Problem reading generated texture from fragment shader using ThreeJS

I'm trying to generate a textue using a Three.WebGLRenderTarget and then access it in a fragment shader in the next stage.

The idea is to run the first stage once to generate a complex and expensive SDF map and then access it in later stages as needed.

At the moment I can generate the test texture if I send it straight to the screen but when i send it to a texture and try to read it I just get a black screen.

I'm guessing it's something simple and appreciate any help.

The test code:

import * as Three from 'three'

const renderer = new Three.WebGLRenderer();
renderer.setSize(window.innerWidth, window.innerHeight)
document.body.appendChild(renderer.domElement)


const buffer = new Three.WebGLRenderTarget(window.innerWidth, window.innerHeight, {
    minFilter: Three.LinearFilter,
    magFilter: Three.LinearFilter,
    stencilBuffer: false,
    depthBuffer: false,
    type: Three.UnsignedByteType
});
buffer.texture.needsUpdate = false;

const camera = new Three.PerspectiveCamera(
    75,
    window.innerWidth / window.innerHeight,
    0.1,
    1000
)
camera.position.set(0, 0, 1000);

//Buffer scene
const bufferScene = new Three.Scene()

const bufferQuad = new Three.Mesh(
    new Three.PlaneGeometry(2, 2),
    new Three.ShaderMaterial({
        uniforms: {
            u_resolution: { value: new Three.Vector2(window.innerWidth, window.innerHeight) }
        },
        blending: Three.NoBlending,
        vertexShader: vertShader,
        fragmentShader: bufferShader,
        depthWrite: false,
        depthTest: false,
    })
);

bufferScene.add(bufferQuad);


//Screen Scene
const screenScene = new Three.Scene()

const screenQuad = new Three.Mesh(
    new Three.PlaneGeometry(2, 2),
    new Three.ShaderMaterial({
        uniforms: {
            buffer: { value: buffer.texture }
        },
        blending: Three.NoBlending,
        vertexShader: vertShader,
        fragmentShader: screenShader,
        depthWrite: false,
        depthTest: false,

    })
);

screenScene.add(screenQuad);


window.addEventListener('resize', onWindowResize, false)

function onWindowResize() {
    camera.aspect = window.innerWidth / window.innerHeight
    camera.updateProjectionMatrix()
    renderer.setSize(window.innerWidth, window.innerHeight)
    render(screenScene)
}

function loop() {
    requestAnimationFrame(loop)
    render(screenScene)
}

function render(scene) {
    renderer.render(scene, camera)
}

function main() {
    render(bufferScene);
    loop();
}

main()

And the shaders

const vertShader = `
varying vec2 vUv;

void main(){
    
    vUv=position.xy*.5+.5;
    gl_Position=vec4(position.xy,1.,1.);
}`;

const bufferShader = `
precision highp float;

uniform vec2 u_resolution;

void main(){
    vec2 uv=gl_FragCoord.xy/u_resolution.xy;
    gl_FragColor=vec4(uv,0.,1.);
}`;

const screenShader = `
precision highp float;

uniform sampler2D buffer;
varying vec2 vUv;

void main(){
    
    vec4 texel=texture2D(buffer,vUv);
    gl_FragColor=texel;
}`;

It seems you set the render target in the renderer. Try to rewrite your main() function like so:

function main() {
    renderer.setRenderTarget(buffer);
    render(bufferScene);
    renderer.setRenderTarget(null);
    loop();
}

Full code:

const vertShader = `
varying vec2 vUv;

void main(){
    
    vUv=position.xy*.5+.5;
    gl_Position=vec4(position.xy,1.,1.);
}`;

const bufferShader = `
precision highp float;

uniform vec2 u_resolution;

void main(){
    vec2 uv=gl_FragCoord.xy/u_resolution.xy;
    gl_FragColor=vec4(uv,0.,1.);
}`;

const screenShader = `
precision highp float;

uniform sampler2D buffer;
varying vec2 vUv;

void main(){
    
    vec4 texel=texture2D(buffer,vUv);
    gl_FragColor=texel;
}`;

const renderer = new THREE.WebGLRenderer();
renderer.setSize(window.innerWidth, window.innerHeight)
document.body.appendChild(renderer.domElement)


const buffer = new THREE.WebGLRenderTarget(window.innerWidth, window.innerHeight, {
  minFilter: THREE.LinearFilter,
  magFilter: THREE.LinearFilter,
  stencilBuffer: false,
  depthBuffer: false,
  type: THREE.UnsignedByteType
});
buffer.texture.needsUpdate = false;

const camera = new THREE.PerspectiveCamera(
  75,
  window.innerWidth / window.innerHeight,
  0.1,
  1000
)
camera.position.set(0, 0, 1000);

//Buffer scene
const bufferScene = new THREE.Scene()

const bufferQuad = new THREE.Mesh(
  new THREE.PlaneGeometry(2, 2),
  new THREE.ShaderMaterial({
    uniforms: {
      u_resolution: {
        value: new THREE.Vector2(window.innerWidth, window.innerHeight)
      }
    },
    blending: THREE.NoBlending,
    vertexShader: vertShader,
    fragmentShader: bufferShader,
    depthWrite: false,
    depthTest: false,
  })
);

bufferScene.add(bufferQuad);


//Screen Scene
const screenScene = new THREE.Scene()

const screenQuad = new THREE.Mesh(
  new THREE.PlaneGeometry(2, 2),
  new THREE.ShaderMaterial({
    uniforms: {
      buffer: {
        value: buffer.texture
      }
    },
    blending: THREE.NoBlending,
    vertexShader: vertShader,
    fragmentShader: screenShader,
    depthWrite: false,
    depthTest: false,

  })
);

screenScene.add(screenQuad);


window.addEventListener('resize', onWindowResize, false)

function onWindowResize() {
  camera.aspect = window.innerWidth / window.innerHeight
  camera.updateProjectionMatrix()
  renderer.setSize(window.innerWidth, window.innerHeight)
  render(screenScene)
}

function loop() {
  requestAnimationFrame(loop)
  render(screenScene)
}

function render(scene) {
  renderer.render(scene, camera)
}

function main() {
  renderer.setRenderTarget(buffer);
  render(bufferScene);
  renderer.setRenderTarget(null);
  loop();
}

main()

body {
      margin: 0;
}

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