How to work with framebuffers in webgl?

I have been trying to understand framebuffer in WebGL/OpenGL-ES. I know that we can blend multiple textures using framebuffer.

So, to understand that I wrote a sample by taking a 1*1 texture and tried to apply framebuffer logic on top of it. But , it didn't work.

See snippet at bottom, if you click on "mix red and blue", the images doesn't get rendered, am I doing anything wrong?

Code : `

var canvas, gl, attrPosition, texture, program, vertexBuffer, textureBuffer, vertices, texVertices, attrPos, attrTexPos, textures = [], framebuffers = [];

canvas = document.getElementById('canvas');
    gl = getWebGL();
    vertices = new Float32Array([
        -1.0, -1.0,  
         1.0, -1.0, 
         1.0,  1.0, 
        -1.0,  1.0, 
        -1.0, -1.0, 
    ]);

    texVertices = new Float32Array([
        0.0, 0.0,
        1.0, 0.0,
        1.0, 1.0,
        0.0, 1.0,
        0.0, 0.0
    ]);
    var getProgram = function () {
        var vs = createVertexShader([
            'attribute vec2 attrPos;',
            'attribute vec2 attrTexPos;',
            'varying highp vec2 vTexCoord;',
            'void main() {',
                '\tgl_Position = vec4(attrPos, 0.0, 1.0);',
            '}'
        ].join('\n'));

        var fs = createFragmentShader([
            'varying highp vec2 vTexCoord;',
            'uniform sampler2D uImage;',
            'void main() {',
                '\tgl_FragColor = texture2D(uImage, vTexCoord);',
            '}'
        ].join('\n'));
        return createAndLinkPrograms(vs, fs);
    };

    var render = function () {
        gl.clear(gl.DEPTH_BUFFER_BIT|gl.COLOR_BUFFER_BIT);
        gl.bindTexture(gl.TEXTURE_2D, texture);
        gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
        gl.vertexAttribPointer(attrPos, 2, gl.FLOAT, gl.FALSE, 0, 0);
        gl.bindBuffer(gl.ARRAY_BUFFER, textureBuffer);
        gl.vertexAttribPointer(attrTexPos, 2, gl.FLOAT, gl.FALSE, 0, 0);
        gl.drawArrays(gl.TRIANGLE_STRIP, 0, 5);
    };
    if (gl) {
        gl.clearColor(0.1, 0.5, 1.0, 1.0);
        render();
        program = getProgram();
        texture = createAndSetupTexture();
        vertexBuffer = createAndBindBuffer(vertices, gl.ARRAY_BUFFER);
        attrPos = gl.getUniformLocation(program, 'attrPos');
        gl.enableVertexAttribArray(attrPos);

        textureBuffer = createAndBindBuffer(texVertices, gl.ARRAY_BUFFER);
        attrTexPos = gl.getUniformLocation(program, 'attrTexPos');
        gl.enableVertexAttribArray(attrTexPos);

        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, new Uint8Array([123, 0, 60, 255]));
        render();
    }

    var initPingPongTextures = function(textures, framebuffers) {
        for (var i = 0; i < 2; ++i) {
            var tex = createAndSetupTexture(gl);
            textures.push(tex);
            // make the texture the same size as the image
            gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
            // Create a framebuffer
            var fbo = gl.createFramebuffer();
            framebuffers.push(fbo);
            gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
            // Attach a texture to it.
            gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, tex, 0);
        }
    }

    var setFramebuffer = function(fbo, width, height) {
        gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
        gl.viewport(0, 0, width, height);
    };

    var mixRedAndBlue = function () {
        gl.activeTexture(gl.TEXTURE0);
        gl.bindTexture(gl.TEXTURE_2D, texture);

        setFramebuffer(framebuffers[0], 1, 1);
        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, new Uint8Array([255, 0, 0, 255]));
        render();
        gl.bindTexture(gl.TEXTURE_2D, textures[0]);

        setFramebuffer(framebuffers[1], 1, 1);
        gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, new Uint8Array([0, 255, 0, 255]));
        render();
        gl.bindTexture(gl.TEXTURE_2D, textures[1]);

