# WebGL Points Tutorial

## Swipe From the Center Outward

### Overview

This tutorial covers *four techniques* to use with WebGL points.
The WebGL Particle Explosion
example combines concepts used in the other
three WebGL point examples.
*First* points react to user interaction.
*Second*
points are mapped with a texture.
*Third* shaders draw circular points.
*Fourth* the WebGL blend function
causes overlapping points to appear brighter.
Brighter overlapping points create the illusion of more heat toward the center of the explosion
when sprites draw over each other.

## JavaScript Constructor

The JavaScript constructor creates a reference of
type `GLPointsExplode`

defined
within JavaScript file GLPointsExplode.js

.

First this tutorial covers JavaScript which loads a square flat plane defined
with sixteen vertices. Each vertex represents a point.
Create three entities which will access the *same*
same set of vertices,
with different matrices for rotation
and translation.
You may
download the JavaScript Model
which is used to create a square plane with sixteen vertices.

The following listing initializes three square planes. The matrices are offset along the X, Y, or Z axes.

var shapes = new PlanePoints(); var aIm = new Array(); // Initialize the // entity with a texture. var e = new GLEntity( s, 0 ); e.nOffset = Number( shapes.aOffset[0] ); e.nCount = Number( shapes.aCount[0] ); e.matrix[14] = -2.5; aIm.push(e); e = new GLEntity( null,1 ); e.nOffset = Number( shapes.aOffset[0] ); e.nCount = Number( shapes.aCount[0] ); e.matrix[14] = -5; e.matrix[12] = -0.2; e.matrix[13] = -0.2; aIm.push(e); e = new GLEntity( null, 2 ); e.nOffset = Number( shapes.aOffset[0] ); e.nCount = Number( shapes.aCount[0] ); e.matrix[14] = -5; e.matrix[12] = 0.2; e.matrix[13] = 0.2; aIm.push(e);

Last the constructor creates a controller, `GLControl`

reference.
The controller initializes a vertex buffer object with the
vertices and element array index declared in
the JavaScript Model.
For more details regarding the exploding points example
see the
JavaScript Constructor.

var controller = new GLControl ( shapes.aVertices, shapes.aIndices, aIm, this );

## JavaScript Initialization

Before displaying the first frame JavaScript prepares some variables for rendering. For the explosion example set the clear color to black.

gl.clearColor( 0.0, 0.0, 0.0, 1.0 );

The plane includes vertices only.
Neither texels nor vertex colors are needed with face
mapped points.
The following call to
`vertexAttribPointer()`

describes the arrangement of
vertices within the vertex buffer object.
Three vertices follow each other sequentially
with no gaps in between.

gl.vertexAttribPointer ( controller.aPosition, 3, gl.FLOAT, gl.FALSE, 0, 0 );

Save the location of three uniforms named
`uf_x, uf_y`

, and `uf_pointsize`

.
Uniforms `uf_x`

and `uf_y`

represent the user's current X and Y touch or mouse
point on the canvas.
Uniform `uf_pointsize`

adjusts the
size of WebGL points before performing a draw operation.

glDemo.ufX = gl.getUniformLocation( controller.program, "uf_x" ); // Uniform to modify Y // coordinates by time. glDemo.ufY = gl.getUniformLocation( controller.program, "uf_y" ); // Change size of // points by time. glDemo.ufPointSize = gl.getUniformLocation( controller.program, "uf_pointsize" );

Prepare to blend points when they overlap.
WebGL methods `enable(gl.BLEND)`

and
`blendFunc(...)`

modify color channels when points
draw *over* each other.
The
destination `gl.ONE_MINUS_CONSTANT_COLOR`

selection,
causes the center of the explosion to appear brighter.
Color channels are added together when areas overlap.
Try the
*free* Colorful WebGL Blending
tool to *see* how different blend functions
render.
The following listing demonstrates preparing
`blendFunc()`

to modify WebGL rendering
output for
the WebGL Particle Explosion
example.

gl.enable( gl.BLEND ); gl.blendFunc( gl.ONE, gl.ONE_MINUS_CONSTANT_COLOR );

See the entire JavaScript initialization source code.

## JavaScript render()

The `render()`

method
accesses properties `nX, nY`

,
and `nMiddle`

to calculate
the amount of rotation for each
plane's matrix.
Properties `nX`

and `nY`

represent the user's current touch or
mouse coordinates over the canvas.
Property `nMiddle`

is just the middle
of the canvas.

See the
Drag a Sprite
example which includes links to *free* tutorials which demonstrate how to track
the user's touch location on the canvas.

Variable `glDemo`

is a
reference to this project's class

named `GLPointsExplode`

.
The following few lines demonstrate
preparing X and Y coordinates for
upload to the vertex shader
using properties `nX, nY`

, and
`nMiddle`

.

