CS 418 - Using video memory

High-end graphics cards have dedicated video memory that is distinct from main memory. There are many reasons why this is desirable for graphics card performance, but we need to know it because graphics APIs, including WebGL, are designed to make the two-memories model work.

This page addresses only attributes, which provide per-vertex information, and their connectivity into primitives. Uniforms and samplers are also stored in video memory, but are out of scope for this page.

Vertex attributes are values supplied per-vertex to the vertex shader. In the vertex shader, they are specified with the in keyword.

Attributes are usually floating-point values or vectors of up to four floating-point values.

The vertex shader can access up to 16 attributes. These are provided in numbered locations, 0 through 15. When you provide a vertex shader you can either specify the location of each attribute directly by in layout(location=3) vec4 p or leave the locations unspecified by in vec4 n and let the shader compiler pick the location for you. We do need locations, though, so if we let the compiler pick them we later ask the compiler where they ended up using gl.getAttribLocation(program, 'n').

Attributes are sent to the GPU in arrays of bytes known as buffers.

CPU setup

Determine the per-vertex data.

Store it in one or more arrays.

Arrays need to be in the same order, but may be interleaved. For example, the following shows two ways to provide the same data:

pos = [ [-1,-1], [ 1,-1], [ 1, 1], [-1, 1] ]
col = [ [0,0,0], [1,0,0], [0,1,0], [0,0,1] ]

interleaved = [
    [-1,-1], [0,0,0],
    [ 1,-1], [1,0,0],
    [ 1, 1], [0,1,0],
    [-1, 1], [0,0,1],
]

Flatten the arrays using the .flat() method. This removes all internal arrays and just keeps the values in order.

pflat = pos.flat() // = [-1,-1,1,-1,1,1,-1,1]
cflat = col.flat() // = [0,0,0,1,0,0,0,1,0,0,0,1]

iflat = interleaved.flat()
// = [-1,-1,0,0,0, 1,-1,1,0,0, 1,1,0,1,0, -1,1,0,0,1]

Encode the values in the arrays into bytes using the new Float32Array constructor.

pbytes = new Float32Array(pflat)
// still looks like [-1,-1,1,-1,1,1,-1,1]
// but now stored as bytes (little-endian on most CPUs):
// 00 00 80 BF  00 00 80 BF  00 00 80 3F  00 00 80 BF
// 00 00 80 3F  00 00 80 3F  00 00 80 BF  00 00 80 3F

Send data to a buffer in video memory

Allocate a buffer to store bytes in video memory. We’ll need one buffer for each byte-encoded array.
```
buf = gl.createBuffer()
```
Prepare a route to send data to that buffer.
```
gl.bindBuffer(gl.ARRAY_BUFFER, buf)
```
Copy bytes into the buffer.
```
gl.bufferData(gl.ARRAY_BUFFER, pbytes, hint)
```
The hint is either gl.DYNAMIC_DRAW which tells the GPU I’m going to send new bytes into this buffer often so place it somewhere easy to modify and don’t waste time optimizing it; of gl.STATIC_DRAW which tells the GPU I’m going to draw this data many many times before I change it so place it somewhere easy to access and optimize it for faster reads.

Tell the GPU how to parse the buffer

Make a vertex array object to store some future setup in.
```
vao = gl.createVertexArray()
```
Record the next parts into the vertex array object. This same function also reactivates what it’s recorded later during drawing.
```
gl.bindVertexArray(voa)
```
Make sure we have the vertex shader locations of the attributes.

We might have these as constants due to using in layout(location = 0) vec4 pos or the like in the vertex shader, or we might retrieve them using gl.getAttribLocation.
For each attribute, use gl.vertexAttribPointer with its six arguments that tell the GPU how to parse the bytes.
1. Pick a buffer using gl.bindBuffer(gl.ARRAY_BUFFER, buf).
  
  This example will assume the interleaved buffer.
2. Pass in an attribute location
```
gl.vertexAttribPointer(pos_loc,
```
3. Pass in how many number are used per vertex for that attribute.
```
gl.vertexAttribPointer(pos_loc,
    2, // because pos is a 2-vector
```
  Note that this is the number of values per vertex for the attribute in the byte-serialized data, which may be larger or smaller than the number in the vertex shader in variable type.
4. Pass in the byte format used.
```
gl.vertexAttribPointer(pos_loc,
    2,
    gl.FLOAT,
```
5. Pass in a normalization argument that only applies to less-common integer-based byte formats and is ignored for floats.
```
gl.vertexAttribPointer(pos_loc,
    2,
    gl.FLOAT,
    false,
```
6. Pass in how many bytes to skip between the end of one attribute and the start of the next.
```
gl.vertexAttribPointer(pos_loc,
    2,
    gl.FLOAT,
    false,
    4*3, // between two pos are 3 4-byte col values
```
7. Pass in how many bytes to skip at the front of the buffer.
```
gl.vertexAttribPointer(pos_loc,
    2,
    gl.FLOAT,
    false,
    4*3,
    0) // the first pos starts on the first byte
```
8. Activate the attribute.
```
gl.enableVertexAttribArray(pos_loc)
```

The details specified as part of the gl.vertexAttribPointer and gl.enableVertexAttribArray are stored in the active VAO, along with prerequisite information like which buffer was bound to the gl.ARRAY_BUFFER when gl.vertexAttritPointer was called. None of the other information above is.

The full non-interleaved example is

pos = [ [-1,-1], [ 1,-1], [ 1, 1], [-1, 1] ]
col = [ [0,0,0], [1,0,0], [0,1,0], [0,0,1] ]
pflat = pos.flat()
cflat = col.flat()
pbytes = new Float32Array(pflat)
cbytes = new Float32Array(cflat)

pbuf = gl.createBuffer()
cbuf = gl.createBuffer()
gl.bindBuffer(gl.ARRAY_BUFFER, pbuf)
gl.bufferData(gl.ARRAY_BUFFER, pbytes, gl.STATIC_DRAW)
gl.bindBuffer(gl.ARRAY_BUFFER, cbuf)
gl.bufferData(gl.ARRAY_BUFFER, cbytes, gl.STATIC_DRAW)

vao = gl.createVertexArray()
gl.bindVertexArray(voa)
gl.bindBuffer(gl.ARRAY_BUFFER, pbuf)
gl.vertexAttribPointer(pos_loc, 2, gl.FLOAT, false, 0, 0)
gl.bindBuffer(gl.ARRAY_BUFFER, cbuf)
gl.vertexAttribPointer(col_loc, 2, gl.FLOAT, false, 0, 0)
gl.enableVertexAttribArray(pos_loc)
gl.enableVertexAttribArray(col_loc)

The full interleaved example is

interleaved = [
    [-1,-1], [0,0,0],
    [ 1,-1], [1,0,0],
    [ 1, 1], [0,1,0],
    [-1, 1], [0,0,1],
]
iflat = pos.flat()
ibytes = new Float32Array(iflat)

ibuf = gl.createBuffer()
gl.bindBuffer(gl.ARRAY_BUFFER, ibuf)
gl.bufferData(gl.ARRAY_BUFFER, ibytes, gl.STATIC_DRAW)

vao = gl.createVertexArray()
gl.bindVertexArray(voa)
// gl.bindBuffer(gl.ARRAY_BUFFER, ibuf) // not needed, still bound
gl.vertexAttribPointer(pos_loc, 2, gl.FLOAT, false, 4*3, 0)
gl.vertexAttribPointer(col_loc, 2, gl.FLOAT, false, 4*2, 4*3)
gl.enableVertexAttribArray(pos_loc)
gl.enableVertexAttribArray(col_loc)