        setFramebuffer(null, 1, 1);
        render();
    };`

var getWebGLContext = function(canvas) {
	var webglContextParams = ['webgl', 'experimental-webgl', 'webkit-3d', 'moz-webgl'];
	var webglContext = null;
	for (var index = 0; index < webglContextParams.length; index++) {
		try {
			webglContext = canvas.getContext(webglContextParams[index]);
			if(webglContext) {
				//breaking as we got our context
				break;
			}
		} catch (E) {
			console.log(E);
		}
	}
	if(webglContext === null) {
		alert('WebGL is not supported on your browser.');
	} else {
		//WebGL is supported in your browser, lets render the texture
	}
	fillGLForCleanUp(webglContext);
	return webglContext;
}
var createVertexShader = function (vertexShaderSource) {
	console.log(vertexShaderSource);
	var vertexShader = gl.createShader(gl.VERTEX_SHADER);
	gl.shaderSource(vertexShader, vertexShaderSource);
	gl.compileShader(vertexShader);
	return vertexShader;
}

var createFragmentShader = function (fragmentShaderSource) {
	console.log(fragmentShaderSource);
	var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
	gl.shaderSource(fragmentShader, fragmentShaderSource);
	gl.compileShader(fragmentShader);
	return fragmentShader;
}


var createAndLinkPrograms = function (vertexShader, fragmentShader) {
	var program = gl.createProgram();
	gl.attachShader(program, vertexShader);
	gl.attachShader(program, fragmentShader);
	gl.linkProgram(program);
    if (!gl.getProgramParameter(program, gl.LINK_STATUS)) {
        alert('Could not initialise shaders');
    }
	gl.useProgram(program);
	return program;
}

var createAndBindBuffer = function (verticesOrIndices, bufferType) {
	var buffer = gl.createBuffer();
	gl.bindBuffer(bufferType, buffer);
	gl.bufferData(bufferType, verticesOrIndices, gl.STATIC_DRAW);
	//clear memory
	gl.bindBuffer(bufferType, null);
	return buffer;
}

var allowAllImageSizes = function() {
	  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
	  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
	  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
	  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
	 // gl.bindTexture(gl.TEXTURE_2D, null);
} 

var createAndSetupTexture = function() {
	var texture = gl.createTexture();
	gl.bindTexture(gl.TEXTURE_2D, texture);
	allowAllImageSizes();
	gl.textures.push(texture);
	return texture;
}

var getWebGL = function (canvas, width, height) {
	if(!canvas) {
		canvas = document.createElement('canvas');
		canvas.id = 'canvas';
		canvas.width = !width ? 512 : width;
		canvas.height = !height ? 512 : height;
		document.body.appendChild(canvas);
	} else {
		canvas.width = !width ? 512 : width;
		canvas.height = !height ? 512 : height;
	}
	return getWebGLContext(canvas);
}

var fillGLForCleanUp = function (gl) {
	gl.textures = [];
	gl.framebuffers = [];
	gl.array_buffer = [];
	gl.element_array_buffers = [];
}


var canvas, gl, attrPosition, texture, program, 
vertexBuffer, textureBuffer, vertices, texVertices,
attrPos, attrTexPos, textures = [], framebuffers = [];
canvas = document.getElementById('canvas');
gl = getWebGL(canvas);
vertices = new Float32Array([
	-1.0, -1.0,  
	 1.0, -1.0, 
	 1.0,  1.0, 
	-1.0,  1.0, 
	-1.0, -1.0, 
]);

texVertices = new Float32Array([
	0.0, 0.0,
	1.0, 0.0,
	1.0, 1.0,
	0.0, 1.0,
	0.0, 0.0
]);
var getProgram = function () {
	var vs = createVertexShader([
		'attribute vec2 attrPos;',
		'attribute vec2 attrTexPos;',
		'varying highp vec2 vTexCoord;',
		'void main() {',
			'\tgl_Position = vec4(attrPos, 0.0, 1.0);',
		'}'
	].join('\n'));

	var fs = createFragmentShader([
		'varying highp vec2 vTexCoord;',
		'uniform sampler2D uImage;',
		'void main() {',
			'\tgl_FragColor = texture2D(uImage, vTexCoord);',
		'}'
	].join('\n'));
	return createAndLinkPrograms(vs, fs);
};

var render = function () {
	gl.clear(gl.DEPTH_BUFFER_BIT|gl.COLOR_BUFFER_BIT);
	gl.bindTexture(gl.TEXTURE_2D, texture);
	gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
	gl.vertexAttribPointer(attrPos, 2, gl.FLOAT, gl.FALSE, 0, 0);
	gl.bindBuffer(gl.ARRAY_BUFFER, textureBuffer);
	gl.vertexAttribPointer(attrTexPos, 2, gl.FLOAT, gl.FALSE, 0, 0);
	gl.drawArrays(gl.TRIANGLE_STRIP, 0, 5);
};
if (gl) {
	gl.clearColor(0.1, 0.5, 1.0, 1.0);
	render();
	program = getProgram();
	texture = createAndSetupTexture();
	vertexBuffer = createAndBindBuffer(vertices, gl.ARRAY_BUFFER);
	attrPos = gl.getUniformLocation(program, 'attrPos');
	gl.enableVertexAttribArray(attrPos);

