根据原始顶点在 WEBGL 片段着色器中进行裁剪

Question

我是 WEBGL 的新手，我阅读了很多关于片段着色器的帖子。但是，我无法弄清楚如何在片段着色器中访问原始顶点（或每个顶点的标志）。

如果片段的至少一个原始顶点被剪裁，我的目标是丢弃片段。

感谢您的帮助

Answer 1

您不能直接访问片段着色器中的顶点。默认情况下，片段着色器获取的唯一数据是屏幕坐标（画布/帧缓冲区坐标）和当前光栅化像素的深度缓冲区值。您必须传入的所有其他数据。

在我的脑海中，在顶点着色器中，您可以计算一个顶点是否被剪裁，并将该信息作为变量传递给片段着色器。如果没有被剪裁，你会传递 0，如果被剪裁，你会传递 1。变量会被插值，所以如果在片段着色器中变量 > 0，那么其中一个顶点被剪裁了。

const vs = `
attribute vec4 position;
uniform mat4 matrix;
varying float clipped;

void main() {
  gl_Position = matrix * position;
  clipped = (
      any(lessThan(gl_Position.xyz, -gl_Position.www)) ||
      any(greaterThan(gl_Position.xyz, gl_Position.www))
    ) ? 1.0
      : 0.0;
}
`;

const fs = `
precision highp float;
varying float clipped;
void main() {
  if (clipped > 0.0) {
    discard;
  }
  gl_FragColor = gl_FrontFacing ? vec4(1, 0, 0, 1) : vec4(0, 0, 1, 1);
}
`;

const m4 = twgl.m4;
const gl = document.querySelector('canvas').getContext('webgl');

// compile shaders, link program, look up locations
const prgInfo = twgl.createProgramInfo(gl, [vs, fs]);

// calls gl.createBuffer, gl.bindBuffer, gl.bufferData for each of
// position, normals, texcoord, indices of a sphere.
const bufferInfo = twgl.primitives.createSphereBufferInfo(gl, 1, 8, 8);

function render(time) {
  twgl.resizeCanvasToDisplaySize(gl.canvas);
  gl.enable(gl.DEPTH_TEST);

  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  // calls gl.bindBuffer, gl.enableVertexAttribArray, gl.vertexAttribPointer for each attribute
  twgl.setBuffersAndAttributes(gl, prgInfo, bufferInfo);

  gl.useProgram(prgInfo.program);

  const fov = 60 * Math.PI / 180;
  const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
  const near = 0.1;
  const far = 5.0;
  const mat = m4.perspective(fov, aspect, near, far);
  m4.translate(mat, [
      Math.sin(time / 1200), 
      Math.sin(time / 1300), 
      Math.sin(time / 1400) - 1.8,
  ], mat);
  m4.rotateX(mat, time / 1000, mat);
  m4.rotateY(mat, time / 1100, mat);

  // calls gl.uniform
  twgl.setUniforms(prgInfo, {
    matrix: mat, 
  });

  // calls gl.drawArrays or gl.drawElements
  twgl.drawBufferInfo(gl, bufferInfo);

  requestAnimationFrame(render);
}
requestAnimationFrame(render);

canvas { border: 1px solid black; }

<script src="https://twgljs.org/dist/4.x/twgl-full.min.js"></script>
<canvas></canvas>

可能有更有效的方法，例如将三角形的所有顶点传递给顶点着色器，如果其中任何一个被裁剪，则将所有顶点设置为裁剪，这样三角形甚至不会被光栅化。

const vs = `
attribute vec4 position;
attribute vec4 position1;  // second vertex for triangle
attribute vec4 position2;  // third vertex for triangle
uniform mat4 matrix;

bool vertexClipped(vec4 clipspace) {
  return any(lessThan(clipspace.xyz, -clipspace.www)) ||
         any(greaterThan(clipspace.xyz, clipspace.www));
}

void main() {
  gl_Position = matrix * position;
  vec4 clipPosition1 = matrix * position1;
  vec4 clipPosition2 = matrix * position2;
  
  bool clipped = vertexClipped(gl_Position) ||
                 vertexClipped(clipPosition1) ||
                 vertexClipped(clipPosition2);
                 
  if (clipped) {
     gl_Position = vec4(vec3(2), 1); // some offscreen value
  }
}
`;

const fs = `
precision highp float;
void main() {
  gl_FragColor = gl_FrontFacing ? vec4(1, 0, 0, 1) : vec4(0, 0, 1, 1);
}
`;

const m4 = twgl.m4;
const gl = document.querySelector('canvas').getContext('webgl');

// compile shaders, link program, look up locations
const prgInfo = twgl.createProgramInfo(gl, [vs, fs]);

const verts = twgl.primitives.deindexVertices(twgl.primitives.createSphereVertices(1, 8, 8));
// copy the positions
const position1 = new Float32Array(verts.position);
const position2 = new Float32Array(verts.position);
// shift the positions so we can pass all 3 vertices for each triangle
// to the vertex shader for each iteration
for (let i = 0; i < position1.length; i += 9) {
  { 
     // 0, 1, 2 => 1, 2, 0
     const temp = position1.slice(i, i + 3);
     position1.set(position1.slice(i + 3, i + 9), i);
     position1.set(temp, i + 6);
  }
  {
     // 0, 1, 2 => 2, 0, 1
     const temp = position2.slice(i + 6, i + 9);
     position2.set(position2.slice(i + 0, i + 6), i + 3);
     position2.set(temp, i);
  }  
}
verts.position1 = position1;
verts.position2 = position2;

// calls gl.createBuffer, gl.bindBuffer, gl.bufferData for each of
// position, normals, texcoord, indices of a sphere.
const bufferInfo = twgl.createBufferInfoFromArrays(gl, verts);

function render(time) {
  twgl.resizeCanvasToDisplaySize(gl.canvas);
  gl.enable(gl.DEPTH_TEST);

  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  // calls gl.bindBuffer, gl.enableVertexAttribArray, gl.vertexAttribPointer for each attribute
  twgl.setBuffersAndAttributes(gl, prgInfo, bufferInfo);

  gl.useProgram(prgInfo.program);

  const fov = 60 * Math.PI / 180;
  const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
  const near = 0.1;
  const far = 5.0;
  const mat = m4.perspective(fov, aspect, near, far);
  m4.translate(mat, [
      Math.sin(time / 1200), 
      Math.sin(time / 1300), 
      Math.sin(time / 1400) - 1.8,
  ], mat);
  m4.rotateX(mat, time / 1000, mat);
  m4.rotateY(mat, time / 1100, mat);

  // calls gl.uniform
  twgl.setUniforms(prgInfo, {
    matrix: mat, 
  });

  // calls gl.drawArrays or gl.drawElements
  twgl.drawBufferInfo(gl, bufferInfo);

  requestAnimationFrame(render);
}
requestAnimationFrame(render);

canvas { border: 1px solid black; }

<script src="https://twgljs.org/dist/4.x/twgl-full.min.js"></script>
<canvas></canvas>

可以说这会更有效，因为我们不是一次拒绝 1000 个像素，而是拒绝整个三角形，甚至跳过每个像素的检查。但它需要更多数据，因为我们需要能够同时将每个三角形的所有 3 个顶点传递给顶点着色器。

注意：如果您是 WebGL 新手，您可能会发现 these articles 很有用。