General computation with offscreen rendering

• Alternate two textures and framebuffers each step to avoid a feedback loop.
• Draw a fullscreen triangle with clearColor false during the compute pass.
• Choose rgba32float only when isTextureFormatSupported confirms availability.

function createField(width, height) {
  return device.createTexture({
    width: width,
    height: height,
    format: 'rgba32float',
    usage: luma.Texture.SAMPLE | luma.Texture.RENDER | luma.Texture.COPY_DST,
    sampler: { minFilter: 'nearest', magFilter: 'nearest', mipmapFilter: 'none' }
  });
}

const SIZE = 512;
let src = createField(SIZE, SIZE);
let dst = createField(SIZE, SIZE);

const fboA = device.createFramebuffer({
  width: SIZE,
  height: SIZE,
  colorAttachments: [src]
});

const fboB = device.createFramebuffer({
  width: SIZE,
  height: SIZE,
  colorAttachments: [dst]
});

let writeFbo = fboB;   // 本轮写入目标，初始写进 B

const sim = new luma.Model(device, {
  vs: fullscreenVS,
  fs: simFS,
  attributes: { positions: quadBuffer },
  bufferLayout: [
    { name: 'positions', stepMode: 'vertex', format: 'float32x2' }
  ],
  bindings: { uState: src },
  vertexCount: 3,          // 全屏三角形
  topology: 'triangle-list'
});

function step() {
  const pass = device.beginRenderPass({
    framebuffer: writeFbo,
    clearColor: false        // 全屏覆盖，不需要清除
  });

  sim.setBindings({ uState: src });
  sim.draw(pass);
  pass.end();
  const texTmp = src;
  src = dst;
  dst = texTmp;

  writeFbo = (writeFbo === fboA) ? fboB : fboA;
}

const loop = new luma.AnimationLoop({
  device: device,
  onRender: function () { step(); }
});

• Binding the current color attachment as a sampler is undefined WebGL behaviour.
• Half-float accumulation shows blocky artifacts after hundreds of iterations.