This is a continuation from WebGL Fundamentals. If you haven't read about how WebGL works you might want to read this first.

We've talked about shaders and GLSL but haven't really given them any specific details. I think I was hoping it would be clear by example but let's try to make it clearer just in case.

As mentioned in how it works WebGL requires 2 shaders every time you draw something. A vertex shader and a fragment shader. Each shader is a function. A vertex shader and fragment shader are linked together into a shader program (or just program). A typical WebGL app will have many shader programs.

Vertex Shader

A Vertex Shader's job is to generate clip space coordinates. It always takes the form

void main() {
   gl_Position = doMathToMakeClipspaceCoordinates
}

Your shader is called once per vertex. Each time it's called you are required to set the special global variable, gl_Position to some clip space coordinates.

Vertex shaders need data. They can get that data in 3 ways.

1. Attributes (data pulled from buffers)
2. Uniforms (values that stay the same for all vertices of a single draw call)
3. Textures (data from pixels/texels)

Attributes

The most common way is through buffers and attributes. How it works covered buffers and attributes. You create buffers,

var buf = gl.createBuffer();

put data in those buffers

gl.bindBuffer(gl.ARRAY_BUFFER, buf);
gl.bufferData(gl.ARRAY_BUFFER, someData, gl.STATIC_DRAW);

Then, given a shader program you made you look up the location of its attributes at initialization time

var positionLoc = gl.getAttribLocation(someShaderProgram, "a_position");

and at render time tell WebGL how to pull data out of those buffers and into the attribute

// turn on getting data out of a buffer for this attribute
gl.enableVertexAttribArray(positionLoc);
 
var numComponents = 3;  // (x, y, z)
var type = gl.FLOAT;    // 32bit floating point values
var normalize = false;  // leave the values as they are
var offset = 0;         // start at the beginning of the buffer
var stride = 0;         // how many bytes to move to the next vertex
                        // 0 = use the correct stride for type and numComponents
 
gl.vertexAttribPointer(positionLoc, numComponents, type, normalize, stride, offset);

In WebGL fundamentals we showed that we can do no math in the shader and just pass the data directly through.

attribute vec4 a_position;
 
void main() {
   gl_Position = a_position;
}

If we put clip space vertices into our buffers it will work.

Attributes can use float, vec2, vec3, vec4, mat2, mat3, and mat4 as types.

Uniforms

For a shader uniforms are values passed to the shader that stay the same for all vertices in a draw call. As a very simple example we could add an offset to the vertex shader above

attribute vec4 a_position;
uniform vec4 u_offset;
 
void main() {
   gl_Position = a_position + u_offset;
}

And now we could offset every vertex by a certain amount. First we'd look up the location of the uniform at initialization time

var offsetLoc = gl.getUniformLocation(someProgram, "u_offset");

And then before drawing we'd set the uniform

gl.uniform4fv(offsetLoc, [1, 0, 0, 0]);  // offset it to the right half the screen

Note that uniforms belong to individual shader programs. If you have multiple shader programs with uniforms of the same name both uniforms will have their own locations and hold their own values. When calling gl.uniform??? you're only setting the uniform for the current program. The current program is the last program you passed to gl.useProgram.

Uniforms can be many types. For each type you have to call the corresponding function to set it.

gl.uniform1f (floatUniformLoc, v);                 // for float
gl.uniform1fv(floatUniformLoc, [v]);               // for float or float array
gl.uniform2f (vec2UniformLoc,  v0, v1);            // for vec2
gl.uniform2fv(vec2UniformLoc,  [v0, v1]);          // for vec2 or vec2 array
gl.uniform3f (vec3UniformLoc,  v0, v1, v2);        // for vec3
gl.uniform3fv(vec3UniformLoc,  [v0, v1, v2]);      // for vec3 or vec3 array
gl.uniform4f (vec4UniformLoc,  v0, v1, v2, v4);    // for vec4
gl.uniform4fv(vec4UniformLoc,  [v0, v1, v2, v4]);  // for vec4 or vec4 array
 
gl.uniformMatrix2fv(mat2UniformLoc, false, [  4x element array ])  // for mat2 or mat2 array
gl.uniformMatrix3fv(mat3UniformLoc, false, [  9x element array ])  // for mat3 or mat3 array
gl.uniformMatrix4fv(mat4UniformLoc, false, [ 16x element array ])  // for mat4 or mat4 array
 
