/*---------------------------------------------------------------------------------------------
* Copyright (c) Bentley Systems, Incorporated. All rights reserved.
* See LICENSE.md in the project root for license terms and full copyright notice.
*--------------------------------------------------------------------------------------------*/
/** @packageDocumentation
 * @module WebGL
 */
import { addEyeSpace, addFrustum } from "./Common";
import { computeLinearDepth } from "./Fragment";
import { addModelViewMatrix } from "./Vertex";
// See Weighted Blended Order-Independent Transparency for examples of different weighting functions:
// http://jcgt.org/published/0002/02/09/
// We are using Equation 10 from the above paper.  Equation 10 directly uses screen-space gl_FragCoord.z.
// flatAlphaWeight bit is set if we want to apply OIT transparency using a constant Z value of 1.
// computeLinearDepth() removes the perspective and puts z in linear [0..1]
// To avoid excessively low weight for fragments close to the far plane, scale depth to [0.15, 1.0].
export const computeAlphaWeight = `
float computeAlphaWeight(float a) {
  float d = computeLinearDepth(v_eyeSpace.z) * .85 + .15;
  float z = (u_shaderFlags[kShaderBit_OITFlatAlphaWeight] ? 1.0 : d);
  return pow(a + 0.01, 4.0) + max(1e-2, 3.0 * 1e3 * pow(z, 3.0));
}
`;
// NB: Our blending algorithm uses pre-multiplied alpha
export const computeOutputs = `
  vec3 Ci = baseColor.rgb * baseColor.a;
  float ai = min(0.99, baseColor.a); // OIT algorithm does not nicely handle a=1
  float wzi = computeAlphaWeight(ai);

  // If we are scaling output into the 0 to 1 range, we use the maximum output of the alpha weight function.
  float outputScale = (u_shaderFlags[kShaderBit_OITScaleOutput] ? 1.0 / 3001.040604 : 1.0);

  vec4 output0 = vec4(Ci * wzi * outputScale, ai);
  vec4 output1 = vec4(ai * wzi * outputScale, 0.0, 0.0, ai * wzi * outputScale);
`;
const assignFragData = `${computeOutputs}
  FragColor0 = output0;
  FragColor1 = output1;
`;
/** @internal */
export function addTranslucency(prog) {
    const frag = prog.frag;
    addEyeSpace(prog);
    addFrustum(prog);
    addModelViewMatrix(prog.vert);
    frag.addFunction(computeLinearDepth);
    frag.addFunction(computeAlphaWeight);
    frag.addDrawBuffersExtension(2);
    frag.set(16 /* FragmentShaderComponent.AssignFragData */, assignFragData);
}
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