Rotating a geometry using vertex shader in threejs

Question

I am trying to rotate a box using vertex shader but its getting sheared for reason I can't figure out, following is the code, please help me. Here is the fiddle and my code of vertex shader

uniform float delta;
void main()
{
    vec4 modelViewPosition = modelViewMatrix * vec4(position, 1.0);
      gl_Position = (projectionMatrix * modelViewPosition);
      float new_x = gl_Position.x*cos(delta) - gl_Position.y*sin(delta);
      float new_y = gl_Position.y*cos(delta) + gl_Position.x*sin(delta);
      gl_Position.x = new_x;
      gl_Position.y = new_y;
}

https://jsfiddle.net/co4vbhye/

Answer 1

If the viewport is rectangular , then this is compensated by the projection matrix. This means that last transformation which has to be applied to the vertex coordinates is the projection matrix:

clip_position = projection * view * model * position

You've to apply the rotation to the vertex coordinate position. After that transform the result by the view matrix and projection matrix:

uniform float delta;
void main()
{
    vec3 p = position.xyz;
    float new_x = p.x*cos(delta) - p.y*sin(delta);
    float new_y = p.y*cos(delta) + p.x*sin(delta);

    gl_Position = projectionMatrix * modelViewMatrix * vec4(new_x, new_y, p.z, 1.0);
}

Furthermore, the aspect ration which is set to the projection matrix (PerspectiveCamera), has to match the aspect ration of the viewport (canvas):

either

//RENDERER
renderer = new THREE.WebGLRenderer();
renderer.setSize(window.innerWidth, window.innerHeight);

//CAMERA
camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.01, 10000);

or

//RENDERER
renderer = new THREE.WebGLRenderer();
renderer.setSize(CANVAS_WIDTH, CANVAS_HEIGHT);

//CAMERA
camera = new THREE.PerspectiveCamera(75, CANVAS_WIDTH / CANVAS_HEIGHT, 0.01, 10000);

See the example:

var renderer,
    scene,
    camera,
    container = document.getElementById('Canvas_3');

//RENDERER
renderer = new THREE.WebGLRenderer();
//renderer.setClearColor(0xffffff);
renderer.setPixelRatio(window.devicePixelRatio);
renderer.setSize(window.innerWidth, window.innerHeight);
container.appendChild(renderer.domElement);

//CAMERA
camera = new THREE.PerspectiveCamera(75, window.innerWidth / window.innerHeight, 0.01, 10000);

//SCENE
scene = new THREE.Scene();

var customUniforms = {
    delta: {
        value: 0
    }
};
var material = new THREE.ShaderMaterial({
    uniforms: customUniforms,
    vertexShader: document.getElementById('vertexShader').textContent,
    fragmentShader: document.getElementById('fragmentShader').textContent
});

var geometry = new THREE.BoxBufferGeometry(1, 1, 1, 0, 0, 0);
var mesh = new THREE.Mesh(geometry, material);
mesh.position.z = -5;
mesh.position.x = 0;
scene.add(mesh);

window.onresize = function() {
    renderer.setSize(window.innerWidth, window.innerHeight);
    camera.aspect = window.innerWidth / window.innerHeight;
    camera.updateProjectionMatrix();
}

//RENDER LOOP
render();

var delta = 0;

function render() {

    delta += 0.006;
    if (delta > 1.57) delta = 0;

    mesh.material.uniforms.delta.value = delta;

    renderer.render(scene, camera);

    requestAnimationFrame(render);
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/three.js/109/three.min.js"></script>
<div id="Canvas_3"></div>

<script type="x-shader/x-vertex" id="vertexShader">
uniform float delta;
void main()
{
    vec3 p = position.xyz;
    float new_x = p.x*cos(delta) - p.y*sin(delta);
    float new_y = p.y*cos(delta) + p.x*sin(delta);
    
    gl_Position = projectionMatrix * modelViewMatrix * vec4(new_x, new_y, p.z, 1.0);
}
</script>

<script type="x-shader/x-fragment" id="fragmentShader">
uniform float delta;
void main() {
    gl_FragColor = vec4(delta, 0.0, 1.0-delta, 1.0);
}
</script>