如何在 OpenGL ES 2 或 3 上进行剪切平面？

Posted 2023-03-23

【中文标题】如何在 OpenGL ES 2 或 3 上进行剪切平面？

【英文标题】：How to do clipping planes on OpenGL ES 2 or 3?

【发布时间】：2018-08-12 01:10:19

【问题描述】：

在 ios 或 android 上，我可以选择切换到 OpenGL ES 2 或 3。 我不确定我为什么要这样做，因为我不是在开发 3D 游戏，但如果我要切换，我将如何实现剪切平面？据称这些已在 ES 2 中删除。

我特别希望避免在剪裁平面的矢量发生变化时重新编译任何着色器代码。

【问题讨论】：

“我不知道为什么要这么做，因为我不是在开发 3D 游戏，但如果我要切换，我将如何实现剪切平面？” - 听起来你是二维的。如果你的剪辑是屏幕对齐的，最简单和最有效的选择是使用剪刀矩形（即使你坚持使用 GLES 1.1，你也应该这样做）。否则，在发布顶点之前在 CPU 上进行裁剪可能比裁剪平面或等效项更有效，但有点繁琐。

【参考方案1】：

在 OpenGL ES 中，您必须使用 EXT_clip_cull_distance 扩展名。

在顶点着色器中，您必须设置gl_ClipDistance[i] 输出。 gl_ClipDistance[i] 为每个用户裁剪平面 i 指定一个裁剪距离。其中 0.0 表示顶点在剪切平面上，> 0.0 表示顶点在剪切平面内。

必须启用剪辑功能。例如

glEnable( CLIP_DISTANCE0_EXT );

使用gl_ClipDistance 的顶点着色器可能如下所示，其中u_clipPlane 是Ax + By + Cz + D 形式的平面方程（将其想象为平面和到原点的距离）：

#extension GL_EXT_clip_cull_distance : enable

precision highp float;

attribute vec3 inPos;   

uniform mat4 u_projectionMat44;
uniform mat4 u_viewMat44;
uniform mat4 u_modelMat44;
uniform vec4 u_clipPlane;

void main()
   
    vec4 modelPos      = u_modelMat44 * vec4( inPos, 1.0 );
    gl_Position        = u_projectionMat44 * u_viewMat44 * modelPos;
    gl_ClipDistance[0] = dot(modelPos, u_clipPlane);

另请参阅 Vertex Post-Processing - User-defined clipping 和 The little Grasshopper - Clip Planes 教程。

---

如果扩展不可用，则可以通过丢弃片段在片段着色器中模拟剪辑。见Fragment Shader - Special operations。

另见OpenGL ES Shading Language 1.00 Specification; 6.4 Jumps; page 58：

discard 关键字只允许在片段着色器中使用。可以在片段着色器中使用它来放弃对当前片段的操作。此关键字导致片段被丢弃，并且不会更新任何缓冲区。它通常在条件语句中使用，例如：

if (intensity < 0.0)
    discard;

以下 WebGL 示例演示了这一点。请注意，WebGL 1.0 上下文非常符合 OpenGL ES 2.0 API。

var readInput = true;
  function changeEventHandler(event)
    readInput = true;
  
  
  (function loadscene() 
  
  var gl, progDraw, vp_size;
  var bufCube = ;
  var clip = 0.0;
  
  function render(delteMS)

      if ( readInput ) 
          //readInput = false;
          clip = (document.getElementById( "clip" ).value - 50) / 50;
      

      Camera.create();
      Camera.vp = vp_size;
          
      gl.viewport( 0, 0, vp_size[0], vp_size[1] );
      gl.enable( gl.DEPTH_TEST );
      gl.clearColor( 0.0, 0.0, 0.0, 1.0 );
      gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );

      // set up draw shader
      ShaderProgram.Use( progDraw );
      ShaderProgram.SetUniformM44( progDraw, "u_projectionMat44", Camera.Perspective() );
      ShaderProgram.SetUniformM44( progDraw, "u_viewMat44", Camera.LookAt() );
      var modelMat = IdentityMat44()
      modelMat = RotateAxis( modelMat, CalcAng( delteMS, 13.0 ), 0 );
      modelMat = RotateAxis( modelMat, CalcAng( delteMS, 17.0 ), 1 );
      ShaderProgram.SetUniformM44( progDraw, "u_modelMat44", modelMat );
      ShaderProgram.SetUniformF4( progDraw, "u_clipPlane", [1.0,-1.0,0.0,clip*1.7321] );
      
