how the vertex shader and fragment shader work?

[TopGun34]

there are two set of shaders in SplatMesh.js

1. vertexShaderSource + fragmentShaderSource in buildMaterial : which is used to render Gaussians, we treat each Gaussian as a instance of four points, in VS transform each point based the Gaussian 2D projection, the four point are similar to AABB of this Gaussian. After rasterization, we can get the pixel's color covered by this Gaussian in FS.
2. vsSource + fsSource in setupDistancesComputationTransformFeedback: which is used for transformfeedback to calculate the distance

For the second one, there has code as below to config and use it:
const vertexShader = createShader(gl, gl.VERTEX_SHADER, vsSource);
const fragmentShader = createShader(gl, gl.FRAGMENT_SHADER, fsSource);
gl.drawArrays(gl.POINTS, 0, this.getSplatCount());

But for the first one, i haven't find similar code , i think we should have some code as below to draw the quads which cover each Gaussian:
gl.drawArraysInstanced(gl.TRIANGLE_FAN, 0, 4, vertexCount);

Anyone can help me to understand the code?

[mkkellogg]

For the first one, which is the shader for rendering the splats, the actual calls to draw the splat geometry are in the Three.js library. I try to use Three.js where ever I can in this project and for the rendering of the splats, all I need to provide to Three.js is the shader code and some setup for the splat quad geometry (which is in the buildGeometry() function). Three.js will internally make a call to gl.drawArraysInstanced(). However, Three.js doesn't have built-in support for using transform feedback functionality, so I had to write all of it myself in raw WebGL; that is why you see the call to functions like gl.drawArrays() for that part.

[TopGun34]

thank you so much, it's clear to me now