如何在 OpenGL ES 2 或 3 上进行剪裁平面?

Question

在iOS或Android上，我可以选择切换到OpenGL ES 2或3。 我不确定为什么我会这样做，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，这些在ES 2中被删除。

我特别想避免在剪切平面的矢量更改时重新编译任何着色器代码。

Answer 1

在OpenGL ES中，你必须使用EXT_clip_cull_distance扩展。

在顶点着色器中，您必须设置gl_ClipDistance[i]输出。gl_ClipDistance[i]指定每个用户剪辑平面i的剪辑距离。其中 0.0 表示顶点位于裁剪平面上，> 0.0 表示顶点位于裁剪平面内。

必须启用剪切功能。
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(将其想象为平面的法线向量和到原点的距离)：

#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);
}

另请参阅顶点后处理 - 用户定义的裁剪和小蚱蜢 - 裁剪平面教程。

---

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。请参阅片段着色器 - 特殊操作。

另请参阅 OpenGL ES 着色语言 1.00 规范; 6.4 跳转; 第 58 页：

丢弃关键字只允许在片段着色器中使用。它可以在片段着色器中用于放弃对当前片段的操作。此关键字会导致片段被丢弃，并且不会对任何缓冲区进行更新。它通常在条件语句中使用，例如：

if (intensity < 0.0)
discard;

下面的WebGL示例演示了这一点。请注意，WebGL 1.0 上下文与 OpenGL ES 2.0 API 非常一致。

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;" width="100%" height="100%"></canvas>