gl.uniform1i (intUniformLoc,   v);                 // for int
gl.uniform1iv(intUniformLoc, [v]);                 // for int or int array
gl.uniform2i (ivec2UniformLoc, v0, v1);            // for ivec2
gl.uniform2iv(ivec2UniformLoc, [v0, v1]);          // for ivec2 or ivec2 array
gl.uniform3i (ivec3UniformLoc, v0, v1, v2);        // for ivec3
gl.uniform3iv(ivec3UniformLoc, [v0, v1, v2]);      // for ivec3 or ivec3 array
gl.uniform4i (ivec4UniformLoc, v0, v1, v2, v4);    // for ivec4
gl.uniform4iv(ivec4UniformLoc, [v0, v1, v2, v4]);  // for ivec4 or ivec4 array
 
gl.uniform1i (sampler2DUniformLoc,   v);           // for sampler2D (textures)
gl.uniform1iv(sampler2DUniformLoc, [v]);           // for sampler2D or sampler2D array
 
gl.uniform1i (samplerCubeUniformLoc,   v);         // for samplerCube (textures)
gl.uniform1iv(samplerCubeUniformLoc, [v]);         // for samplerCube or samplerCube array

There's also types bool, bvec2, bvec3, and bvec4. They use either the gl.uniform?f? or gl.uniform?i? functions.

Note that for an array you can set all the uniforms of the array at once. For example

// in shader
uniform vec2 u_someVec2[3];
 
// in JavaScript at init time
var someVec2Loc = gl.getUniformLocation(someProgram, "u_someVec2");
 
// at render time
gl.uniform2fv(someVec2Loc, [1, 2, 3, 4, 5, 6]);  // set the entire array of u_someVec2

But if you want to set individual elements of the array you must look up the location of each element individually.

// in JavaScript at init time
var someVec2Element0Loc = gl.getUniformLocation(someProgram, "u_someVec2[0]");
var someVec2Element1Loc = gl.getUniformLocation(someProgram, "u_someVec2[1]");
var someVec2Element2Loc = gl.getUniformLocation(someProgram, "u_someVec2[2]");
 
// at render time
gl.uniform2fv(someVec2Element0Loc, [1, 2]);  // set element 0
gl.uniform2fv(someVec2Element1Loc, [3, 4]);  // set element 1
gl.uniform2fv(someVec2Element2Loc, [5, 6]);  // set element 2

Similarly if you create a struct

struct SomeStruct {
  bool active;
  vec2 someVec2;
};
uniform SomeStruct u_someThing;

you have to look up each field individually

var someThingActiveLoc = gl.getUniformLocation(someProgram, "u_someThing.active");
var someThingSomeVec2Loc = gl.getUniformLocation(someProgram, "u_someThing.someVec2");

Textures in Vertex Shaders

See Textures in Fragment Shaders.

Fragment Shader

A Fragment Shader's job is to provide a color for the current pixel being rasterized. It always takes the form

precision mediump float;
 
void main() {
   gl_FragColor = doMathToMakeAColor;
}

Your fragment shader is called once per pixel. Each time it's called you are required to set the special global variable, gl_FragColor to some color.

Fragment shaders need data. They can get data in 3 ways

1. Uniforms (values that stay the same for every pixel of a single draw call)
2. Textures (data from pixels/texels)
3. Varyings (data passed from the vertex shader and interpolated)

Uniforms in Fragment Shaders

See Uniforms in Shaders.

Textures in Fragment Shaders

Getting a value from a texture in a shader we create a sampler2D uniform and use the GLSL function texture2D to extract a value from it.

precision mediump float;
 
uniform sampler2D u_texture;
 
void main() {
   vec2 texcoord = vec2(0.5, 0.5);  // get a value from the middle of the texture
   gl_FragColor = texture2D(u_texture, texcoord);
}

What data comes out of the texture is dependent on many settings. At a minimum we need to create and put data in the texture, for example

var tex = gl.createTexture();
gl.bindTexture(gl.TEXTURE_2D, tex);
var level = 0;
var width = 2;
var height = 1;
var data = new Uint8Array([
   255, 0, 0, 255,   // a red pixel
   0, 255, 0, 255,   // a green pixel
]);
gl.texImage2D(gl.TEXTURE_2D, level, gl.RGBA, width, height, 0, gl.RGBA, gl.UNSIGNED_BYTE, data);

And set the texture's filtering

gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);

At initialization time look up the uniform location in the shader program

var someSamplerLoc = gl.getUniformLocation(someProgram, "u_texture");

At render time bind the texture to a texture unit

var unit = 5;  // Pick some texture unit
gl.activeTexture(gl.TEXTURE0 + unit);
gl.bindTexture(gl.TEXTURE_2D, tex);

And tell the shader which unit you bound the texture to

gl.uniform1i(someSamplerLoc, unit);

Varyings

A varying is a way to pass a value from a vertex shader to a fragment shader which we covered in how it works.