      // draw scene
      VertexBuffer.Draw( bufCube );

      requestAnimationFrame(render);
  
  
  function resize() 
      //vp_size = [gl.drawingBufferWidth, gl.drawingBufferHeight];
      vp_size = [window.innerWidth, window.innerHeight]
      canvas.width = vp_size[0];
      canvas.height = vp_size[1];
  
  
  function initScene() 
  
      canvas = document.getElementById( "canvas");
      gl = canvas.getContext( "experimental-webgl" );
      //gl = canvas.getContext( "webgl2" );
      if ( !gl )
        return null;
      
      /*
      var ext_frag_depth = gl.getExtension( "EXT_clip_cull_distance" );  // gl_ClipDistance gl_CullDistance
      if (!ext_frag_depth)
          alert('no gl_ClipDistance and gl_CullDistance support');
      */

      progDraw = ShaderProgram.Create( 
        [  source : "draw-shader-vs", stage : gl.VERTEX_SHADER ,
           source : "draw-shader-fs", stage : gl.FRAGMENT_SHADER 
        ] );
      if ( !progDraw.progObj )
          return null;
      progDraw.inPos = ShaderProgram.AttributeIndex( progDraw, "inPos" );
      progDraw.inNV  = ShaderProgram.AttributeIndex( progDraw, "inNV" );
      progDraw.inCol = ShaderProgram.AttributeIndex( progDraw, "inCol" );
      
      // create cube
      var cubePos = [
        -1.0, -1.0,  1.0,  1.0, -1.0,  1.0,  1.0,  1.0,  1.0, -1.0,  1.0,  1.0,
        -1.0, -1.0, -1.0,  1.0, -1.0, -1.0,  1.0,  1.0, -1.0, -1.0,  1.0, -1.0 ];
      var cubeCol = [ 1.0, 0.0, 0.0, 1.0, 0.5, 0.0, 1.0, 0.0, 1.0, 1.0, 1.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0 ];
      var cubeHlpInx = [ 0, 1, 2, 3, 1, 5, 6, 2, 5, 4, 7, 6, 4, 0, 3, 7, 3, 2, 6, 7, 1, 0, 4, 5 ];  
      var cubePosData = [];
      for ( var i = 0; i < cubeHlpInx.length; ++ i ) 
        cubePosData.push( cubePos[cubeHlpInx[i]*3], cubePos[cubeHlpInx[i]*3+1], cubePos[cubeHlpInx[i]*3+2] );
      
      var cubeNVData = [];
      for ( var i1 = 0; i1 < cubeHlpInx.length; i1 += 4 ) 
      var nv = [0, 0, 0];
      for ( i2 = 0; i2 < 4; ++ i2 ) 
          var i = i1 + i2;
          nv[0] += cubePosData[i*3]; nv[1] += cubePosData[i*3+1]; nv[2] += cubePosData[i*3+2];
      
      for ( i2 = 0; i2 < 4; ++ i2 )
        cubeNVData.push( nv[0], nv[1], nv[2] );
      
      var cubeColData = [];
      for ( var is = 0; is < 6; ++ is ) 
        for ( var ip = 0; ip < 4; ++ ip ) 
         cubeColData.push( cubeCol[is*3], cubeCol[is*3+1], cubeCol[is*3+2] ); 
        
      
      var cubeInxData = [];
      for ( var i = 0; i < cubeHlpInx.length; i += 4 ) 
        cubeInxData.push( i, i+1, i+2, i, i+2, i+3 );   
      
      bufCube = VertexBuffer.Create(
      [  data : cubePosData, attrSize : 3, attrLoc : progDraw.inPos ,
         data : cubeNVData,  attrSize : 3, attrLoc : progDraw.inNV ,
         data : cubeColData, attrSize : 3, attrLoc : progDraw.inCol  ],
        cubeInxData );
        
      window.onresize = resize;
      resize();
      requestAnimationFrame(render);
  
