检测鼠标碰撞画布文本（JS）

Question

如何检测我的鼠标是否位于画布上呈现的一段文本上？例如：

<canvas id="myCanvas" width="200" height="100" style="border:1px solid #000000;"></canvas>

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.font = '30px Arial'
ctx.fillText('Test', 100, 50)

如果无法检测鼠标是否在实际文本上，我曾考虑使用.measureText 来查找渲染文本的边界框，并改用它。但这对于旋转的文本效果不是很好，我不确定如何找到旋转的边界框。

总结一下：

1. 是否可以检测画布上呈现的文本上的鼠标悬停事件？
2. 如果没有，有什么方法可以使用.measureText 找到某种旋转的边界框吗？

提前谢谢你！

Answer 1

getTransform() 方法确实返回一个代表上下文当前转换的DOMMatrix 对象，它本身有一个transformPoint() 方法可以用来转换DOMPoints（实际上是任何带有@ 987654327@、y、z 或 w 数字属性）。

因此，如果您想检查一个点是否在转换后的 BBox 中，您只需使用反向当前上下文的转换来转换该点，并检查该转换后的点是否适合原始 BBox。

function checkCollision() {
  const mat = ctx.getTransform().inverse()
  const pt = mat.transformPoint( viewport_point );
  const x = pt.x - text_pos.x;
  const y = pt.y - text_pos.y;
  const collides =  x >= text_bbox.left &&
                    x <= text_bbox.right &&
                    y >= text_bbox.top &&
                    y <= text_bbox.bottom;
  //...
}

const canvas = document.querySelector( "canvas" );
const ctx = canvas.getContext( "2d" );
const width = canvas.width = 500;
const height = canvas.height = 180;
const text = "Hello world";

ctx.font = "800 40px sans-serif";
ctx.textAlign = "center";
ctx.textBaseline = "middle";

const text_pos = { x: width / 2, y: height / 2 };
const text_bbox = getTextBBox( ctx, text );
const bbox_path = drawBBoxPath( text_bbox, text_pos );

const start_time = performance.now();

ctx.strokeStyle = "red";

let viewport_point = new DOMPoint(0, 0);
onmousemove = ( { clientX, clientY } ) => {
  const canvas_bbox = canvas.getBoundingClientRect();
  viewport_point = new DOMPoint(
    clientX - canvas_bbox.left,
    clientY - canvas_bbox.top
  );
};

anim();

function getTextBBox( ctx, text ) {
  const metrics = ctx.measureText( text );
  const left = metrics.actualBoundingBoxLeft * -1;
  const top = metrics.actualBoundingBoxAscent * -1;
  const right = metrics.actualBoundingBoxRight;
  const bottom = metrics.actualBoundingBoxDescent;
  const width = right - left;
  const height = bottom - top;
  return { left, top, right, bottom, width, height };
}
function drawBBoxPath( bbox, offset = { x: 0, y: 0 } ) {
  const path = new Path2D();
  const { left, top, width, height } = bbox;
  path.rect( left + offset.x, top + offset.y, width, height );
  return path;
}
function anim( t ) {
  clear();
  updateTransform( t );
  checkCollision();
  drawText();
  drawBoundingBox();
  requestAnimationFrame( anim );
}
function clear() {
  ctx.setTransform( 1, 0, 0, 1, 0, 0 );
  ctx.clearRect( 0, 0, width, height );
}
function updateTransform( t ) {
  const dur = 10000;
  const delta = (t - start_time) % dur;

  const pos = Math.PI * 2 / dur * delta;
  const angle = Math.cos( pos );
  const scale = Math.sin( pos ) * 4;
  ctx.translate( width / 2, height / 2 );
  ctx.rotate( angle );
  ctx.scale( scale, 1 );
  ctx.translate( -width / 2, -height / 2 );
}
function checkCollision() {
  const mat = ctx.getTransform().inverse()
  const pt = mat.transformPoint( viewport_point );
  const x = pt.x - text_pos.x;
  const y = pt.y - text_pos.y;
  const collides =  x >= text_bbox.left &&
                    x <= text_bbox.right &&
                    y >= text_bbox.top &&
                    y <= text_bbox.bottom;
  ctx.fillStyle = collides ? "green" : "blue";  
}
function drawText() {
  ctx.fillText( text, text_pos.x, text_pos.y );
}
function drawBoundingBox() {
  ctx.stroke( bbox_path );
}

<canvas></canvas>

当然也可以更懒一点，让上下文的isPointInPath() 方法为我们完成所有这些工作：

function checkCollision() {
  const collides = ctx.isPointInPath( bbox_path, viewport_point.x, viewport_point.y );
  //...
}

const canvas = document.querySelector( "canvas" );
const ctx = canvas.getContext( "2d" );
const width = canvas.width = 500;
const height = canvas.height = 180;
const text = "Hello world";

ctx.font = "800 40px sans-serif";
ctx.textAlign = "center";
ctx.textBaseline = "middle";

const text_pos = { x: width / 2, y: height / 2 };
const text_bbox = getTextBBox( ctx, text );
const bbox_path = drawBBoxPath( text_bbox, text_pos );

const start_time = performance.now();

ctx.strokeStyle = "red";

let viewport_point = new DOMPoint(0, 0);
onmousemove = ( { clientX, clientY } ) => {
  const canvas_bbox = canvas.getBoundingClientRect();
  viewport_point = new DOMPoint(
    clientX - canvas_bbox.left,
    clientY - canvas_bbox.top
  );
};

