Question:

I'm using three.js to create an interactive data visualisation. This visualisation involves rendering 68000 nodes, where each different node has a different size and color.

Initially I tried to do this by rendering meshes, but that proved to be very expensive. My current attempt is to use a three.js particle system, with each point being a node in the visualisation.

I can control the color * size of the point, but only to a certain point. On my card, the maximum size for a gl point seems to be 63. As I zoom in to the visualisation, points get larger - to a point, and then remain at 63 pixels.

I'm using a vertex & fragment shader currently:

vertex shader:

attribute float size;
attribute vec3 ca;
varying vec3 vColor;

void main() {
    vColor = ca;
    vec4 mvPosition = modelViewMatrix * vec4( position, 1.0 );
    gl_PointSize = size * ( 300.0 / length( mvPosition.xyz ) );
    gl_Position = projectionMatrix * mvPosition;
}

Fragment shader:

uniform vec3 color;
uniform sampler2D texture;

varying vec3 vColor;

void main() {
    gl_FragColor = vec4( color * vColor, 1.0 );
    gl_FragColor = gl_FragColor * texture2D( texture, gl_PointCoord );
}

These are copied almost verbatim from one of the three.js examples.

I'm totally new to GLSL, but I'm looking for a way to draw points larger than 63 pixels. Can I do something like draw a mesh for any points larger than a certain size, but use a gl_point otherwise? Are there any other work-arounds I can use to draw points larger than 63 pixels?

Answer:

You can make your own point system by making arrays of unit quads + the center point then expanding by size in GLSL.

So, you'd have 2 buffers. One buffer is just a 2D unitQuad repeated for how ever many points you want to draw.

var unitQuads = new Float32Array([
  -0.5, 0.5, 0.5, 0.5, -0.5, -0.5, 0.5, -0.5,
  -0.5, 0.5, 0.5, 0.5, -0.5, -0.5, 0.5, -0.5,
  -0.5, 0.5, 0.5, 0.5, -0.5, -0.5, 0.5, -0.5,
  -0.5, 0.5, 0.5, 0.5, -0.5, -0.5, 0.5, -0.5,
  -0.5, 0.5, 0.5, 0.5, -0.5, -0.5, 0.5, -0.5,
];

The second one is your points except the positions need to be repeated 4 times each

var points = new Float32Array([
  p1.x, p1.y, p1.z, p1.x, p1.y, p1.z, p1.x, p1.y, p1.z, p1.x, p1.y, p1.z,
  p2.x, p2.y, p2.z, p2.x, p2.y, p2.z, p2.x, p2.y, p2.z, p2.x, p2.y, p2.z,
  p3.x, p3.y, p3.z, p3.x, p3.y, p3.z, p3.x, p3.y, p3.z, p3.x, p3.y, p3.z,
  p4.x, p4.y, p4.z, p4.x, p4.y, p4.z, p4.x, p4.y, p4.z, p4.x, p4.y, p4.z,
  p5.x, p5.y, p5.z, p5.x, p5.y, p5.z, p5.x, p5.y, p5.z, p5.x, p5.y, p5.z,
]);

Setup your buffers and attributes

var buf = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buf);
gl.bufferData(gl.ARRAY_BUFFER, unitQuads, gl.STATIC_DRAW);
gl.enableVertexAttribArray(unitQuadLoc);
gl.vertexAttribPointer(unitQuadLoc, 2, gl.FLOAT, false, 0, 0);

var buf = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, buf);
gl.bufferData(gl.ARRAY_BUFFER, points, gl.STATIC_DRAW);
gl.enableVertexAttribArray(pointLoc);
gl.vertexAttribPointer(pointLoc, 3, gl.FLOAT, false, 0, 0);

In your GLSL shader, compute the gl_PointSize you want then multiply the unitQuad by that size in view space or screen space. Screen space would match what gl_Point does but often people want their points to scale in 3D like normal stuff in which case view space is what you want.

attribute vec2 a_unitQuad;
attribute vec4 a_position;
uniform mat4 u_view;
uniform mat4 u_viewProjection;

void main() {
   float fake_gl_pointsize = 150;

   // Get the xAxis and yAxis in view space
   // these are unit vectors so they represent moving perpendicular to the view.
   vec3 x_axis = view[0].xyz;
   vec3 y_axis = view[1].xyz;

   // multiply them by the desired size 
   x_axis *= fake_gl_pointsize;
   y_axis *= fake_gl_pointsize;

   // multiply them by the unitQuad to make a quad around the origin
   vec3 local_point = vec3(x_axis * a_unitQuad.x + y_axis * a_unitQuad.y);

   // add in the position you actually want the quad.
   local_point += a_position;

   // now do the normal math you'd do in a shader.
   gl_Position = u_viewProjection * local_point;
}

I'm not sure that made any sense but there's more complicated but a working sample here