  
  function Fract( val )  
      return val - Math.trunc( val );
  
  function CalcAng( deltaTime, intervall ) 
      return Fract( deltaTime / (1000*intervall) ) * 2.0 * Math.PI;
  
  function CalcMove( deltaTime, intervall, range ) 
      var pos = self.Fract( deltaTime / (1000*intervall) ) * 2.0
      var pos = pos < 1.0 ? pos : (2.0-pos)
      return range[0] + (range[1] - range[0]) * pos;
      
  function EllipticalPosition( a, b, angRag ) 
      var a_b = a * a - b * b
      var ea = (a_b <= 0) ? 0 : Math.sqrt( a_b );
      var eb = (a_b >= 0) ? 0 : Math.sqrt( -a_b );
      return [ a * Math.sin( angRag ) - ea, b * Math.cos( angRag ) - eb, 0 ];
  
  
  glArrayType = typeof Float32Array !="undefined" ? Float32Array : ( typeof WebGLFloatArray != "undefined" ? WebGLFloatArray : Array );
  
  function IdentityMat44() 
    var m = new glArrayType(16);
    m[0]  = 1; m[1]  = 0; m[2]  = 0; m[3]  = 0;
    m[4]  = 0; m[5]  = 1; m[6]  = 0; m[7]  = 0;
    m[8]  = 0; m[9]  = 0; m[10] = 1; m[11] = 0;
    m[12] = 0; m[13] = 0; m[14] = 0; m[15] = 1;
    return m;
  ;
  
  function RotateAxis(matA, angRad, axis) 
      var aMap = [ [1, 2], [2, 0], [0, 1] ];
      var a0 = aMap[axis][0], a1 = aMap[axis][1]; 
      var sinAng = Math.sin(angRad), cosAng = Math.cos(angRad);
      var matB = new glArrayType(16);
      for ( var i = 0; i < 16; ++ i ) matB[i] = matA[i];
      for ( var i = 0; i < 3; ++ i ) 
          matB[a0*4+i] = matA[a0*4+i] * cosAng + matA[a1*4+i] * sinAng;
          matB[a1*4+i] = matA[a0*4+i] * -sinAng + matA[a1*4+i] * cosAng;
      
      return matB;
  
  
  function Cross( a, b )  return [ a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0], 0.0 ]; 
  function Dot( a, b )  return a[0]*b[0] + a[1]*b[1] + a[2]*b[2]; 
  function Normalize( v ) 
      var len = Math.sqrt( v[0] * v[0] + v[1] * v[1] + v[2] * v[2] );
      return [ v[0] / len, v[1] / len, v[2] / len ];
  
  
  var Camera = ;
  Camera.create = function() 
      this.pos    = [0, 3, 0.0];
      this.target = [0, 0, 0];
      this.up     = [0, 0, 1];
      this.fov_y  = 90;
      this.vp     = [800, 600];
      this.near   = 0.5;
      this.far    = 100.0;
  
  Camera.Perspective = function() 
      var fn = this.far + this.near;
      var f_n = this.far - this.near;
      var r = this.vp[0] / this.vp[1];
      var t = 1 / Math.tan( Math.PI * this.fov_y / 360 );
      var m = IdentityMat44();
      m[0]  = t/r; m[1]  = 0; m[2]  =  0;                              m[3]  = 0;
      m[4]  = 0;   m[5]  = t; m[6]  =  0;                              m[7]  = 0;
      m[8]  = 0;   m[9]  = 0; m[10] = -fn / f_n;                       m[11] = -1;
      m[12] = 0;   m[13] = 0; m[14] = -2 * this.far * this.near / f_n; m[15] =  0;
      return m;
  
