如何正确地将鼠标坐标传递给 WebGL？

Question

我想将 canvas 鼠标坐标传递给一个函数，该函数以交互方式生成以鼠标坐标为中心的圆。因此，我使用以下函数进行标准化：

var mousePositionX = (2*ev.clientX/canvas.width) - 1;
var mousePositionY = (2*ev.clientY/(canvas.height*-1)) + 1;

但是，这仅适用于屏幕中心。当围绕光标移动鼠标时，光标不再位于圆的中心： see the picture here

鼠标光标离屏幕中心越远，越偏离圆心。这是一些相关代码：

HTML

  body {
    border: 0;
    margin: 0;
  }
  /* make the canvas the size of the viewport */
  canvas {
    width: 100vw;
    height: 100vh;
    display: block;
  }
  ...
  <body onLoad="main()">
        <canvas id="glContext"></canvas>
  </body>

着色器

<script id="vShaderCircle" type="notjs">
    attribute vec4 a_position;
    uniform mat4 u_viewMatrix;

    void main(){
        gl_Position = u_viewMatrix * a_position;
    }
</script>

JS

function main(){

    // PREPARING CANVAS AND WEBGL-CONTEXT
    var canvas = document.getElementById("glContext");
    var gl_Original = initWebGL(canvas);
    var gl = WebGLDebugUtils.makeDebugContext(gl_Original);

    resize(canvas);
    gl.viewport(0, 0, canvas.width, canvas.height);
    // ----------------------------------
    ...
    // MATRIX SETUP
    var viewMatrix = new Matrix4();
      viewMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100);
      viewMatrix.lookAt(0, 0, 5, 0, 0, 0, 0, 1, 0);
    // ----------------------------------
    canvas.addEventListener("mousemove", function(){stencilTest(event)});

    function stencilTest(ev){
        var mousePositionX = (2*ev.clientX/canvas.width) - 1;
        var mousePositionY = (2*ev.clientY/(canvas.height*(-1))) + 1;
        ...
        ...
        drawCircle(..., mousePositionX, mousePositionY, viewMatrix);
        ...
        drawCube(...);
    }
}

我该如何解决这个问题？

Answer 1

假设这在 mousemove 回调事件中被调用并且 canvas 被定义为对 HTML CANVAS 元素的正确引用，鼠标的相对位置指向 canvas space 的指针应该是：

var rect = gl.canvas.getBoundingClientRect();
var mousePositionX = ev.clientX - rect.left;
var mousePositionY = ev.clientY - rect.top;

从像素坐标转换到 WebGL 坐标系：

var rect = gl.canvas.getBoundingClientRect();
var x = (ev.clientX - rect.left) / canvas.width *  2 - 1;
var y = (ev.clientY - rect.top) / canvas.height * -2 + 1;

Answer 2

只需将您的 mousemove 事件绑定到 canvas 本身并使用 offsetX and offsetY

var mouseX = (e.offsetX / canvas.clientWidth)*2-1;
var mouseY = ((canvas.clientHeight - e.offsetY) / canvas.clientHeight)*2-1;

请注意，这完全取决于您在着色器中进行的变换。

Answer 3

这其实是一个far more complicated issue than it sounds。您的 canvas 的显示大小是否与其绘图缓冲区相同？你的 canvas 上有边框吗？

假设您的 canvas.

上没有边框或任何填充，这里有一些代码将为您提供 canvas 相对像素坐标

function getRelativeMousePosition(event, target) {
  target = target || event.target;
  var rect = target.getBoundingClientRect();

  return {
    x: event.clientX - rect.left,
    y: event.clientY - rect.top,
  }
}

// assumes target or event.target is canvas
function getNoPaddingNoBorderCanvasRelativeMousePosition(event, target) {
  target = target || event.target;
  var pos = getRelativeMousePosition(event, target);

  pos.x = pos.x * target.width  / target.clientWidth;
  pos.y = pos.y * target.height / target.clientHeight;

  return pos;  
}

将其转换为 WebGL 坐标

  var pos = getRelativeMousePosition(event, target);
  const x = pos.x / gl.canvas.width  *  2 - 1;
  const y = pos.y / gl.canvas.height * -2 + 1;

工作示例：

function getRelativeMousePosition(event, target) {
  target = target || event.target;
  var rect = target.getBoundingClientRect();

  return {
    x: event.clientX - rect.left,
    y: event.clientY - rect.top,
  }
}

// assumes target or event.target is canvas
function getNoPaddingNoBorderCanvasRelativeMousePosition(event, target) {
  target = target || event.target;
  var pos = getRelativeMousePosition(event, target);

  pos.x = pos.x * target.width  / target.clientWidth;
  pos.y = pos.y * target.height / target.clientHeight;

  return pos;  
}

const vs = `
attribute vec4 position;
void main() {
  gl_Position = position;
  gl_PointSize = 20.;
}
`;
const fs = `
void main() {
  gl_FragColor = vec4(1,0,1,1);
}
`;
const gl = document.querySelector("canvas").getContext("webgl");
// compiles and links shaders and assigns position to location 
const program = twgl.createProgramFromSources(gl, [vs, fs]);
const positionLoc = gl.getAttribLocation(program, "position");

window.addEventListener('mousemove', e => {

  const pos = getNoPaddingNoBorderCanvasRelativeMousePosition(e, gl.canvas);

