【问题标题】：Webgl binding of a Framebuffer and RenderbufferFramebuffer 和 Renderbuffer 的 Webgl 绑定
【发布时间】：2015-04-11 13:42:20
【问题描述】：

我想从帧缓冲区中获取像素颜色。我尝试绑定帧缓冲区，但收到消息“此附件组合不起作用”。

RenderingEngine.prototype.getPixel = function(x, y, drawObject){
var framebuffer = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
framebuffer.width = this.canvas.width;
framebuffer.height = this.canvas.height;

var depthBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);

// allocate renderbuffer
gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, this.canvas.width, this.canvas.height);  

// attach renderebuffer
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, depthBuffer);

if (gl.checkFramebufferStatus(gl.FRAMEBUFFER) != gl.FRAMEBUFFER_COMPLETE) {
   alert("this combination of attachments does not work");
}
gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);

    drawObject.draw();

var pixel = new Uint8Array(4);
gl.readPixels(x, y, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
console.log(pixel);
return pixel;

}

更新 #1

我像这样更改了我的代码。但我总是得到黑色（[0, 0, 0, 0]）。你能说现在是什么问题吗？

RenderingEngine.prototype.getPixel = function(x, y, drawObject){
var framebuffer = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
framebuffer.width = this.canvas.width;
framebuffer.height = this.canvas.height;

var depthBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);


// allocate renderbuffer
gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, framebuffer.width, framebuffer.height);  

// attach renderebuffer
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, depthBuffer);

var colorBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, colorBuffer);
// allocate colorBuffer
gl.renderbufferStorage(gl.RENDERBUFFER, gl.RGBA4, framebuffer.width, framebuffer.height);  

// attach colorbuffer
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.RENDERBUFFER, colorBuffer);

if (gl.checkFramebufferStatus(gl.FRAMEBUFFER) != gl.FRAMEBUFFER_COMPLETE) {
   alert("this combination of attachments does not work");
}

renderingEngine.draw(drawObject);

var pixel = new Uint8Array(4);
gl.readPixels(x, y, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
console.log(pixel);
return pixel;

}

更新 #2

我的错误是绘画。此代码有效。但是有一些奇怪的东西。渲染的场景被水平镜像。你知道为什么吗？

RenderingEngine.prototype.getPixel = function(x, y, drawObject){
var framebuffer = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);
framebuffer.width = this.canvas.width;
framebuffer.height = this.canvas.height;

var depthBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);


// allocate renderbuffer
gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, framebuffer.width, framebuffer.height);  

// attach renderebuffer
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, depthBuffer);

var colorBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, colorBuffer);
// allocate colorBuffer
gl.renderbufferStorage(gl.RENDERBUFFER, gl.RGBA4, framebuffer.width, framebuffer.height);  

// attach colorbuffer
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.RENDERBUFFER, colorBuffer);

if (gl.checkFramebufferStatus(gl.FRAMEBUFFER) != gl.FRAMEBUFFER_COMPLETE) {
   alert("this combination of attachments does not work");
}

renderingEngine.draw(drawObject);

var pixel = new Uint8Array(4);
gl.readPixels(x, y, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
console.log(pixel);
return pixel;
}

【问题讨论】：

您只附加了一个深度渲染缓冲区。您还需要附加颜色渲染缓冲区或颜色纹理。我认为您不能只渲染到深度渲染缓冲区。
我这样修改了我的代码：
我将您的更新复制到您上面的问题。您能否删除下面的2个答案？将您的问题的更新发布为 SO 的答案并不合适。否则，至于为什么翻转，我假设您的意思是垂直而不是水平？请记住，WebGL 中的 0,0 是左下角。

标签： colors webgl pixel framebuffer

【解决方案1】：

这对我有用

function log(msg) {
  var pre = document.createElement("pre");
  pre.appendChild(document.createTextNode(msg));
  document.body.appendChild(pre);
}

var gl = document.getElementById("c").getContext("webgl");

var framebuffer = gl.createFramebuffer();
gl.bindFramebuffer(gl.FRAMEBUFFER, framebuffer);

var depthBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, depthBuffer);

// allocate renderbuffer
gl.renderbufferStorage(gl.RENDERBUFFER, gl.DEPTH_COMPONENT16, gl.canvas.width, gl.canvas.height);  

// attach renderebuffer
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.DEPTH_ATTACHMENT, gl.RENDERBUFFER, depthBuffer);

var colorBuffer = gl.createRenderbuffer();
gl.bindRenderbuffer(gl.RENDERBUFFER, colorBuffer);
// allocate colorBuffer
gl.renderbufferStorage(gl.RENDERBUFFER, gl.RGBA4, gl.canvas.width, gl.canvas.height);  

// attach colorbuffer
gl.framebufferRenderbuffer(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.RENDERBUFFER, colorBuffer);

if (gl.checkFramebufferStatus(gl.FRAMEBUFFER) != gl.FRAMEBUFFER_COMPLETE) {
   alert("this combination of attachments does not work");
}

gl.clearColor(0.5, 0.25, 1, 1);
gl.clear(gl.COLOR_BUFFER_BIT);
             
var pixel = new Uint8Array(4);
gl.readPixels(0, 0, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, pixel);
log(pixel[0] + "," + pixel[1] + "," + pixel[2] + "," + pixel[3]);

canvas { border: 1px solid black; }

<body>
  <canvas id="c"></canvas>  
</body>

虽然如果是我，我会为颜色附件附加纹理而不是渲染缓冲区。

【讨论】：

为什么是 136 ？第一种颜色是 0.5，或者 255 * 0.5 不是 136
ahh ok RGBA 是 4 位浮点数（整数中的 4 位 = 0..15）。 136 = (8/15) * 255 ; 68 = (4/15) * 255 ！

【解决方案2】：

引自WebGLFundamentals：重要的是要注意 WebGL 只承诺 3 种附件组合有效。根据the spec，唯一保证的附件组合是：

COLOR_ATTACHMENT0 = RGBA/UNSIGNED_BYTE texture
COLOR_ATTACHMENT0 = RGBA/UNSIGNED_BYTE texture + DEPTH_ATTACHMENT = DEPTH_COMPONENT16 renderbuffer
COLOR_ATTACHMENT0 = RGBA/UNSIGNED_BYTE texture + DEPTH_STENCIL_ATTACHMENT = DEPTH_STENCIL renderbuffer

【讨论】：