This query could be deemed as quite broad.
Suppose you implement something along these lines:
var renderer = new THREE.WebGLRenderer();
var scene = new THREE.Scene();
var texture = new THREE.Texture();
var color = new THREE.Color();
var material = new THREE.MeshBasicMaterial({color: color, map: texture});
var coloredAndTexturedCube = new THREE.Mesh( new THREE.CubeGeometry(), material);
var camera = new THREE.Camera();
By connecting all these elements, a cube will be displayed on your screen, showcasing both color and texture if provided.
However, a lot of intricate processes happen behind the scenes. Three.js communicates instructions to the GPU using the WebGL API, involving very low-level commands like 'prepare this shader to process this chunk of memory', 'set blending mode for this call', and 'take this chunk of memory and have it ready to be drawn'.
I am confused about whether gl_FragColor sets the color for the complete object or if the coloring is executed in a specific order that allows for manipulating the coordinates in the shader to achieve random coloring?
If so, how is the shape and coloring sequence determined?
It would be beneficial to have a basic understanding of the rendering pipeline, even if it may not be entirely comprehensible at first, as it can bring clarity to certain aspects.
gl_FragColor establishes the color for the pixel in a buffer, which can be your screen or an offscreen texture. While it sets the color for the 'entire object', this object can potentially be a particle cloud, interpreted as multiple entities. You could have a grid of 10x10 cubes, each with a unique color, yet rendered with just one draw call (as one object).
Returning to your shader:
//You may not see this, but Three.js incorporates this for you. Try introducing an intentional mistake to your shader, and when your debugger flags it, you will witness the entire shader along with these lines in it.
uniform mat4 projectionMatrix; //One mat4 shared across all vertices/pixels
uniform mat4 modelViewMatrix; //One mat4 shared across all vertices/pixels
attribute vec3 position; //Actual vertex, with a different value in each vertex
//Try adding this
varying vec2 vUv;
void main()
{
vUv = uv; //uv, similar to position, is another attribute automatically generated for you, facilitating its transmission to the pixel shader via the varying vec2 vUv.
//This represents the transformation
//The projection matrix transforms space into perspective (vanishing points, shrinking objects as they recede from the camera)
//The modelViewMatrix comprises two matrices - viewMatrix (part of the camera, depicting its rotation and movement against the world) and modelMatrix (indicating the size, orientation, and placement of the object)
gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );
gl_Position = projectionMatrix * viewMatrix * modelMatrix * vec4( position , 1.0 ); //will yield the same result
}
Every material formed with Three.js contains this section of the shader. However, this alone is insufficient for implementing lighting, as it lacks normals.
Consider this fragment shader:
varying vec2 vUv; //Received from the vertex shader
void main(){
gl_FragColor = vec4( vUv , 0.0 , 1.0);
}
Or alternatively, let's illustrate the world position of the object in color:
Vertex shader:
varying vec3 vertexWorldPosition;
void main(){
vec4 worldPosition = modelMatrix * vec4( position , 1.0 ); //Compute the world position and retain it
//The model matrix transforms the object from object space to world space, while vec4( vec3 , 1.0 ) creates a point rather than a direction in "homogeneous coordinates"
//Extracting the vec3 component for working with mat4, ignoring .w
vertexWorldPosition = worldPosition.xyz;
//Carry out the remaining transformations - perceptual perspective within the world space viewed from the camera's standpoint
gl_Position = viewMatrix * worldPosition;
//Writing the prior outcome to gl_Position, which could also be accomplished using a new vec4 cameraSpace, yet writing to gl_Position is viable
gl_Position = projectionMatrix * gl_Position; //Applying perspective distortion
}
Fragment shader:
varying vec3 vertexWorldPosition; //Fetched from the vertex shader
void main(){
gl_FragColor = vec4( vertexWorldPosition , 1.0 );
}
If a sphere is positioned at 0,0,0 and remains stationary, one hemisphere will be dark while the other will be colored. Based on the scale, it might appear white. For instance, with a radius of 100, a gradient from 0 to 1 will be observed, with the remaining portion appearing white (or RGB values clamped to 1.0). Experiment with something like this next:
gl_FragColor = vec4( vec3( sin( vertexWorldPosition.x ) ), 1.0 );