Dual Depth Peeling

Reference: Dual Depth Peeling on CFF Libraries

Introduction

Dual Depth Peeling is a technique used in computer graphics to achieve order-independent transparency (OIT). It allows rendering of transparent objects without the need to sort them by depth, which can be computationally expensive and error-prone. This technique is particularly useful in scenarios where multiple transparent surfaces overlap, such as in medical imaging, scientific visualization, or complex 3D scenes.

In this article, we will explore the Dual Depth Peeling technique and how it can be implemented using Three.js, a popular JavaScript library for 3D rendering.

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What is Dual Depth Peeling?

Dual Depth Peeling is an advanced transparency rendering technique that works by peeling away layers of depth from both the front and back of the scene simultaneously. It operates in multiple passes:

1. Initialization Pass: The depth of the nearest and farthest fragments is captured.
2. Peeling Passes: In each pass, the next nearest and farthest fragments are peeled away, and their contributions to the final color are accumulated.
3. Final Composition: The accumulated colors are blended to produce the final transparent rendering.

This approach avoids the need for sorting and ensures accurate blending of overlapping transparent objects.

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Implementing Dual Depth Peeling

To implement Dual Depth Peeling using the provided example, you can follow these steps:

Example:

Below is an example implementation of Dual Depth Peeling using the ComposerWithDualDepthPeeling class and the AdvancedOpacityModule from the @stg-oneportal/web-3d-stdlib library. This example demonstrates how to create a Three.js scene with interactive controls for adjusting the number of peeling layers and the opacity of objects.

import { VanillaThreeCanvas } from "../../tools/vanillaThreeCanvas";
import { ComposerWithDualDepthPeeling } from "./ComposerWithDualDepthPeeling";
import { AdvancedOpacityModule } from "@stg-oneportal/web-3d-stdlib";
import { createBasicThreeScene } from "../../tools/createBasicThreeScene";
import {
  Group,
  Mesh,
  MeshPhysicalMaterial,
  TorusKnotGeometry,
  DoubleSide,
} from "three";

const ExampleScene = () => {
  const onCreate = (canvas) => {
    let composer;

    // Install advanced opacity extensions
    AdvancedOpacityModule.installAdvancedOpacityExtensions();

    const threeState = createBasicThreeScene(
      canvas,
      (renderer, scene, camera, size) => {
        if (!composer) {
          composer = new ComposerWithDualDepthPeeling(renderer, camera, scene);
        }
        composer.setSize(size.x, size.y, size.z);
        composer.render();
      }
    );

    // Add objects to the scene
    const createObject = (color) => {
      const material = new MeshPhysicalMaterial({
        color,
        transparent: true,
        opacity: 0.5,
        side: DoubleSide,
      });
      return new Mesh(new TorusKnotGeometry(0.4, 0.1, 200, 32), material);
    };

    const redObject = createObject("red");
    const greenObject = createObject("green");
    const blueObject = createObject("blue");

    const group = new Group();
    group.add(redObject, greenObject, blueObject);
    scene.add(group);

    return {
      dispose: () => {
        composer.dispose();
        threeState.dispose();
      },
    };
  };

  return <VanillaThreeCanvas onCreate={onCreate} />;
};

export default ExampleScene;

Key Features of the Example

1. Advanced Opacity Module: The AdvancedOpacityModule is used to enable advanced opacity handling for materials.
2. ComposerWithDualDepthPeeling: This class manages the rendering pipeline for Dual Depth Peeling.
3. Interactive Controls: The example includes sliders and switches to adjust the number of peeling layers and the opacity of objects dynamically.

This example provides a starting point for integrating Dual Depth Peeling into your Three.js projects. You can customize it further based on your application's requirements.

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Challenges and Optimization

• Performance: Dual Depth Peeling can be computationally expensive due to multiple rendering passes. Optimizations such as reducing the number of passes or using lower-resolution render targets can help.
• Shader Complexity: Writing custom shaders for depth peeling requires a good understanding of GLSL and Three.js's rendering pipeline.

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Dual Depth Peeling is a powerful technique for achieving accurate transparency in 3D scenes. While it requires custom implementation in Three.js, the results are well worth the effort for applications that demand high-quality transparency rendering. By leveraging render targets and custom shaders, you can implement this technique to enhance the visual fidelity of your Three.js projects.