  Camera.LookAt = function() 
      var mz = Normalize( [ this.pos[0]-this.target[0], this.pos[1]-this.target[1], this.pos[2]-this.target[2] ] );
      var mx = Normalize( Cross( this.up, mz ) );
      var my = Normalize( Cross( mz, mx ) );
      var tx = Dot( mx, this.pos );
      var ty = Dot( my, this.pos );
      var tz = Dot( [-mz[0], -mz[1], -mz[2]], this.pos ); 
      var m = IdentityMat44();
      m[0]  = mx[0]; m[1]  = my[0]; m[2]  = mz[0]; m[3]  = 0;
      m[4]  = mx[1]; m[5]  = my[1]; m[6]  = mz[1]; m[7]  = 0;
      m[8]  = mx[2]; m[9]  = my[2]; m[10] = mz[2]; m[11] = 0;
      m[12] = tx;    m[13] = ty;    m[14] = tz;    m[15] = 1; 
      return m;
   
  
  var ShaderProgram = ;
  ShaderProgram.Create = function( shaderList ) 
      var shaderObjs = [];
      for ( var i_sh = 0; i_sh < shaderList.length; ++ i_sh ) 
          var shderObj = this.CompileShader( shaderList[i_sh].source, shaderList[i_sh].stage );
          if ( shderObj == 0 )
              return 0;
          shaderObjs.push( shderObj );
      
      var prog = 
      prog.progObj = this.LinkProgram( shaderObjs )
      if ( prog.progObj ) 
          prog.attribIndex = ;
          var noOfAttributes = gl.getProgramParameter( prog.progObj, gl.ACTIVE_ATTRIBUTES );
          for ( var i_n = 0; i_n < noOfAttributes; ++ i_n ) 
              var name = gl.getActiveAttrib( prog.progObj, i_n ).name;
              prog.attribIndex[name] = gl.getAttribLocation( prog.progObj, name );
          
          prog.unifomLocation = ;
          var noOfUniforms = gl.getProgramParameter( prog.progObj, gl.ACTIVE_UNIFORMS );
          for ( var i_n = 0; i_n < noOfUniforms; ++ i_n ) 
              var name = gl.getActiveUniform( prog.progObj, i_n ).name;
              prog.unifomLocation[name] = gl.getUniformLocation( prog.progObj, name );
          
      
      return prog;
  
  ShaderProgram.AttributeIndex = function( prog, name )  return prog.attribIndex[name];  
  ShaderProgram.UniformLocation = function( prog, name )  return prog.unifomLocation[name];  
  ShaderProgram.Use = function( prog )  gl.useProgram( prog.progObj );  
  ShaderProgram.SetUniformI1  = function( prog, name, val )  if(prog.unifomLocation[name]) gl.uniform1i( prog.unifomLocation[name], val ); 
  ShaderProgram.SetUniformF1  = function( prog, name, val )  if(prog.unifomLocation[name]) gl.uniform1f( prog.unifomLocation[name], val ); 
  ShaderProgram.SetUniformF2  = function( prog, name, arr )  if(prog.unifomLocation[name]) gl.uniform2fv( prog.unifomLocation[name], arr ); 
  ShaderProgram.SetUniformF3  = function( prog, name, arr )  if(prog.unifomLocation[name]) gl.uniform3fv( prog.unifomLocation[name], arr ); 
  ShaderProgram.SetUniformF4  = function( prog, name, arr )  if(prog.unifomLocation[name]) gl.uniform4fv( prog.unifomLocation[name], arr ); 
  ShaderProgram.SetUniformM33 = function( prog, name, mat )  if(prog.unifomLocation[name]) gl.uniformMatrix3fv( prog.unifomLocation[name], false, mat ); 
  ShaderProgram.SetUniformM44 = function( prog, name, mat )  if(prog.unifomLocation[name]) gl.uniformMatrix4fv( prog.unifomLocation[name], false, mat ); 
  ShaderProgram.CompileShader = function( source, shaderStage ) 
      var shaderScript = document.getElementById(source);
      if (shaderScript)
        source = shaderScript.text;
      var shaderObj = gl.createShader( shaderStage );
      gl.shaderSource( shaderObj, source );
      gl.compileShader( shaderObj );
      var status = gl.getShaderParameter( shaderObj, gl.COMPILE_STATUS );
      if ( !status ) alert(gl.getShaderInfoLog(shaderObj));
      return status ? shaderObj : null;
   
