你很难找到他们:(
在HTML画布中没有等价的。你必须用简单的老数学找到你自己的中点。
我为你做了一个如何找到贝塞尔曲线中点的例子。在JSfiddle现场观看
here
. 下面粘贴了一个javascript副本。
实际曲线是红色的,中点是绿色的小矩形。其他一切只是视觉辅助。
var ctx = $("#test")[0].getContext("2d")
function mid(a,b) {
return (a+b) / 2;
}
var cp1x = 100;
var cp1y = 150;
var cp2x = 175;
var cp2y = 175;
var x = 200;
var y = 0;
ctx.lineWidth = 4;
ctx.strokeStyle = "red";
ctx.fillStyle = "rgba(0,0,0,0.6)";
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y);
ctx.stroke();
//line goes from start to control point 1
ctx.strokeStyle = "rgba(0,0,200,0.4)";
ctx.beginPath();
ctx.moveTo(0, 0);
ctx.lineTo(cp1x , cp1y);
ctx.stroke();
//line goes from end to control point 2
ctx.beginPath();
ctx.moveTo(x, y);
ctx.lineTo(cp2x , cp2y);
ctx.stroke();
//line goes from control point to control point
ctx.strokeStyle = "rgba(200,0,200,0.4)";
ctx.beginPath();
ctx.moveTo(cp1x, cp1y);
ctx.lineTo(cp2x , cp2y);
ctx.stroke();
// now find the midpoint of each of those 3 lines
var ax = mid(cp1x,0);
var bx = mid(cp2x,x)
var cx = mid(cp1x,cp2x)
var ay = mid(cp1y,0)
var by = mid(cp2y,y)
var cy = mid(cp1y,cp2y)
// draw midpoints for visual aid
// not gonna look exact 'cause square
// will be drawn from top-right instead of center
ctx.fillRect(ax, ay, 4, 4);
ctx.fillRect(bx, by, 4, 4);
ctx.fillRect(cx, cy, 4, 4);
//now draw lines between those three points. These are green
ctx.strokeStyle = "rgba(0,200,0,0.4)";
ctx.beginPath();
ctx.moveTo(ax, ay);
ctx.lineTo(cx , cy);
ctx.stroke();
ctx.beginPath();
ctx.moveTo(bx, by);
ctx.lineTo(cx , cy);
ctx.stroke();
//now the midpoint of the green lines:
// so g1 and g2 are the green line midpoints
var g1x = mid(ax,cx);
var g2x = mid(bx,cx);
var g1y = mid(ay,cy);
var g2y = mid(by,cy);
//draw them to make sure:
ctx.fillRect(g1x , g1y, 4, 4);
ctx.fillRect(g2x , g2y, 4, 4);
//now one final line, in gray
ctx.strokeStyle = "rgba(20,20,20,0.4)";
ctx.beginPath();
ctx.moveTo(g1x , g1y);
ctx.lineTo(g2x , g2y);
ctx.stroke();
//whew! We made it!
var FinallyTheMidpointx = mid(g1x,g2x);
var FinallyTheMidpointy = mid(g1y,g2y);
//draw something at the midpoint to celebrate
ctx.fillStyle = "rgba(0,255,0,1)";
ctx.fillRect(FinallyTheMidpointx, FinallyTheMidpointy, 4, 4);
艾斯
.
