布上画线(任意多)。 你可以在代码中看到它: https : //jsfiddle.net/saipavan579/a6L3ka8p/ 。
var ctx = tempcanvas.getContext('2d'),
mainctx = canvas.getContext('2d'),
w = canvas.width,
h = canvas.height,
x1,
y1,
isDown = false;
tempcanvas.onmousedown = function(e) {
var pos = getPosition(e, canvas);
x1 = pos.x;
y1 = pos.y;
isDown = true;
}
tempcanvas.onmouseup = function() {
isDown = false;
mainctx.drawImage(tempcanvas, 0, 0);
ctx.clearRect(0, 0, w, h);
}
tempcanvas.onmousemove = function(e) {
if (!isDown) return;
var pos = getPosition(e, canvas);
x2 = pos.x;
y2 = pos.y;
ctx.clearRect(0, 0, w, h);
drawEllipse(x1, y1, x2, y2);
}
function drawEllipse(x1, y1, x2, y2) {
var radiusX = (x2 - x1) * 0.5,
radiusY = (y2 - y1) * 0.5,
centerX = x1 + radiusX,
centerY = y1 + radiusY,
step = 0.01,
a = step,
pi2 = Math.PI * 2 - step;
ctx.beginPath();
ctx.moveTo(x1,y1);
for(; a < pi2; a += step) {
ctx.lineTo(x2,y2);
}
ctx.closePath();
ctx.strokeStyle = '#000';
ctx.stroke();
}
function getPosition(e, gCanvasElement) {
var x;
var y;
x = e.pageX;
y = e.pageY;
x -= gCanvasElement.offsetLeft;
y -= gCanvasElement.offsetTop;
return {x:x, y:y};
};
现在我想以与绘制线条相同的方式绘制箭头线(用于指向图像上的某个特定点)。 怎么能这样? 先感谢您。
您可以像这样在线段 [p0,p1] 的末尾绘制一个箭头:
使用 Math.atan2 计算从 p0 到 p1 的角度。
箭头的每一边都从 p1 开始,因此使用三角法计算 2 个箭头端点。
绘制 [p0,p1] 线段和 2 条箭头线段。
这是示例代码和演示:
var canvas=document.getElementById("canvas"); var ctx=canvas.getContext("2d"); var p0={x:50,y:100}; var p1={x:250,y:50}; drawLineWithArrowhead(p0,p1,15); function drawLineWithArrowhead(p0,p1,headLength){ // constants (could be declared as globals outside this function) var PI=Math.PI; var degreesInRadians225=225*PI/180; var degreesInRadians135=135*PI/180; // calc the angle of the line var dx=p1.x-p0.x; var dy=p1.y-p0.y; var angle=Math.atan2(dy,dx); // calc arrowhead points var x225=p1.x+headLength*Math.cos(angle+degreesInRadians225); var y225=p1.y+headLength*Math.sin(angle+degreesInRadians225); var x135=p1.x+headLength*Math.cos(angle+degreesInRadians135); var y135=p1.y+headLength*Math.sin(angle+degreesInRadians135); // draw line plus arrowhead ctx.beginPath(); // draw the line from p0 to p1 ctx.moveTo(p0.x,p0.y); ctx.lineTo(p1.x,p1.y); // draw partial arrowhead at 225 degrees ctx.moveTo(p1.x,p1.y); ctx.lineTo(x225,y225); // draw partial arrowhead at 135 degrees ctx.moveTo(p1.x,p1.y); ctx.lineTo(x135,y135); // stroke the line and arrowhead ctx.stroke(); }
body{ background-color: ivory; } canvas{border:1px solid red;}
<canvas id="canvas" width=300 height=300></canvas>
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