	textureBuffer = createAndBindBuffer(texVertices, gl.ARRAY_BUFFER);
	attrTexPos = gl.getUniformLocation(program, 'attrTexPos');
	gl.enableVertexAttribArray(attrTexPos);

	gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, new Uint8Array([123, 0, 60, 255]));
	render();
}

var initPingPongTextures = function(textures, framebuffers) {
	for (var i = 0; i < 2; ++i) {
		var tex = createAndSetupTexture(gl);
		textures.push(tex);
		// make the texture the same size as the image
		gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
		// Create a framebuffer
		var fbo = gl.createFramebuffer();
		framebuffers.push(fbo);
		gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
		// Attach a texture to it.
		gl.framebufferTexture2D(gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, tex, 0);
	}
}

var setFramebuffer = function(fbo, width, height) {
	gl.bindFramebuffer(gl.FRAMEBUFFER, fbo);
	gl.viewport(0, 0, width, height);
};

var mixRedAndBlue = function () {
	gl.activeTexture(gl.TEXTURE0);
	gl.bindTexture(gl.TEXTURE_2D, texture);

	setFramebuffer(framebuffers[0], 1, 1);
	gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, new Uint8Array([255, 0, 0, 255]));
	render();
	gl.bindTexture(gl.TEXTURE_2D, textures[0]);

	setFramebuffer(framebuffers[1], 1, 1);
	gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, 1, 1, 0, gl.RGBA, gl.UNSIGNED_BYTE, new Uint8Array([0, 255, 0, 255]));
	render();
	gl.bindTexture(gl.TEXTURE_2D, textures[1]);

	setFramebuffer(null, 1, 1);
	render();
};

<button id="redImg" onclick="mixRedAndBlue()">Mix Red and blue</button><hr/>
<canvas id="canvas" width=512 height=512></canvas>

Edit 1 :

I am trying to achieve the same for multiple programs with multiple fragment shaders because having if/else statements within the fragment shader is not recommended as it runs for each pixel.

`
Shaders.prototype.VS_Base = [ 'attribute vec3 verticesPosition;', 'attribute vec2 texturePosition;', 'varying highp vec2 vTextureCoord;', 'void main(void) {', '\tgl_Position = vec4(verticesPosition * vec3(1.0, -1.0, 1.0), 0.5);', '\tvTextureCoord = texturePosition;', '}' ].join('\n');

Shaders.prototype.FS_Base_Image_RED = [
        '#ifdef GL_ES',
        'precision highp float;',
        '#endif',
        'uniform sampler2D uImage;',
        'varying highp vec2 vTextureCoord;',
        'void main (void) {',
        '\tgl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);//texture2D(uImage, vTextureCoord);',
        '}'
].join('\n');

Shaders.prototype.FS_Base_Image_BLUE = [
        '#ifdef GL_ES',
        'precision highp float;',
        '#endif',
        'uniform sampler2D uImage;',
        'varying highp vec2 vTextureCoord;',
        'void main (void) {',
        '\tgl_FragColor = vec4(0.0, 0.0, 1.0, 1.0);//texture2D(uImage, vTextureCoord);',
        '}'
].join('\n');`

Now I have 2 separate programs for both the fragment shader and I need to use framebuffers for mixing Red and Blue. I am not looking for mix() as the actual scenario is very complex and that's the reason I am using multiple programs with fragment shaders for avoiding conditional if/else statements.

标签： javascript webgl

1条回答

放我归山

2楼-- · 2019-07-27 06:00

It's not clear what you're trying to do. Framebuffers are just a list of attachments (textures and renderbuffers). You use them to render to a texture and/or renderbuffer. Then you can use the texture you just rendered to as input to some other render.

Here's an example with NO framebuffers. It blends 2 textures.

var vs = `
attribute vec4 position;

varying vec2 v_texcoord;

void main() {
  gl_Position = position;
  v_texcoord = position.xy * .5 + .5;
}
`;

var fs = `
precision mediump float;

varying vec2 v_texcoord;

uniform sampler2D tex1;
uniform sampler2D tex2;

void main() {
  vec4 color1 = texture2D(tex1, v_texcoord);
  vec4 color2 = texture2D(tex2, v_texcoord);
  gl_FragColor = mix(color1, color2, 0.5);
}
`;

const gl = document.querySelector("canvas").getContext("webgl");
const program = twgl.createProgramFromSources(gl, [vs, fs]);