anim();

function getTextBBox( ctx, text ) {
  const metrics = ctx.measureText( text );
  const left = metrics.actualBoundingBoxLeft * -1;
  const top = metrics.actualBoundingBoxAscent * -1;
  const right = metrics.actualBoundingBoxRight;
  const bottom = metrics.actualBoundingBoxDescent;
  const width = right - left;
  const height = bottom - top;
  return { left, top, right, bottom, width, height };
}
function drawBBoxPath( bbox, offset = { x: 0, y: 0 } ) {
  const path = new Path2D();
  const { left, top, width, height } = bbox;
  path.rect( left + offset.x, top + offset.y, width, height );
  return path;
}
function anim( t ) {
  clear();
  updateTransform( t );
  checkCollision();
  drawText();
  drawBoundingBox();
  requestAnimationFrame( anim );
}
function clear() {
  ctx.setTransform( 1, 0, 0, 1, 0, 0 );
  ctx.clearRect( 0, 0, width, height );
}
function updateTransform( t ) {
  const dur = 10000;
  const delta = (t - start_time) % dur;

  const pos = Math.PI * 2 / dur * delta;
  const angle = Math.cos( pos );
  const scale = Math.sin( pos ) * 4;
  ctx.translate( width / 2, height / 2 );
  ctx.rotate( angle );
  ctx.scale( scale, 1 );
  ctx.translate( -width / 2, -height / 2 );
}
function checkCollision() {
  const collides = ctx.isPointInPath( bbox_path, viewport_point.x, viewport_point.y );
  ctx.fillStyle = collides ? "green" : "blue";  
}
function drawText() {
  ctx.fillText( text, text_pos.x, text_pos.y );
}
function drawBoundingBox() {
  ctx.stroke( bbox_path );
}

<canvas></canvas>

---

关于与文本绘制像素的碰撞，
canvas API 不公开文本的跟踪，因此如果您想知道鼠标何时真正位于由fillText() 方法绘制的像素上，则必须在绘制此文本后读取像素数据。

一旦我们得到像素数据，我们只需使用与第一个 sn-p 中相同的方法，并检查变换点坐标处的像素是否被绘制。

// at init, draw once, untransformed,
// ensure it's the only thing being painted on the canvas
clear();
drawText();
// grab the pixels data, once
const img_data = ctx.getImageData( 0, 0, width, height );
const pixels_data = new Uint32Array( img_data.data.buffer );

function checkCollision() {
  const mat = ctx.getTransform().inverse();
  const { x, y } = mat.transformPoint( viewport_point );
  const index =  (Math.floor( y ) * width) + Math.floor( x );
  const collides = !!pixels_data[ index ];
  //...
}

const canvas = document.querySelector( "canvas" );
const ctx = canvas.getContext( "2d" );
const width = canvas.width = 500;
const height = canvas.height = 180;
const text = "Hello world";

const font_settings = {
  font: "800 40px sans-serif",
  textAlign: "center",
  textBaseline: "middle"
};
Object.assign( ctx, font_settings );

const text_pos = {
  x: width / 2,
  y: height / 2
};
let clicked = false;
onclick = e => clicked = true;

// grab the pixels data
const pixels_data = (() => {
  // draw once, untransformed on a new context
  // getting the image data of a context will mark it as
  // deaccelerated and since we only want to do this once
  // it's not a good idea to do it on our visible canvas
  // also it helps ensure it's the only thing being painted on the canvas
  const temp_canvas = canvas.cloneNode();
  const temp_ctx = temp_canvas.getContext("2d");
  Object.assign( temp_ctx, font_settings);  

  drawText(temp_ctx);
  const img_data = temp_ctx.getImageData( 0, 0, width, height );
  // Safari has issues releasing canvas buffers...
  temp_canvas.width = temp_canvas.height = 0;
  return new Uint32Array( img_data.data.buffer );
})();

const start_time = performance.now();

let viewport_point = new DOMPoint( 0, 0 );
onmousemove = ( { clientX, clientY } ) => {
  const canvas_bbox = canvas.getBoundingClientRect();
  viewport_point = new DOMPoint(
    clientX - canvas_bbox.left,
    clientY - canvas_bbox.top
  );
};

anim();

function anim( t ) {
  clear();
  updateTransform( t );
  checkCollision();
  drawText();
  requestAnimationFrame( anim );
}
function clear() {
  ctx.setTransform( 1, 0, 0, 1, 0, 0 );
  ctx.clearRect( 0, 0, width, height );
}
function updateTransform( t ) {
  const dur = 10000;
  const delta = (t - start_time) % dur;

  const pos = Math.PI * 2 / dur * delta;
  const angle = Math.cos( pos );
  const scale = Math.sin( pos ) * 4;
  ctx.translate( width / 2, height / 2 );
  ctx.rotate( angle );
  ctx.scale( scale, 1 );
  ctx.translate( -width / 2, -height / 2 );
}
function checkCollision() {
  const mat = ctx.getTransform().inverse();
  const { x, y } = mat.transformPoint( viewport_point );
  const index =  (Math.floor( y ) * width) + Math.floor( x );
  const collides = !!pixels_data[ index ];
  ctx.fillStyle = collides ? "green" : "blue";
}
function drawBoundingBox() {
  ctx.stroke( bbox_path );
}
// used by both 'ctx' and 'temp_ctx'
function drawText(context = ctx) {
  context.fillText( text, text_pos.x, text_pos.y );
}

<canvas></canvas>