To use a varying we need to declare matching varyings in both a vertex and fragment shader. We set the varying in the vertex shader with some value per vertex. When WebGL draws pixels it will interpolate between those values and pass them to the corresponding varying in the fragment shader

Vertex shader

attribute vec4 a_position;
 
uniform vec4 u_offset;
 
varying vec4 v_positionWithOffset;
 
void main() {
  gl_Position = a_position + u_offset;
  v_positionWithOffset = a_position + u_offset;
}

Fragment shader

precision mediump float;
 
varying vec4 v_positionWithOffset;
 
void main() {
  // convert from clip space (-1 <-> +1) to color space (0 -> 1).
  vec4 color = v_positionWithOffset * 0.5 + 0.5;
  gl_FragColor = color;
}

The example above is a mostly nonsense example. It doesn't generally make sense to directly copy the clip space values to the fragment shader and use them as colors. Nevertheless it will work and produce colors.

GLSL

GLSL stands for Graphics Library Shader Language. It's the language shaders are written in. It has some special semi unique features that are certainly not common in JavaScript. It's designed to do the math that is commonly needed to compute things for rasterizing graphics. So for example it has built in types like vec2, vec3, and vec4 which represent 2 values, 3 values, and 4 values respectively. Similarly it has mat2, mat3 and mat4 which represent 2x2, 3x3, and 4x4 matrices. You can do things like multiply a vec by a scalar.

vec4 a = vec4(1, 2, 3, 4);
vec4 b = a * 2.0;
// b is now vec4(2, 4, 6, 8);

Similarly it can do matrix multiplication and vector to matrix multiplication

mat4 a = ???
mat4 b = ???
mat4 c = a * b;
 
vec4 v = ???
vec4 y = c * v;

It also has various selectors for the parts of a vec. For a vec4

vec4 v;

• v.x is the same as v.s and v.r and v[0].
• v.y is the same as v.t and v.g and v[1].
• v.z is the same as v.p and v.b and v[2].
• v.w is the same as v.q and v.a and v[3].

It is able to swizzle vec components which means you can swap or repeat components.

v.yyyy

is the same as

vec4(v.y, v.y, v.y, v.y)

Similarly

v.bgra

is the same as

vec4(v.b, v.g, v.r, v.a)

when constructing a vec or a mat you can supply multiple parts at once. So for example

vec4(v.rgb, 1)

Is the same as

vec4(v.r, v.g, v.b, 1)

Also

vec4(1)

Is the same as

vec4(1, 1, 1, 1)

One thing you'll likely get caught up on is that GLSL is very type strict.

float f = 1;  // ERROR 1 is an int. You can't assign an int to a float

The correct way is one of these

float f = 1.0;      // use float
float f = float(1)  // cast the integer to a float

The example above of vec4(v.rgb, 1) doesn't complain about the 1 because vec4 is casting the things inside just like float(1).

GLSL has a bunch of built in functions. Many of them operate on multiple components at once. So for example

T sin(T angle)

Means T can be float, vec2, vec3 or vec4. If you pass in vec4 you get vec4 back which the sine of each of the components. In other words if v is a vec4 then

vec4 s = sin(v);

is the same as

vec4 s = vec4(sin(v.x), sin(v.y), sin(v.z), sin(v.w));

Sometimes one argument is a float and the rest is T. That means that float will be applied to all the components. For example if v1 and v2 are vec4 and f is a float then

vec4 m = mix(v1, v2, f);

is the same as

vec4 m = vec4(
  mix(v1.x, v2.x, f),
  mix(v1.y, v2.y, f),
  mix(v1.z, v2.z, f),
  mix(v1.w, v2.w, f));

You can see a list of all the GLSL functions on the last page of the WebGL Reference Card. If you like really dry and verbose stuff you can try the GLSL spec.

Putting it all together

That's the point of this entire series of posts. WebGL is all about creating various shaders, supplying the data to those shaders and then calling gl.drawArrays or gl.drawElements to have WebGL process the vertices by calling the current vertex shader for each vertex and then render pixels by calling the current fragment shader for each pixel.

Actually creating the shaders requires several lines of code. Since those lines are the same in most WebGL programs and since once written you can pretty much ignore them. How to compile GLSL shaders and link them into a shader program is covered here.

If you're just starting from here you can go in 2 directions. If you are interested in image processing I'll show you how to do some 2D image processing. If you are interesting in learning about translation, rotation, scale and eventually 3D then start here.