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WebGL not displaying content

i am mainly developing in C++ and decided to compile one of my projects to WebAssembly and build a website upon it. Since I have written some 3D-Engine in C++ before, I decided to use WebGL on the website. I have translated my shader template class. I have reduced the problem to a two-dimensional problem.
Task
First, I will describe what I am trying to do.
I am going to render a 2D FEM-grid which consists of FEM-elements which is any type of polygon. The nodes of those polygons contain values which I am trying to display. I already wrote code to break the polygons down to triangles. Initially I am just trying to render two dimensional triangles with their node values being interpolated.
Shader Code
I wrote some template shader class in WebGL which handles the construction and compilation of the shaders and has been copied from my C++ 3D-Engine. Since the code itself is somewhat long, I am not going to post it here but eventually show a list of executed OpenGL commands.
The shaders I am currently using to debug my problem are the following:
Vertex-Shader:
precision mediump float;
attribute vec2 position;
varying vec2 fragPos;
void main(){
gl_Position = vec4(position,0,1.0);
fragPos = position;
}
---------------------------------------------------------------
Fragment-Shader:
precision mediump float;
varying vec2 fragPos;
uniform sampler2D elementValues;
uniform int elementValuesDimension;
void main(){
gl_FragColor = vec4(0.8,0.2,0.0,1.0);
}
As you can see, I am not going for any interpolation in this debug case but just trying to some red-ish color within my fragment shader.
Executed Commands
I went ahead and used webgl-debug.js to show all the operations done. For this case, you can see the vertices and indices matching a simple quad spanning [0,0.6]x[0,0.6] which should be well within the clip space.
This can be broken into a few parts:
Enable uint32 as indexing
gl.getExtension(OES_element_index_uint)
Create two buffers for vertex coordinates and indices
gl.createBuffer()
gl.createBuffer()
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, [object WebGLBuffer])
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, 0,1,2,0,2,3, gl.STATIC_DRAW)
gl.bindBuffer(gl.ARRAY_BUFFER, [object WebGLBuffer])
gl.bufferData(gl.ARRAY_BUFFER, 0,0,0,0.6000000238418579,0.6000000238418579,0.6000000238418579,0.6000000238418579,0, gl.STATIC_DRAW)
Create and compile the shaders
gl.createShader(gl.VERTEX_SHADER)
gl.shaderSource(vertex_shader_src);
gl.compileShader([object WebGLShader]);
gl.getShaderParameter([object WebGLShader], gl.COMPILE_STATUS);
gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fragment_shader_src);
gl.compileShader([object WebGLShader]);
gl.getShaderParameter([object WebGLShader], gl.COMPILE_STATUS);
gl.createProgram()
gl.attachShader([object WebGLProgram], [object WebGLShader])
gl.attachShader([object WebGLProgram], [object WebGLShader])
gl.linkProgram([object WebGLProgram])
gl.validateProgram([object WebGLProgram])
Get Locations of the uniforms (which are unused) as well as the location of the attribute
gl.getUniformLocation([object WebGLProgram], elementValues)
gl.getUniformLocation([object WebGLProgram], elementValuesDimension)
gl.getAttribLocation([object WebGLProgram], position)
Verify that linking has worked
gl.getProgramParameter([object WebGLProgram], gl.LINK_STATUS)
Main Rendering Loop. Initially clear the displayed stuff
gl.clearColor(0.5, 0.5, 0.5, 1)
gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT)
gl.disable(gl.CULL_FACE)
gl.viewport(0, 0, 700, 600)
Activate the shader and bind the buffers
gl.useProgram([object WebGLProgram])
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, [object WebGLBuffer])
gl.bindBuffer(gl.ARRAY_BUFFER, [object WebGLBuffer])
Connect the vertices to the first attribute
gl.vertexAttribPointer(0, 2, gl.FLOAT, false, 0, 0)
Render Call.
gl.drawElements(gl.TRIANGLES, 4, gl.UNSIGNED_INT, 0)
Sadly when the commands above are executed, I am unable to see anything except a gray screen:
I am very happy for any help!
Greetings
Finn

Thanks to teddybeard and a lot more time on this problem spent than I was hoping, two issues have shown up:
l.bufferData(gl.ELEMENT_ARRAY_BUFFER, 0,1,2,0,2,3, gl.STATIC_DRAW) contains 6 vertices but the drawElements command only took 4 values. This is obviously a bug but should still have rendered atleast one of the two triangles.
Secondly, I forgot to enable the vertex attribute in the shader. gl.enableVertexAttribArray(1); solved the problem.

