Question

在WebGL中使用纹理时，有时我需要使它们比原来更大。当我这样做时，它会使纹理以不同的方式显示，尤其是在较浅的背景上。

我有以下图像（256 x 256）：

original image

在WebGL中渲染时，它略大于原始图像。以下是图像在两个不同背景上的显示方式：

enter image description here image on light background

如您所见，图像在深色背景上正确显示，但在浅色背景上时，图像呈现白色轮廓。

我的设置代码：

gl.clearColor(0x22 / 0xFF, 0x22 / 0xFF, 0x22 / 0xFF, 1); // set background color
gl.enable(gl.BLEND); // enable transparency
gl.disable(gl.DEPTH_TEST); // disable depth test (causes problems with alpha if enabled)
gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA); //set up blending
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT); //clear the gl canvas
gl.viewport(0, 0, canvas.width, canvas.height); //set the viewport

这是每次加载纹理时调用的代码：

function handleTextureLoaded(image, texture) {
  gl.bindTexture(gl.TEXTURE_2D, texture);
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR_MIPMAP_NEAREST);
  gl.generateMipmap(gl.TEXTURE_2D);
  gl.bindTexture(gl.TEXTURE_2D, null);
  loadCount++;
}

导致轮廓出现的原因，以及如何解决？

注意：当我将原始图像放在这两个相同的背景上时，即使调整图像大小，也不会出现此问题。

我尝试在WebGL上下文中禁用alpha（由@zfedoran讲述）：

gl = canvas.getContext('webgl', {antialias: false, alpha: false }) 
  || canvas.getContext('experimental-webgl', {antialias: false, alpha: false });

图像周围会出现一个小的空白边框，如下所示（放大）：

enter image description here

Answer 1

在@zfedoran提到的画布上的alpha之上，你如何制作原始图像？

我认为问题如下

假设您有这样的抗锯齿边缘。这个像素是什么颜色的？

enter image description here

假设主要颜色（右下角的像素颜色）为1,0,0（纯红色）。理想情况下，箭头指向的像素为（1,0,0,0.5）。换句话说，纯红色的α为0.5。但是，根据图像的创建方式来生成抗锯齿像素，它可能已与其旁边的纯透明像素混合，因此它不再是纯红色。那些纯透明像素可能是（0,0,0,0）透明黑色。

即使您的绘图程序正确处理此问题，GL也可能不会。当您使用纹理过滤（gl.LINEAR等）绘制图像时，GL会对彼此靠近的像素求平均值，其中一些像素是透明的黑色。混合黑色和红色会产生深红色。因此你会得到一个黑色边框。

Here you can see the issue

"use strict";

function main() {
  var planeVertices = [
       -1, -1,
        1, -1,
       -1,  1,
        1,  1,
    ];
      
  var texcoords = [
     0, 1,
     1, 1,
     0, 0,
     1, 0,
  ];
    
  var indices = [
     0, 1, 2,
     2, 1, 3,
  ];
    

  var canvas = document.getElementById("c");
  var gl = canvas.getContext("webgl", {alpha:false});

  var programs = {}
  programs.normalProgram = twgl.createProgramFromScripts(
      gl, ["2d-vertex-shader", "2d-fragment-shader"], ["a_position", "a_texcoord"]);
  programs.preMultiplyAlphaProgram = twgl.createProgramFromScripts(
      gl, ["2d-vertex-shader", "pre-2d-fragment-shader"], ["a_position", "a_texcoord"]);
    
  var positionLoc = 0;  // assigned in createProgramsFromScripts
  var texcoordLoc = 1;  // assigned in createProgramsFromScripts

  var buffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
  gl.bufferData(
      gl.ARRAY_BUFFER,
      new Float32Array(planeVertices),
      gl.STATIC_DRAW);
  gl.enableVertexAttribArray(positionLoc);
  gl.vertexAttribPointer(positionLoc, 2, gl.FLOAT, false, 0, 0);
    
  var buffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
  gl.bufferData(
      gl.ARRAY_BUFFER,
      new Float32Array(texcoords),
      gl.STATIC_DRAW);
  gl.enableVertexAttribArray(texcoordLoc);
  gl.vertexAttribPointer(texcoordLoc, 2, gl.FLOAT, false, 0, 0);
    
  var buffer = gl.createBuffer();
  gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, buffer);
  gl.bufferData(
      gl.ELEMENT_ARRAY_BUFFER,
      new Uint16Array(indices),
      gl.STATIC_DRAW);    

  var img = new Image();
  img.onload = createTextures;
  img.src = document.getElementById("i").text;
    
  function createTexture() {
    var tex = gl.createTexture();
    gl.bindTexture(gl.TEXTURE_2D, tex);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, img);
    gl.generateMipmap(gl.TEXTURE_2D); // assuming power-of-2 
    return tex;
  }
 