// make 2 textures with canvas 2d
const ctx = document.createElement("canvas").getContext("2d");
ctx.canvas.width = 64;
ctx.canvas.height = 64;

// first texture has a circle
ctx.fillStyle = "blue";
ctx.fillRect(0, 0, 64, 64);
ctx.strokeStyle = "yellow";
ctx.beginPath();
ctx.arc(32, 32, 20, 0, Math.PI * 2, false);
ctx.lineWidth = 12;
ctx.stroke();

const tex1 = createTextureFromCanvas(gl, ctx.canvas);

// second texture has a diamond (diagonal square)
ctx.fillStyle = "red";
ctx.fillRect(0, 0, 64, 64);
ctx.fillStyle = "cyan";
ctx.beginPath();
ctx.moveTo(32, 6);
ctx.lineTo(58, 32);
ctx.lineTo(32, 58);
ctx.lineTo(6, 32);
ctx.lineTo(32, 6);
ctx.fill();

const tex2 = createTextureFromCanvas(gl, ctx.canvas);

const buf = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buf);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
  -1, -1,
   1, -1,
  -1,  1,
  -1,  1,
   1, -1,
   1,  1,
]), gl.STATIC_DRAW);


const positionLoc = gl.getAttribLocation(program, "position");
gl.enableVertexAttribArray(positionLoc);
gl.vertexAttribPointer(positionLoc, 2, gl.FLOAT, false, 0, 0);

const tex1Loc = gl.getUniformLocation(program, "tex1");  
const tex2Loc = gl.getUniformLocation(program, "tex2");  
  
gl.useProgram(program);

gl.uniform1i(tex1Loc, 0);
gl.uniform1i(tex2Loc, 1);
gl.activeTexture(gl.TEXTURE0 + 0);
gl.bindTexture(gl.TEXTURE_2D, tex1);
gl.activeTexture(gl.TEXTURE0 + 1);
gl.bindTexture(gl.TEXTURE_2D, tex2);

gl.drawArrays(gl.TRIANGLES, 0, 6);

function createTextureFromCanvas(gl, canvas) {
  const tex = gl.createTexture();
  gl.bindTexture(gl.TEXTURE_2D, tex);
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, ctx.canvas);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
  return tex;
}

canvas { border: 1px solid black; }

<script src="https://twgljs.org/dist/2.x/twgl.min.js"></script>
<canvas></canvas>

For your purpose there is no difference about the blending part, the only difference is where the textures come from. Above the textures were created by using a 2d canvas. Instead you can use framebuffer to render to a texture. AFTER you've rendered to a texture you can then use that texture in some other render just like above.

To render to a texture first you create a framebuffer

var fb = gl.createFramebuffer();

Then you attach a texture to it

gl.bindFramebuffer(gl.FRAMEBUFFER, fb);  
gl.framebufferTexture2D(
    gl.FRAMEBUFFER, 
    gl.COLOR_ATTACHMENT0,  // attach texture as COLOR_ATTACHMENT0
    gl.TEXTURE_2D,         // attach a 2D texture
    someTexture,           // the texture to attach
    0);                    // the mip level to render to (must be 0 in WebGL1)

Depending on your attachments you should check if they work.

if (gl.checkFramebufferStatus(gl.FRAMEBUFFER) !== gl.FRAMEBUFFER_COMPLETE) {
  // these attachments don't work
}

The WebGL spec lists 3 combinations of attachments that are guaranteed to work. The example below is using one of those 3 so there's no need to check

Now if you bind the framebuffer

gl.bindFramebuffer(gl.FRAMEBUFFER, fb);

Then when you call any gl.drawXXX function or gl.clear it will be drawing to the someTexture instead of the canvas. To start drawing to the canvas again bind null

gl.bindFramebuffer(gl.FRAMEBUFFER, null);

Remember that if the canvas and the texture are different sizes you'll need to call gl.viewport to render correctly

var vs = `
attribute vec4 position;

uniform mat4 matrix;

varying vec2 v_texcoord;

void main() {
  gl_Position = matrix * position;
  v_texcoord = position.xy * .5 + .5;
}
`;

var colorFS = `
precision mediump float;

uniform vec4 color;

void main() {
  gl_FragColor = color;
}
`;

var mixFS = `
precision mediump float;

varying vec2 v_texcoord;

uniform sampler2D tex1;
uniform sampler2D tex2;

void main() {
  // probably should use different texture coords for each
  // texture for more flexibility but I'm lazy
  vec4 color1 = texture2D(tex1, v_texcoord);
  vec4 color2 = texture2D(tex2, v_texcoord);
  gl_FragColor = mix(color1, color2, 0.5);
}
`;

const gl = document.querySelector("canvas").getContext("webgl");
const colorProgram = twgl.createProgramFromSources(gl, [vs, colorFS]);
const mixProgram = twgl.createProgramFromSources(gl, [vs, mixFS]);

