
Notes for September 9 class  Introduction to Shaders
3D Coordinate system
WebGL exists in a 3D world:
 x goes to the right
 y goes up
 z goes forward (out of the screen)
This is called a right hand coordinate system.
For now we are going to be doing all
of our work starting with the x,y plane.
Specifically we will be working with a square
that extends from 1 → +1 in both x and y.



Square as a triangle strip
Our geometry will be a square.
In WebGL everything is made of triangles, so we will need two triangles.
We define these as a triangle strip.
In a triangle strip,
every three successive vertices makes a new triangle,
so we will need to specify a total of four vertices,
as in the figure to the right.
 

Zbuffer algorithm
The GPU (Graphics Processing Unit) renders using a "Zbuffer algorithm"
For each animation frame,
this algorithm consists of two successive steps:

For each vertex:
The GPU runs a vertex shader to:
 Find which pixel of the image contains this vertex;
 Set up "varying" values to be interpolated.

For each triangle:
The GPU interpolates from vertices to pixels.
For each pixel:
The GPU runs a fragment shader to:
 Compute color;
 If this is the nearest fragment at the pixel, replace color and depth at this pixel in the image.
 

Vector3 object
One data structure that will be very useful is a vector of length 3,
which we can use to represent x,y,z coordinates, as seen in the
figure to the right.
In Javascript we can define this object via a constructor,
which contains all of its properties that can change per instance,
as well as a prototype, which contains properties that do not
change from one instance to another (such as functions to
do such operations as setting values).
 

The Vector3 object
will grow in capability as the semester progresses, but
to the right is a starter version.
Note that the x,y and z properties, which change
from instance to intance, are defined in the
constructor itself.
The set property, which will be the same function
for all instances, is defined in the prototype.
 
function Vector3() {
this.x = 0;
this.y = 0;
this.z = 0;
}
Vector3.prototype = {
set : function(x,y,z) {
this.x = x;
this.y = y;
this.z = z;
},
}

Uniform variables
GLSL (for "GL Shading Language") is the Clike language that is used for
shaders on the GPU.
One of its key constructs
is a uniform variable.
Uniform variables on the GPU have the same value at every pixel.
They can (and often do) change over time.
By convention, a uniform variable name starts with the letter 'u'.
For your assignment, I have create some useful uniform variables:
float uTime; // time elapsed, in seconds
vec3 uCursor; // mouse position and state
// uCursor.x goes from 1 to +1
// uCursor.y goes from 1 to +1
// uCursor.z is 1 when mouse down, 0 when mouse up.
 

Vertex shaders
A vertex shader is a program that you (the application programmer/artist)
writes which gets run on the GPU at every vertex.
It is written in the special purpose language GLSL.
To the right is a very simple vertex shader program.
An "attribute" is a constant value that is passed in from
the CPU. In this case, it is aPosition , the x,y,z position of
each vertex in the scene.
wIt is of type vec3 ,
which means that it consists of three GLSL floating point numbers.
 
attribute vec3 aPosition;
varying vec3 vPosition;
void main() {
gl_Position = vec4(aPosition, 1.0);
vPosition = aPosition;
}

One of the most powerful things that a vertex shader can do
is set "varying" variables.
These values are then interpolated by the GPU across the
pixels of
any triangles that use this vertex.
That interpolated value will then be available
to fragment shaders at each pixel.
For example, "varying" variable vPosition
is set by this vertex shader.
By convention, the names of varying variables start with the letter 'v'.
This vertex shader does two things:

By setting
gl_Position ,
it determines at which pixel of the image this vertex will appear.

It sets varying variable
vPosition
to equal the attribute position aPosition for this vertex.

Fragment shaders
A fragment shader is a program that you
(the application programmer) writes which is
run at every pixel.
Because pieces of different triangles can be
visible at each pixel (eg: when triangles are very small,
or pixels where an edge of one
triangle partly obscures another triangle),
in general we are really defining the colors of
fragments of pixels, which is why these are called
fragment shaders.
Since our vertex shader has set a value for vPosition ,
any fragment shader we write will be able to make use of this
variable, whose values will now have been
interpolated down to the pixel level.
Guides to WebGL
There are many guides online to WebGL.
I find this Tutorial on WebGL Fundamentals to be particularly clear and easy to follow
for people who are new to WebGL.
I also find this Quick Reference Card
to be very useful when writing WebGL shaders.
I think you will find the list of builtin functions on the last page to be particularly useful.

Assignment 1
Your assignment for this week is to start with
the code in this zip file,
which we went over in class, and replace the fragment shader
in index.html with your own fascinating and wonderful
and original fragment shader.
Important: Create something of your own.
Don't just make a minor variation of what I did in class.
Have fun with it.
Your homework is due before the start of next week's class.

 