Bitwise OR Texels in WebGL2 fragment shader

I am attempting to use use WebGL2 for some GPGPU computations. A key component of this is setting the value of a texel to the bitwise-OR of itself and the new value computed in the fragment shader. Is there a way of applying this bitwise operation to each fragment instead of overwriting the value completely?
Here is the relevant code:
precision mediump float;
varying float id; // integer passed though the vertex shader, value is in range [1, 32].
void main() {
if (id == 1.0) {
gl_FragColor = vec4(0.0039, 0.0, 0.0, 0.0);
} else if (id == 2.0) {
gl_FragColor = vec4(0.0078, 0.0, 0.0, 0.0);
} else if (id == 3.0) {
...
}
}
To clarify the desired behaviour, let's say the value of a texel that we are writing to is 0b01. I perform some computation in the fragment shader and write the value 0b10. I would like the result to be 0b11.
I know that reading and writing operations to a single texture are mutually exclusive, so I am wondering if there is a way to configure WebGL2 to always perform a bitwise-OR when writing to a texture

there is no bitwise-OR writing. Typically you read from some texture, bitwise or in the shader, write to a new texture
BTW, webgl2 has signed and unsigned integer textures.

Passing arrays of off-length to gl.uniform3fv

It seems that when using WebGL uniform3fv you MUST pass an array of length equal to 3 if your shader uniform is vec3 OR an array having length equal to multiple of 3 if your shader uniform is an array of vec3's.
Doing this:
var data = new Float32Array([ 1, 2, 3, 4 ]);
gl.uniform3fv( uniformLocation, data );
when your uniform is declared as:
uniform vec3 some_uniform;
will result in some_uniform getting (0,0,0) value.
I searched the web and SO and MDN and forums and stuff (one of the stuff being WebGL specification) and I can't find a requirement (or mention) for this limitation.
My question is: is this required by WebGL specification (and if yes, can you please point me to it) or is it just some undocumented behaviour you are supposed to know about?
If it is required, we'll change code to support it as requirement, if it is undocumented quirk, we'll change code to support that quirk with an option to disable/remove the support once the quirk is gone.

From the WebGL spec section 5.14.10
If the array passed to any of the vector forms (those ending in v) has an invalid length, an INVALID_VALUE error will be generated. The length is invalid if it is too short for or is not an integer multiple of the assigned type.
Did you check your JavaScript console? When I tried it I clearly saw the INVALID_VALUE error in the console.
The code below
const gl = document.querySelector('canvas').getContext('webgl');
const vs = `
attribute vec4 position;
void main() {
gl_Position = position;
}
`;
const fs = `
precision mediump float;
uniform vec3 color;
void main() {
gl_FragColor = vec4(color, 1);
}
`;
const program = twgl.createProgram(gl, [vs, fs]);
const loc = gl.getUniformLocation(program, 'color');
gl.useProgram(program);
gl.uniform3fv(loc, [1, 2, 3, 4]); // generates INVALID_VALUE
<script src="https://twgljs.org/dist/4.x/twgl.min.js"></script>
<canvas></canvas>
prints
in the JavaScript console
In other words, your uniform was not set to 0,0,0. Instead your gl.uniform3fv function failed to execute since it got an error and so the uniform was left at whatever value it already was. Uniforms default to 0 so that's where the 0s came from
If you want to catch this kind of error during debugging consider using this helper library. Generally I find just looking at the JavaScript console is enough for me to figure out where the issue though.

WebGL fragment-shader for multiple light sources?

I want to be able to append multiple light sources to each node of my scene graph, but I have no clue how to do it!
From the tutorials on learningwebgl.com I learned to either use directional or position lighting, but I couldn't find a good explanation of how to implement multiple light sources.
So, the objective should be, to have the option to append an arbitrary number of light sources to each node, which type can be either directional or position lighting, and if possible and advisable, this should be realized by using only one shader-program (if this isn't the only possibility to do it anyway), because I automatically create the program for each node depending on their specific needs (unless there is already a program on the stack with equal settings).
Based on the tutorials on learningwebgl.com, my fragment-shader source for a node object using lighting without preset binding to one of the lighting-types could look like this...
precision highp float;
uniform bool uUsePositionLighting;
uniform bool uUseDirectionalLighting;
uniform vec3 uLightPosition;
uniform vec3 uLightDirection;
uniform vec3 uAmbientColor;
uniform vec3 uDirectionalColor;
uniform float uAlpha;
varying vec4 vPosition;
varying vec3 vTransformedNormal;
varying vec3 vColor;
void main (void) {
float directionalLightWeighting;
if (uUseDirectionalLighting) {
directionalLightWeighting = max(dot(vTransformedNormal, uLightDirection), 0.0);
else if (uUsePositionLighting) {
vec3 lightDirection = normalize(uLightPosition, vPosition.xyz);
directionalLightWeighting = max(dot(normalize(vTransformedNormal, lightDirection), 0.0);
}
vec3 lightWeighting = uAmbientColor + uDirectionalColor * directionalLightWeighting;
gl_FragColor = vec4(vColor * lightWeighting, uAlpha);
}
...so, that's basically my poor state of knowledge concerning this subject.
I also ask myself, how adding more light sources would affect the lighting-colors:
I mean, do uAmbientColor and uDirectionalColor have to sum up to 1.0? In this case (and particularly when using more than one light source) it surely would be good to precalculate these values before passing them to the shader, wouldn't it?

