// lib/post/webgl/difference.ts
// Difference blend — shows the absolute per-channel difference between the
// incoming scene and a second reference texture. WebGL ShaderPass equivalent of
// the WebGPU `webgpu/difference.ts` effect (used for visual diffing / comparing
// two render states). Bind the comparison texture with `setCompareTexture`.

import * as THREE from 'three'
import { ShaderPass } from 'three/addons/postprocessing/ShaderPass.js'


const DIFFERENCE_SHADER = {
  uniforms: {
    tDiffuse: { value: null },
    tCompare: { value: null },
    uScale:   { value: 1 },
  },
  vertexShader: /* glsl */`
    varying vec2 vUv;
    void main () {
      vUv = uv;
      gl_Position = projectionMatrix * modelViewMatrix * vec4(position, 1.0);
    }
  `,
  fragmentShader: /* glsl */`
    uniform sampler2D tDiffuse;
    uniform sampler2D tCompare;
    uniform float uScale;
    varying vec2 vUv;
    void main () {
      vec4 a = texture2D(tDiffuse, vUv);
      vec4 b = texture2D(tCompare, vUv);
      gl_FragColor = vec4(abs(a.rgb - b.rgb) * uScale, 1.0);
    }
  `,
}

export interface DifferenceOptions {
  // The texture to diff the incoming scene against.
  compare?: THREE.Texture | null
  // Amplify small differences for visibility.
  scale?:   number
}

export interface DifferencePass extends ShaderPass {
  setCompareTexture (texture: THREE.Texture | null): void
}

export function createDifference (options: DifferenceOptions = {}): DifferencePass {
  const { compare = null, scale = 1 } = options
  const pass                          = new ShaderPass(DIFFERENCE_SHADER) as DifferencePass
  pass.uniforms.tCompare.value        = compare
  pass.uniforms.uScale.value          = scale
  pass.setCompareTexture              = texture => {
    pass.uniforms.tCompare.value = texture
  }
  return pass
}

// perf: low. Two taps + abs.