  var textures = {};    
  function createTextures() {
    gl.pixelStorei(gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL, false);
    textures.unpremultipliedAlphaTexture = createTexture();
    gl.pixelStorei(gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL, true);
    textures.premultipliedAlphaTexture = createTexture();
    document.body.appendChild(document.createElement("hr"));
    insert("original image");
    document.body.appendChild(img);
    render();
  }
    
  function insert(text) {
    var pre = document.createElement("pre");
    pre.appendChild(document.createTextNode(text));
    document.body.appendChild(pre);
  };
    
  function grabImage(prg, blend, texName) {
     document.body.appendChild(document.createElement("hr"));
     insert(
       "gl.useProgram(" + prg + ")\n" +
       "gl.blendFunc(gl." + blend.src + ", gl." + blend.dst + ")\n" +
       "gl.bindTexture(gl.TEXTURE2D, " + texName + ")");
     var img = new Image();
     img.src = gl.canvas.toDataURL();
     document.body.appendChild(img);
  };
  
  function render() { 
      gl.enable(gl.BLEND);
      
      Object.keys(programs).forEach(function(p, pndx) {
          gl.useProgram(programs[p]);
          
          [
              { src: "SRC_ALPHA", dst: "ONE_MINUS_SRC_ALPHA" },
              { src: "ONE", dst: "ONE_MINUS_SRC_ALPHA" },
          ].forEach(function(b, bndx) {
               gl.blendFunc(gl[b.src], gl[b.dst]);
           
               Object.keys(textures).forEach(function(texName, tndx) {
                  gl.bindTexture(gl.TEXTURE_2D, textures[texName]);
                  gl.clearColor(0x3D/0xFF, 0x87/0xFF, 0xEA/0xFF, 1);
                  gl.clear(gl.COLOR_BUFFER_BIT);
                  gl.drawElements(gl.TRIANGLES, 6, gl.UNSIGNED_SHORT, 0);
                  grabImage(p, b, texName);
              });
          });
      });
   }
}

main();

canvas {
    border: 1px solid black;
    display: none;
}
img {
    background-color: #3D87EA;
    border: 1px solid black;
    width: 256px;
    height: 256px;
}

<script src="https://twgljs.org/dist/3.x/twgl.min.js"></script>
<!-- vertex shader -->
<script id="2d-vertex-shader" type="x-shader/x-vertex">
attribute vec4 a_position;
attribute vec2 a_texcoord;

varying vec2 v_texcoord;
    
void main() {
   gl_Position = a_position;
   v_texcoord = a_texcoord;
}
</script>
<!-- fragment shaders -->
<script id="2d-fragment-shader" type="x-shader/x-fragment">
precision mediump float;
    
varying vec2 v_texcoord;

uniform sampler2D u_texture;
void main() {
    gl_FragColor = texture2D(u_texture, v_texcoord);
}
</script>
<script id="pre-2d-fragment-shader" type="x-shader/x-fragment">
precision mediump float;
    
varying vec2 v_texcoord;

uniform sampler2D u_texture;
void main() {
    vec4 textureColor = texture2D(u_texture, v_texcoord);
    gl_FragColor = vec4(textureColor.rgb * textureColor.a, textureColor.a);
}
</script>
<canvas id="c" width="32" height="32"></canvas>
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有一些解决方案

确保透明区域实际上有颜色。

换句话说，如果上图中左上方的所有像素都是 RED 且带有0 alpha，那么当像素被过滤时，它们将会混合（1,0,0,0））透明红色代替（0,0,0,0）透明黑色。不幸的是，在大多数绘图程序中没有简单的方法可以做到这一点。

有a plugin for Photoshop that lets you do it called SuperPNG它可以让您为alpha创建第4个频道，而不是使用photoshop的透明度。这使您可以将alpha设置为与图像分开。

在你的情况下，你最终会得到一个像这样的图像的图像

现在没有坏颜色可以融合。
切换到预乘Alpha

在此情况下，请先致电gl.texImage2D上传图片电话
```
gl.pixelStorei(UNPACK_PREMULTIPLY_ALPHA_WEBGL, true);
```
在致电gl.texImage2D之前。这告诉WebGL在加载图像时将颜色乘以alpha。然后使用
混合
```
gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
```
关闭GL中的过滤
```
gl.texParameter(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
gl.texParameter(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
```
假设您的源图像没有任何不良颜色，这意味着GL不会在过滤时产生新的坏颜色，但当然这也意味着如果缩放或旋转图像，您将会出现锯齿。
创建自己的mips

大多数应用使用gl.genereateMipmap来生成mips，但您可以离线生成它们并自行上传。这不是一个完美的解决方案，但它确实让你使用`gl.texParameteri（gl.TEXTURE_2D，gl.TEXTURE_MIN_FILTER，gl.LINEAR_MIPMAP_NEAREST）;

以上的组合

Answer 2

您是否尝试在WebGL上下文中禁用alpha？

var gl = this.canvas.getContext('webgl', {antialias: false, alpha: false }) 
      || this.canvas.getContext('experimental-webgl', {antialias: false, alpha: false });

WebGL纹理调整意外输出的大小

2 个答案: