旋转任意2D向量90度
如果您记得2D向量的旋转90度规则，则可以简单计算垂线。
向量{x，y}可以沿顺时针方向旋转90度{-y，x}或逆时针方向旋转90度{y，-x}。交换x和y，并对顺时针旋转或逆时针旋转的y取反，对于逆时针旋转则取反的是x。
因此，对于线段x1，y1到x2，y2，请将其转换为向量，对该向量进行归一化处理，并按以下方式旋转90度：
function getPerpOfLine(x1,y1,x2,y2){ // the two points can not be the same
var nx = x2 - x1; // as vector
var ny = y2 - y1;
const len = Math.sqrt(nx * nx + ny * ny); // length of line
nx /= len; // make one unit long
ny /= len; // which we call normalising a vector
return [-ny, nx]; // return the normal rotated 90 deg
}
假设你想在线段的两端画出一条长度为10像素的线。
const canvas = document.createElement("canvas");
const ctx = canvas.getContext("2d");
document.body.append(canvas);
ctx.strokeStyle = "black";
ctx.lineJoin = ctx.lineCap = "round";
ctx.lineWidth = 3;
const x1 = 40, y1 = 40, x2 = 260, y2 = 110;
const endLen = 10;
var px = y1 - y2;
var py = x2 - x1;
const len = endLen / Math.hypot(px, py);
px *= len;
py *= len;
ctx.beginPath();
ctx.lineTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.moveTo(x1 + px, y1 + py);
ctx.lineTo(x1 - px, y1 - py);
ctx.moveTo(x2 + px, y2 + py);
ctx.lineTo(x2 - px, y2 - py);
ctx.stroke();
更新
渲染沿着一条线的内容的简单方法，使用相同的90度规则。
有一种使用相同向量旋转的替代渲染方法，但不是通过向量乘法设置垂直轴线，而是将变换的y轴设置为与沿线的x轴垂直的90度。将原点设置为该线的起点，您可以简单地相对于该线进行渲染。
setTransformToLine(x1, y1, x2, y2)
以下功能将设置画布变换沿着该线。
// Set 2D context current transform along the line x1,y1,x2,y2 and origin to
// start of line. y Axis is rotated clockwise 90 from the line.
// Returns the line length as that is most frequently required when
// using the method saving some time.
function setTransformToLine(x1, y1, x2, y2) {
const vx = x2 - x1; // get the line as vector
const vy = y2 - y1;
const len = Math.hypot(vx, vy); // For <= IE11 use Math.sqrt(vx * vx + vy * vy)
const nx = vx / len; // Normalise the line vector. Making it one
const ny = vy / len; // pixel long. This sets the scale
// The transform is the normalised line vector for x axis, y at 90 deg
// and origin at line start
ctx.setTransform(nx, ny, -ny, nx, x1, y1); // set transform
return len;
}
如何使用
这个例子展示了如何使用transform来在同一行上添加注释长度。
const canvas = document.createElement("canvas");
const ctx = canvas.getContext("2d");
document.body.append(canvas);
ctx.strokeStyle = "black";
ctx.lineJoin = ctx.lineCap = "round";
ctx.lineWidth = 3;
ctx.font = "16px arial";
ctx.textBaseline = "middle";
ctx.textAlign = "center";
const x1 = 40, y1 = 40, x2 = 260, y2 = 110;
const endLen = 10;
function setTransformToLine(x1, y1, x2, y2) {
const vx = x2 - x1;
const vy = y2 - y1;
const len = Math.hypot(vx, vy);
const nx = vx / len;
const ny = vy / len;
ctx.setTransform(nx, ny, -ny, nx, x1, y1);
return len;
}
const lineLen = setTransformToLine(x1, y1, x2, y2);
const lineLenStr = Math.round(lineLen) + "px";
const textWidth = ctx.measureText(lineLenStr).width;
const rlen = lineLen - textWidth - 16;
ctx.beginPath();
ctx.lineTo(0, -endLen);
ctx.lineTo(0, endLen);
ctx.moveTo(lineLen, -endLen);
ctx.lineTo(lineLen, endLen);
ctx.moveTo(0,0);
ctx.lineTo(rlen / 2, 0);
ctx.moveTo(lineLen - rlen / 2,0);
ctx.lineTo(lineLen, 0);
ctx.stroke();
ctx.fillText(lineLenStr, lineLen / 2, 0);
ctx.setTransform(1, 0, 0, 1, 0, 0);