  // pos is in pixel coordinates for the canvas.
  // so convert to WebGL clip space coordinates
  const x = pos.x / gl.canvas.width  *  2 - 1;
  const y = pos.y / gl.canvas.height * -2 + 1;

  gl.clear(gl.COLOR_BUFFER_BIT);
  gl.useProgram(program);
  // only drawing a single point so no need to use a buffer
  gl.vertexAttrib2f(positionLoc, x, y);
  gl.drawArrays(gl.POINTS, 0, 1);
});

canvas { 
  display: block;
  width: 400px;
  height: 100px;
}
div {
  display: inline-block;
  border: 1px solid black;
}

<div><canvas></canvas></div>
<p>move the mouse over the canvas</p>
<script src="https://twgljs.org/dist/3.x/twgl-full.min.js"></script>

注意这里没有涉及矩阵。如果您使用的是矩阵，那么您已经定义了自己的 space，而不是 WebGL 的 space，后者始终是剪辑 space。在这种情况下，您要么需要乘以矩阵的逆矩阵，然后在 -1 和 +1 之间选择您想要的任何 Z 值。这样，当您的位置乘以着色器中使用的矩阵时，它会将位置反转回正确的 webgl 剪辑 space 坐标。或者，您需要摆脱矩阵或将它们设置为恒等式。

这是一个例子，注意我没有 have/know 你的数学库，所以你必须翻译成你的

function getRelativeMousePosition(event, target) {
  target = target || event.target;
  var rect = target.getBoundingClientRect();

  return {
    x: event.clientX - rect.left,
    y: event.clientY - rect.top,
  }
}

// assumes target or event.target is canvas
function getNoPaddingNoBorderCanvasRelativeMousePosition(event, target) {
  target = target || event.target;
  var pos = getRelativeMousePosition(event, target);

  pos.x = pos.x * target.width  / target.clientWidth;
  pos.y = pos.y * target.height / target.clientHeight;

  return pos;  
}

const vs = `
attribute vec4 position;
uniform mat4 matrix;
void main() {
  gl_Position = matrix * position;
}
`;
const fs = `
void main() {
  gl_FragColor = vec4(1,0,0,1);
}
`;
const m4 = twgl.m4;
const gl = document.querySelector("canvas").getContext("webgl");
const programInfo = twgl.createProgramInfo(gl, [vs, fs]);
const bufferInfo = twgl.primitives.createSphereBufferInfo(gl, .5, 12, 8);

window.addEventListener('mousemove', e => {

  const pos = getNoPaddingNoBorderCanvasRelativeMousePosition(e, gl.canvas);

  // pos is in pixel coordinates for the canvas.
  // so convert to WebGL clip space coordinates
  const x = pos.x / gl.canvas.width  *  2 - 1;
  const y = pos.y / gl.canvas.height * -2 + 1;
  
  // use a projection and view matrix
  const projection = m4.perspective(
     30 * Math.PI / 180, 
     gl.canvas.clientWidth / gl.canvas.clientHeight, 
     1, 
     100);
  const camera = m4.lookAt([0, 0, 15], [0, 0, 0], [0, 1, 0]);
  const view = m4.inverse(camera);
  const viewProjection = m4.multiply(projection, view);
  
  // pick a clipsace Z value between -1 and 1 
  // we'll zNear to zFar and convert back to clip space
  const viewZ = -5;  // 5 units back from the camera
  const clip = m4.transformPoint(projection, [0, 0, viewZ]);
  const z = clip[2];
  
  // compute the world space position needed to put the sphere
  // under the cursor at this clipspace position
  const inverseViewProjection = m4.inverse(viewProjection);
  const worldPos = m4.transformPoint(inverseViewProjection, [x, y, z]);

  // add that world position to our matrix
  const mat = m4.translate(viewProjection, worldPos);

  gl.clear(gl.COLOR_BUFFER_BIT);
  gl.useProgram(programInfo.program);
  
  twgl.setBuffersAndAttributes(gl, programInfo, bufferInfo);
  twgl.setUniforms(programInfo, {
    matrix: mat,
  });
  gl.drawElements(gl.LINES, bufferInfo.numElements, gl.UNSIGNED_SHORT, 0);
});

canvas { 
  display: block;
  width: 400px;
  height: 100px;
}
div {
  display: inline-block;
  border: 1px solid black;
}

<div><canvas></canvas></div>
<p>move the mouse over the canvas</p>
<script src="https://twgljs.org/dist/3.x/twgl-full.min.js"></script>

另请注意，我故意使 canvas 的显示大小与其绘图缓冲区大小不匹配，以显示数学计算结果。

如何正确地将鼠标坐标传递给 WebGL？

How to correctly pass mouse coordinates to WebGL?

javascript

webgl

coordinate-systems