OffscreenCanvas.getContext().fillText() 产生颠倒的文本。如何翻转它们？

Question

我使用OffscreenCanvas.getContext().fillText() 生成一些文本，然后使用OffscreenCanvas.transferToImageBitmap() 获取地图，但是当我将其用作threejs 项目的纹理时，它们被颠倒了。请查看显示字母“R”如何错误显示的图像。

我想我可以使用createImageBitmap() 和flipY 选项来更改文本的方向，但我没有从该功能中得到任何信息。没有崩溃，但没有显示任何文本。

你能告诉我其他的技巧吗？

Answer 1

使用 OffscreenCanvas 并将其传递给 THREE.CanvasTexture 似乎在工作人员中工作正常。设置texture.flipY = false也可以（默认为true）

在下面的示例中，texture.flipY = false 被注释掉了，但如果你取消注释，你会看到纹理翻转

// Three.js - OffscreenCanvas
// from https://threejsfundamentals.org/threejs/threejs-offscreencanvas.html

'use strict';  // eslint-disable-line

function main() {  /* eslint consistent-return: 0 */
  const canvas = document.querySelector('#c');
  if (!canvas.transferControlToOffscreen) {
    canvas.style.display = 'none';
    document.querySelector('#noOffscreenCanvas').style.display = '';
    return;
  }
  const offscreen = canvas.transferControlToOffscreen();
  const worker = new Worker(getWorkerBlob());
  worker.postMessage({type: 'main', canvas: offscreen}, [offscreen]);

  function sendSize() {
    worker.postMessage({
      type: 'size',
      width: canvas.clientWidth,
      height: canvas.clientHeight,
    });
  }

  window.addEventListener('resize', sendSize);
  sendSize();
}
main();





// ------
// Creates Blobs for the Worker Scripts so things can be self contained for snippets/JSFiddle/Codepen
//
function getWorkerBlob() {
  const idsToUrls = [];
  const scriptElements = [...document.querySelectorAll('script[type=x-worker]')];
  for (const scriptElement of scriptElements) {
    let text = scriptElement.text;
    for (const {id, url} of idsToUrls) {
      text = text.split(id).join(url);
    }
    const blob = new Blob([text], {type: 'application/javascript'});
    const url = URL.createObjectURL(blob);
    const id = scriptElement.id;
    idsToUrls.push({id, url});
  }
  return idsToUrls.pop().url;
}

body {
  margin: 0;
}
#c {
  width: 100vw;
  height: 100vh;
  display: block;
}
#noOffscreenCanvas {
  display: flex;
  width: 100vw;
  height: 100vh;
  align-items: center;
  justify-content: center;
  background: red;
  color: white;
}

<script id="worker-offscreencanvas-cubes.js" type="x-worker">
'use strict';  // eslint-disable-line

/* global importScripts, THREE */

importScripts('https://threejsfundamentals.org/threejs/resources/threejs/r112/build/three.min.js');

const state = {
  width: 300,   // canvas default
  height: 150,  // canvas default
};

function main(data) {
  const {canvas} = data;
  const renderer = new THREE.WebGLRenderer({canvas});

  state.width = canvas.width;
  state.height = canvas.height;

  const fov = 75;
  const aspect = 2; // the canvas default
  const near = 0.1;
  const far = 100;
  const camera = new THREE.PerspectiveCamera(fov, aspect, near, far);
  camera.position.z = 4;

  const scene = new THREE.Scene();

  {
    const color = 0xFFFFFF;
    const intensity = 1;
    const light = new THREE.DirectionalLight(color, intensity);
    light.position.set(-1, 2, 4);
    scene.add(light);
  }

  const boxWidth = 1;
  const boxHeight = 1;
  const boxDepth = 1;
  const geometry = new THREE.BoxGeometry(boxWidth, boxHeight, boxDepth);

  const offscreenCanvas = new OffscreenCanvas(256, 256);
  const ctx = offscreenCanvas.getContext('2d');
  ctx.fillStyle = '#FDB';
  ctx.fillRect(0, 0, 256, 256);
  ctx.font = '200px bold sans-serif';
  ctx.textAlign = 'center';
  ctx.textBaseline = 'middle';
  ctx.fillStyle = '#666';
  ctx.fillText('F', 128, 128);
  const texture = new THREE.CanvasTexture(offscreenCanvas);

// texture.flipY = false;

  function makeInstance(geometry, color, x) {
    const material = new THREE.MeshPhongMaterial({
      map: texture,
    });

    const cube = new THREE.Mesh(geometry, material);
    scene.add(cube);

    cube.position.x = x;

    return cube;
  }

  const cubes = [
    makeInstance(geometry, 0x44aa88, 0),
    makeInstance(geometry, 0x8844aa, -2),
    makeInstance(geometry, 0xaa8844, 2),
  ];

  function resizeRendererToDisplaySize(renderer) {
    const canvas = renderer.domElement;
    const width = state.width;
    const height = state.height;
    const needResize = canvas.width !== width || canvas.height !== height;
    if (needResize) {
      renderer.setSize(width, height, false);
    }
    return needResize;
  }

  function render(time) {
    time *= 0.001;

    if (resizeRendererToDisplaySize(renderer)) {
      camera.aspect = state.width / state.height;
      camera.updateProjectionMatrix();
    }

    cubes.forEach((cube, ndx) => {
      const speed = 1 + ndx * .1;
      const rot = time * speed;
      cube.rotation.x = rot;
      cube.rotation.y = rot;
    });

    renderer.render(scene, camera);

    requestAnimationFrame(render);
  }

  requestAnimationFrame(render);
}

function size(data) {
  state.width = data.width;
  state.height = data.height;
}

const handlers = {
  main,
  size,
};

self.onmessage = function(e) {
  const fn = handlers[e.data.type];
  if (!fn) {
    throw new Error('no handler for type: ' + e.data.type);
  }
  fn(e.data);
};

</script><canvas id="c"></canvas>
    <div id="noOffscreenCanvas" style="display:none;">
      <div>no OffscreenCanvas support</div>
    </div>
  

Answer 2

在您的threejs 纹理上设置 .flipY = false。由于涉及扫描线光栅的历史原因，OpenGL 纹理默认垂直翻转。