Before upload to the GPU coordinates `nX`

and `nY`

need conversion from canvas coordinates to GPU coordinates.
Canvas coordinates are in the range `{0..canvas.width}`

.
GPU coordinates are in the range `{-1.0..0..+1.0}`

.

var glDemo = controller.glDemo; var aEntities = controller.aEntities; // Normalize the current X,Y touch point. // Range [-1.0...+1.0] var nX = Number( glDemo.nX - glDemo.nMiddle ); var nXA = Math.abs(nX); nX /= glDemo.nMiddle; var nY = Number( glDemo.nY - glDemo.nMiddle ); var nYA = Math.abs(nY); nY /= glDemo.nMiddle; var nAvg = (nXA+nYA)/glDemo.nMiddle; var nPointSize = (1 - nAvg) * 128;

Calculate the point size based on the user's touch position on the canvas using converted GPU coordinates.

var nAvg = (nXA+nYA)/glDemo.nMiddle; var nPointSize = (1 - nAvg) * 128;

Upload the X, Y, and point size values to the vertex shader.

gl.uniform1f ( glDemo.ufX, nX ); gl.uniform1f ( glDemo.ufY, nY ); gl.uniform1f ( glDemo.ufPointSize, nPointSize );

*Last* iterate over the list of
entities. For each entity rotate the matrix,
upload the matrix to the shader,
then draw the matrix.

Three sets of points are drawn with different rotations. Mouse or touch interaction offsets the size and coordinates of points. For more exciting realistic explosions apply random numbers to point coordinates.

mat4.set( e.matrix, matrix ); if (i % 2 == 1){ mat4.rotate( matrix, controller.nRad, [0,0,1] ); } else { mat4.rotate( matrix, -controller.nRad, [0,0,1] ); } gl.uniformMatrix4fv ( controller.uMatrixTransform, false, new Float32Array ( matrix ) );

The following call to the WebGL method `drawElements()`

,
draws a set of points.
Number `e.nCount`

contains the number of points to draw.
Number `e.nOffset`

indicates *where*, to begin
drawing, within the vertex buffer object.
Notice the first parameter is `gl.POINTS`

.
Each vertex represents one point.
WebGL examples often pass `gl.TRIANGLES`

as the
first parameter, which draws triangles.

### WebGL drawElements()

gl.drawElements ( gl.POINTS, e.nCount, gl.UNSIGNED_SHORT, e.nOffset );

See the entire rendering source code.

### Vertex Shader

The vertex shader copies the
current vertex coordinates from attribute `a_position`

to local vector `pos`

.
Next the shader multiplies the current vertex X and Y coordinates by
uniforms `uf_x`

and `uf_y`

.
The previous section titled JavaScript render()

,
demonstrated uploading
values to uniforms `uf_x`

and `uf_y`

during rendering.

vec4 pos = a_position; pos.x *= uf_x; pos.y *= uf_y;

Assign the product of the perspective matrix
`um4_pmatrix`

, the transformation
matrix `um4_matrix`

,
and the modified vertex coordinates,
to built in vector `gl_Position`

.

gl_Position = um4_pmatrix * um4_matrix * pos;

*Last* assign the value from
uniform `uf_pointsize`

to the built in variable
`gl_PointSize`

.
Modifying `gl_PointSize`

allows different particles

to
render at different sizes.
However not all browsers support it.
See the entire vertex shader
for more details.

### Fragment Shader

The fragment shader *first*
tests to determine if the fragment's
coordinate is within the area defined by
a circle.

The `distance()`

function
returns a floating point value
representing the distance between
two values.
The following function call tests
the distance from the center of the
point's texture to the current
coordinate on the point.

float f_center_distance = distance( gl_PointCoord, vec2(0.5,0.5) );

If the distance is greater than `0.5`

just return. Don't render this fragment.
The corners of a square point would return
a value of `0.707`

. The following
code avoids drawing the corners of the square,
eventually rendering a round area for each point.

if ( f_center_distance >= 0.5 ) { discard; }

Next sample the texture at the current point coordinate.

vec4 color0 = texture2D( u_sampler0, gl_PointCoord );

*Last* multiply the sampled color
by constant `cf_dark`

.

See the entire fragment shader for more details.

### Tips

Some older Android Chrome and Firefox browsers *lock up* with texture mapped points.
Additionally older Microsoft browsers haven't implemented point size modifications.
Points display as one pixel only.
However Microsoft Edge does display texture mapped points
at various point sizes.

### Summary

This tutorial covered *four techniques* to use with WebGL points.
The WebGL Particle Explosion
example combines concepts used in the other
three WebGL point examples.
*First* points react to user interaction.
*Second*
points are mapped with a texture.
*Third* shaders draw circular points.
*Fourth* the WebGL blend function
causes overlapping points to appear brighter.
Brighter overlapping points create the illusion of more heat toward the center of the explosion
when sprites draw over each other.

It's assumed readers know how to load texture maps, vertices, and texels. See more WebGL point examples. Have fun and love learning!