在WebGL polygonOffset中是否存在錯誤或者我錯過了什麼？

Question

我看到polygonOffset最初工作時很奇怪的行爲，但如果我重新渲染它停止工作。

我舉了一個簡單的例子來說明它。我從WebGL編程指南（https://sites.google.com/site/webglbook/）的Ch7中的z-fighting示例開始。然後我分離出了​​渲染部分並將其包裝在一個函數中。然後，我點擊一個HTML按鈕來調用render（）函數。在第一次點擊時，三角形呈現正確，沒有問題。在第二次點擊時，就像polygonOffset再次關閉。

我已經嘗試了許多不同的變化，包括每次重新啓用，禁用和重新啓用，更改偏移，但我不斷得到相同的行爲。有任何想法嗎？

我包括代碼，雖然代碼片段不會運行，我不會運行沒有本書的庫。

// Zfighting.js (c) 2012 matsuda 
 
// Vertex shader program 
 
var VSHADER_SOURCE = 
 
    'attribute vec4 a_Position;\n' + 
 
    'attribute vec4 a_Color;\n' + 
 
    'uniform mat4 u_ViewProjMatrix;\n' + 
 
    'varying vec4 v_Color;\n' + 
 
    'void main() {\n' + 
 
    ' gl_Position = u_ViewProjMatrix * a_Position;\n' + 
 
    ' v_Color = a_Color;\n' + 
 
    '}\n'; 
 

 
// Fragment shader program 
 
var FSHADER_SOURCE = 
 
    '#ifdef GL_ES\n' + 
 
    'precision mediump float;\n' + 
 
    '#endif\n' + 
 
    'varying vec4 v_Color;\n' + 
 
    'void main() {\n' + 
 
    ' gl_FragColor = v_Color;\n' + 
 
    '}\n'; 
 

 
function main() { 
 
    // Retrieve <canvas> element 
 
    var canvas = document.getElementById('webgl'); 
 

 
    // Get the rendering context for WebGL 
 
    var gl = getWebGLContext(canvas); 
 
    if (!gl) { 
 
    console.log('Failed to get the rendering context for WebGL'); 
 
    return; 
 
    } 
 

 
    // Initialize shaders 
 
    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) { 
 
    console.log('Failed to intialize shaders.'); 
 
    return; 
 
    } 
 

 
    // Set the vertex coordinates and color (the blue triangle is in the front) 
 
    var n = initVertexBuffers(gl); 
 
    if (n < 0) { 
 
    console.log('Failed to set the vertex information'); 
 
    return; 
 
    } 
 

 
    //Set clear color and enable the hidden surface removal function 
 
    gl.clearColor(0, 0, 0, 1); 
 
    gl.enable(gl.DEPTH_TEST); 
 

 
    // Get the storage locations of u_ViewProjMatrix 
 
    var u_ViewProjMatrix = gl.getUniformLocation(gl.program, 'u_ViewProjMatrix'); 
 
    if (!u_ViewProjMatrix) { 
 
    console.log('Failed to get the storage locations of u_ViewProjMatrix'); 
 
    return; 
 
    } 
 

 
    var viewProjMatrix = new Matrix4(); 
 
    // Set the eye point, look-at point, and up vector. 
 
    viewProjMatrix.setPerspective(30, canvas.width/canvas.height, 1, 100); 
 
    viewProjMatrix.lookAt(3.06, 2.5, 10.0, 0, 0, -2, 0, 1, 0); 
 

 
    // Pass the view projection matrix to u_ViewProjMatrix 
 
    gl.uniformMatrix4fv(u_ViewProjMatrix, false, viewProjMatrix.elements); 
 

 
    // Enable the polygon offset function 
 
    gl.enable(gl.POLYGON_OFFSET_FILL); 
 
    
 
    function render() { 
 
    // Clear color and depth buffer 
 
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT); 
 

 
    
 
    // Draw the triangles 
 
    gl.drawArrays(gl.TRIANGLES, 0, n/2); // The green triangle 
 
    gl.polygonOffset(1.0, 1.0);   // Set the polygon offset 
 
    gl.drawArrays(gl.TRIANGLES, n/2, n/2); // The yellow triangle 
 
} 
 
    
 
document.getElementById("button").onclick = render; 
 
} 
 

 
function initVertexBuffers(gl) { 
 
    var verticesColors = new Float32Array([ 
 
    // Vertex coordinates and color 
 
    0.0, 2.5, -5.0, 0.4, 1.0, 0.4, // The green triangle 
 
    -2.5, -2.5, -5.0, 0.4, 1.0, 0.4, 
 
    2.5, -2.5, -5.0, 1.0, 0.4, 0.4, 
 

 
    0.0, 3.0, -5.0, 1.0, 0.4, 0.4, // The yellow triagle 
 
    -3.0, -3.0, -5.0, 1.0, 1.0, 0.4, 
 
    3.0, -3.0, -5.0, 1.0, 1.0, 0.4, 
 
    ]); 
 
    var n = 6; 
 

 
    // Create a buffer object 
 
    var vertexColorbuffer = gl.createBuffer(); 
 
    if (!vertexColorbuffer) { 
 
    console.log('Failed to create the buffer object'); 
 
    return -1; 
 
    } 
 

 
    // Write the vertex coordinates and color to the buffer object 
 
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexColorbuffer); 
 
    gl.bufferData(gl.ARRAY_BUFFER, verticesColors, gl.STATIC_DRAW); 
 

 
    var FSIZE = verticesColors.BYTES_PER_ELEMENT; 
 
    // Assign the buffer object to a_Position and enable the assignment 
 
    var a_Position = gl.getAttribLocation(gl.program, 'a_Position'); 
 
    if(a_Position < 0) { 
 
    console.log('Failed to get the storage location of a_Position'); 
 
    return -1; 
 
    } 
 
    gl.vertexAttribPointer(a_Position, 3, gl.FLOAT, false, FSIZE * 6, 0); 
 
    gl.enableVertexAttribArray(a_Position); 
 
    // Assign the buffer object to a_Color and enable the assignment 
 
    var a_Color = gl.getAttribLocation(gl.program, 'a_Color'); 
 
    if(a_Color < 0) { 
 
    console.log('Failed to get the storage location of a_Color'); 
 
    return -1; 
 
    } 
 
    gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, FSIZE * 6, FSIZE * 3); 
 
    gl.enableVertexAttribArray(a_Color); 
 

 
    return n; 
 
}

<canvas id="webgl" width="400" height="400"> 
 
    Please use a browser that supports "canvas" 
 
    </canvas> 
 
<input type="button" id="button" />

Answer 1

需要重啓PolygonOffset或關閉/重新啓用它，否則兩個三角形是由相同的偏移量。

GPU是狀態機，你負責管理的狀態（變量）：

function render() { 
    // Clear color and depth buffer 
    gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT); 

    // Draw the triangles 
    gl.polygonOffset(0.0, 0.0);   // Reset the polygon offset 
    gl.drawArrays(gl.TRIANGLES, 0, n/2); // The green triangle 
    gl.polygonOffset(1.0, 1.0);   // Set the polygon offset 
    gl.drawArrays(gl.TRIANGLES, n/2, n/2); // The yellow triangle 
}