Put your lights into an array and loop over them for each fragment. Start with a fixed array of light sources, unbounded arrays are not supported until OpenGL 4.3 and are more complicated to work with.
Something along the lines of:
uniform vec3 uLightPosition[16];
uniform vec3 uLightColor[16];
uniform vec3 uLightDirection[16];
uniform bool uLightIsDirectional[16];
....
void main(void) {
vec3 reflectedLightColor;
// Calculate incoming light for all light sources
for(int i = 0; i < 16; i++) {
vec3 lightDirection = normalize(uLightPosition[i], vPosition.xyz);
if (lightIsDirectional[i]) {
reflectedLightColor += max(dot(vTransformedNormal, uLightDirection[i]), 0.0) * uLightColor[i];
}
else {
reflectedLightColor += max(dot(normalize(vTransformedNormal, lightDirection), 0.0) * uLightColor[i];
}
}
glFragColor = vec4(uAmbientColor + reflectedLightColor * vColor, uAlpha);
}
Then you can enable/disable the light sources by setting uLightColor to (0,0,0) for the entries you don't use.
Ambient and directional don't have to sum up to 1, actually the light source can have a strength much stronger than 1.0, but then you will need to do tonemapping to get back to a range of values that can be displayed on a screen, I would suggest playing around to get a feel for what is happening (e.g. what happens when a light source have negative colors, or colors above 1.0?).
uAmbientColor is just a (poor) way to simulate light that has bounced several times in the scene. Otherwise things in shadow becomes completely black Which looks unrealistic.
Reflectance should typically be between 0 and 1 (in this example it would be the parts returned by the 'max' computations), otherwise a lightsource will get stronger when looked at via the material.

#ErikMan's answer is great, but may involve a lot of extra work on the part of the GPU since you're checking every light per fragment, which isn't strictly necessary.
Rather than an array, I'd suggest building a clip-space quadtree. (You can do this in a compute shader if it's supported by your target platform / GL version.)
A node might have a structure such as (pseudocode as my JS is rusty):
typedef struct
{
uint LightMask; /// bitmask - each light has a bit indicating whether it is active for this node. uint will allow for 32 lights.
bool IsLeaf;
} Node;
const uint maxLevels = 4;
const uint maxLeafCount = pow(4,maxLevels);
const uint maxNodeCount = (4 * maLeafCount - 1) / 3;
/// linear quadtree - node offset = 4 * parentIndex + childIndex;
Node tree[maxNodeCount];
When building the tree, just check each light's clip-space bounding box against the implicit node bounds. (Root goes from (-1,-1) to (+1,+1). Each child is half that size on each dimension. So, you don't really need to store node bounds.)
If the light touches the node, set a bit in Node.LightMask corresponding to the light. If the light completely contains the node, stop recursing. If it intersects the node, subdivide and continue.
In your fragment shader, find which leaf node contains your fragment, and apply all lights whose bit is set in the leaf node's mask.
You could also store your tree in a mipmap pyramid if you expect it to be dense.
Keep your tiles to a size that is a multiple of 32, preferably square.
vec2 minNodeSize = vec2(2.f / 32);
Now, if you have a small number of lights, this may be overkill. You would probably have to have a lot of lights to see any real performance benefit. Also, a normal loop may help reduce data divergence in your shader, and makes it easier to eliminate branching.
This is one way to implement a simple tiled renderer, and opens the door to having hundreds of lights.

How to cut an object using WebGL shaders?

I would like to cut an object (a box) in WebGL (fragment shaders / vertex shaders) without using Boolean operations (union, difference, etc..).
I want to use shaders to hide some part of the object (so it is therefore not really a "real cuts" since it simply hides the object).
EDIT

First, make sure that the vertex shader passes through to the fragment shader the position in world space (or rather, whichever coordinate space you wish the clipping to be fixed relative to). Example (written from memory, not tested):
varying vec3 positionForClip;
...
void main(void) {
...
vec4 worldPos = modelMatrix * vertexPosition;
positionForClip = worldPos.xyz / worldPos.w; // don't need homogeneous coordinates, so do the divide early
gl_Position = viewMatrix * worldPos;
}
And in your fragment shader, you can then discard based on an arbitrary plane, or any other kind of test you want:
varying vec3 positionForClip;
uniform vec3 planeNormal;
uniform float planeDistance;
...
void main(void) {
if (dot(positionForClip, planeNormal) > planeDistance) {
// or if (positionForClip.x > 10.0), or whatever
discard;
}
...
gl_FragColor = ...;
}
Note that using discard may cause a performance reduction as the GPU cannot optimize based on knowing that all fragments will be written.
Disclaimer: I haven't researched this myself, and only just wrote down a possible way to do it based on the 'obvious solution'. There may be better ways I haven't heard of.
Regarding your question about multiple objects: There are many different ways to handle this — it's all custom code in the end. But you certainly can use a different shader for different objects in your scene, as long as they're in different vertex arrays.
gl.useProgram(programWhichCuts);
gl.drawArrays();
gl.useProgram(programWhichDoesNotCut);
gl.drawArrays();
If you're new to using multiple programs, it's pretty much just like using one program except that you do all the setup (compile, attach, link) once. The main thing to watch out for is each program has its own uniforms, so you have to initialize your uniforms for each program separately.