  ShaderProgram.LinkProgram = function( shaderObjs ) 
      var prog = gl.createProgram();
      for ( var i_sh = 0; i_sh < shaderObjs.length; ++ i_sh )
          gl.attachShader( prog, shaderObjs[i_sh] );
      gl.linkProgram( prog );
      status = gl.getProgramParameter( prog, gl.LINK_STATUS );
      if ( !status ) alert("Could not initialise shaders");
      gl.useProgram( null );
      return status ? prog : null;
  
  
  var VertexBuffer = ;
  VertexBuffer.Create = function( attributes, indices ) 
      var buffer = ;
      buffer.buf = [];
      buffer.attr = []
      for ( var i = 0; i < attributes.length; ++ i ) 
          buffer.buf.push( gl.createBuffer() );
          buffer.attr.push(  size : attributes[i].attrSize, loc : attributes[i].attrLoc  );
          gl.bindBuffer( gl.ARRAY_BUFFER, buffer.buf[i] );
          gl.bufferData( gl.ARRAY_BUFFER, new Float32Array( attributes[i].data ), gl.STATIC_DRAW );
      
      buffer.inx = gl.createBuffer();
      gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, buffer.inx );
      gl.bufferData( gl.ELEMENT_ARRAY_BUFFER, new Uint16Array( indices ), gl.STATIC_DRAW );
      buffer.inxLen = indices.length;
      gl.bindBuffer( gl.ARRAY_BUFFER, null );
      gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, null );
      return buffer;
  
  VertexBuffer.Draw = function( bufObj ) 
    for ( var i = 0; i < bufObj.buf.length; ++ i ) 
          gl.bindBuffer( gl.ARRAY_BUFFER, bufObj.buf[i] );
          gl.vertexAttribPointer( bufObj.attr[i].loc, bufObj.attr[i].size, gl.FLOAT, false, 0, 0 );
          gl.enableVertexAttribArray( bufObj.attr[i].loc );
      
      gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufObj.inx );
      gl.drawElements( gl.TRIANGLES, bufObj.inxLen, gl.UNSIGNED_SHORT, 0 );
      for ( var i = 0; i < bufObj.buf.length; ++ i )
         gl.disableVertexAttribArray( bufObj.attr[i].loc );
      gl.bindBuffer( gl.ARRAY_BUFFER, null );
      gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, null );
  
  
  initScene();
  
  )();

html,body 
    height: 100%;
    width: 100%;
    margin: 0;
    overflow: hidden;


#gui 
    position : absolute;
    top : 0;
    left : 0;

<script id="draw-shader-vs" type="x-shader/x-vertex">
    precision highp float;
    
    attribute vec3 inPos;
    attribute vec3 inNV;
    attribute vec3 inCol;
  
    varying vec3  vertPos;
    varying vec3  vertNV;
    varying vec3  vertCol;
    varying float clip_distance;
    
    uniform mat4 u_projectionMat44;
    uniform mat4 u_viewMat44;
    uniform mat4 u_modelMat44;
    uniform vec4 u_clipPlane;
    
    void main()
       
        mat4 mv       = u_viewMat44 * u_modelMat44; 
        vertCol       = inCol;
        vertNV        = normalize(mat3(mv) * inNV);
        vec4 viewPos  = mv * vec4( inPos, 1.0 );
        vertPos       = viewPos.xyz;
        gl_Position   = u_projectionMat44 * viewPos;

        vec4 modelPos  = u_modelMat44 * vec4( inPos, 1.0 );
        vec4 clipPlane = vec4(normalize(u_clipPlane.xyz), u_clipPlane.w); 
        clip_distance  = dot(modelPos, clipPlane);
    
</script>
  
<script id="draw-shader-fs" type="x-shader/x-fragment">
    precision mediump float;

    varying vec3  vertPos;
    varying vec3  vertNV;
    varying vec3  vertCol;
    varying float clip_distance;
    
    void main()
    
        if ( clip_distance < 0.0 )
            discard;
        vec3 color   = vertCol;
        gl_FragColor = vec4( color.rgb, 1.0 );
     
</script>

<div>
    <form id="gui" name="inputs">
        <table>
            <tr> <td> <font color= #CCF>clipping</font> </td> 
                 <td> <input type="range" id="clip" min="0" max="100" value="50" onchange="changeEventHandler(event);"/></td> </tr>
        </table>
    </form>
</div>


<canvas id="canvas" style="border: none;"  ></canvas>