// make 2 textures by attaching them to framebuffers and rendering to them
const texFbPair1 = createTextureAndFramebuffer(gl, 64, 64);
const texFbPair2 = createTextureAndFramebuffer(gl, 64, 64);

const buf = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buf);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
  -1, -1,
   1, -1,
  -1,  1,
  -1,  1,
   1, -1,
   1,  1,
]), gl.STATIC_DRAW);

function setAttributes(buf, positionLoc) {
  gl.enableVertexAttribArray(positionLoc);
  gl.vertexAttribPointer(positionLoc, 2, gl.FLOAT, false, 0, 0);
}
  
const colorPrgPositionLoc = gl.getAttribLocation(colorProgram, "position");
setAttributes(buf, colorPrgPositionLoc);
const colorLoc = gl.getUniformLocation(colorProgram, "color");
const colorProgMatrixLoc = gl.getUniformLocation(colorProgram, "matrix");

// draw red rect to first texture through the framebuffer it's attached to
gl.useProgram(colorProgram);
  
gl.bindFramebuffer(gl.FRAMEBUFFER, texFbPair1.fb);
gl.viewport(0, 0, 64, 64);
gl.uniform4fv(colorLoc, [1, 0, 0, 1]);
gl.uniformMatrix4fv(colorProgMatrixLoc, false, [
  0.5, 0, 0, 0,
    0,.25, 0, 0,
    0, 0, 1, 0,
   .2,.3, 0, 1,
]);

gl.drawArrays(gl.TRIANGLES, 0, 6);

// Draw a blue rect to the second texture through the framebuffer it's attached to
gl.bindFramebuffer(gl.FRAMEBUFFER, texFbPair2.fb);
gl.viewport(0, 0, 64, 64);
gl.uniform4fv(colorLoc, [0, 0, 1, 1]);
gl.uniformMatrix4fv(colorProgMatrixLoc, false, [
  0.25, 0, 0, 0,
    0,.5, 0, 0,
    0, 0, 1, 0,
   .2,.3, 0, 1,
]);

gl.drawArrays(gl.TRIANGLES, 0, 6);

// Draw both textures to the canvas
gl.bindFramebuffer(gl.FRAMEBUFFER, null);
gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);
  
const mixPrgPositionLoc = gl.getAttribLocation(mixProgram, "position");
setAttributes(buf, mixPrgPositionLoc);
  
const mixProgMatrixLoc = gl.getUniformLocation(mixProgram, "matrix");

const tex1Loc = gl.getUniformLocation(mixProgram, "tex1");  
const tex2Loc = gl.getUniformLocation(mixProgram, "tex2");  
  
gl.useProgram(mixProgram);

gl.uniform1i(tex1Loc, 0);
gl.uniform1i(tex2Loc, 1);
gl.activeTexture(gl.TEXTURE0 + 0);
gl.bindTexture(gl.TEXTURE_2D, texFbPair1.tex);
gl.activeTexture(gl.TEXTURE0 + 1);
gl.bindTexture(gl.TEXTURE_2D, texFbPair2.tex);
gl.uniformMatrix4fv(mixProgMatrixLoc, false, [
    1, 0, 0, 0,
    0, 1, 0, 0,
    0, 0, 1, 0,
    0, 0, 0, 1,
]);  

gl.drawArrays(gl.TRIANGLES, 0, 6);

function createTextureAndFramebuffer(gl, width, height) {
  const tex = gl.createTexture();
  gl.bindTexture(gl.TEXTURE_2D, tex);
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, width, height, 0, gl.RGBA, gl.UNSIGNED_BYTE, null);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
  gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
  const fb = gl.createFramebuffer();
  gl.bindFramebuffer(gl.FRAMEBUFFER, fb);
  gl.framebufferTexture2D(
     gl.FRAMEBUFFER, gl.COLOR_ATTACHMENT0, gl.TEXTURE_2D, tex, 0);
  return {tex: tex, fb: fb};
}

canvas { border: 1px solid black; }

<script src="https://twgljs.org/dist/2.x/twgl.min.js"></script>
<canvas></canvas>

The only functional difference between the first program and the second is how the textures got their data. In the first example the textures got their data from a canvas 2d. In the 2nd example the textures got their data by rendering to them using WebGL.

As for why your example doesn't blend textures, in order to blend 2 textures you need a shader that uses two textures.

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How to work with framebuffers in webgl?

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