SSAARenderPass.js

import {
	AdditiveBlending,
	Color,
	HalfFloatType,
	ShaderMaterial,
	UniformsUtils,
	WebGLRenderTarget
} from 'three';
import { Pass, FullScreenQuad } from './Pass.js';
import { CopyShader } from '../shaders/CopyShader.js';

/**
*
* Supersample Anti-Aliasing Render Pass
*
* This manual approach to SSAA re-renders the scene ones for each sample with camera jitter and accumulates the results.
*
* References: https://en.wikipedia.org/wiki/Supersampling
*
*/

class SSAARenderPass extends Pass {

	constructor( scene, camera, clearColor, clearAlpha ) {

		super();

		this.scene = scene;
		this.camera = camera;

		this.sampleLevel = 4; // specified as n, where the number of samples is 2^n, so sampleLevel = 4, is 2^4 samples, 16.
		this.unbiased = true;

		// as we need to clear the buffer in this pass, clearColor must be set to something, defaults to black.
		this.clearColor = ( clearColor !== undefined ) ? clearColor : 0x000000;
		this.clearAlpha = ( clearAlpha !== undefined ) ? clearAlpha : 0;
		this._oldClearColor = new Color();

		const copyShader = CopyShader;
		this.copyUniforms = UniformsUtils.clone( copyShader.uniforms );

		this.copyMaterial = new ShaderMaterial(	{
			uniforms: this.copyUniforms,
			vertexShader: copyShader.vertexShader,
			fragmentShader: copyShader.fragmentShader,
			transparent: true,
			depthTest: false,
			depthWrite: false,
			premultipliedAlpha: true,
			blending: AdditiveBlending
		} );

		this.fsQuad = new FullScreenQuad( this.copyMaterial );

	}

	dispose() {

		if ( this.sampleRenderTarget ) {

			this.sampleRenderTarget.dispose();
			this.sampleRenderTarget = null;

		}

		this.copyMaterial.dispose();

		this.fsQuad.dispose();

	}

	setSize( width, height ) {

		if ( this.sampleRenderTarget )	this.sampleRenderTarget.setSize( width, height );

	}

	render( renderer, writeBuffer, readBuffer ) {

		if ( ! this.sampleRenderTarget ) {

			this.sampleRenderTarget = new WebGLRenderTarget( readBuffer.width, readBuffer.height, { type: HalfFloatType } );
			this.sampleRenderTarget.texture.name = 'SSAARenderPass.sample';

		}

		const jitterOffsets = _JitterVectors[ Math.max( 0, Math.min( this.sampleLevel, 5 ) ) ];

		const autoClear = renderer.autoClear;
		renderer.autoClear = false;

		renderer.getClearColor( this._oldClearColor );
		const oldClearAlpha = renderer.getClearAlpha();

		const baseSampleWeight = 1.0 / jitterOffsets.length;
		const roundingRange = 1 / 32;
		this.copyUniforms[ 'tDiffuse' ].value = this.sampleRenderTarget.texture;

		const viewOffset = {

			fullWidth: readBuffer.width,
			fullHeight: readBuffer.height,
			offsetX: 0,
			offsetY: 0,
			width: readBuffer.width,
			height: readBuffer.height

		};

		const originalViewOffset = Object.assign( {}, this.camera.view );

		if ( originalViewOffset.enabled ) Object.assign( viewOffset, originalViewOffset );

		// render the scene multiple times, each slightly jitter offset from the last and accumulate the results.
		for ( let i = 0; i < jitterOffsets.length; i ++ ) {

			const jitterOffset = jitterOffsets[ i ];

			if ( this.camera.setViewOffset ) {

				this.camera.setViewOffset(

					viewOffset.fullWidth, viewOffset.fullHeight,

					viewOffset.offsetX + jitterOffset[ 0 ] * 0.0625, viewOffset.offsetY + jitterOffset[ 1 ] * 0.0625, // 0.0625 = 1 / 16

					viewOffset.width, viewOffset.height

				);

			}

			let sampleWeight = baseSampleWeight;

			if ( this.unbiased ) {

				// the theory is that equal weights for each sample lead to an accumulation of rounding errors.
				// The following equation varies the sampleWeight per sample so that it is uniformly distributed
				// across a range of values whose rounding errors cancel each other out.

				const uniformCenteredDistribution = ( - 0.5 + ( i + 0.5 ) / jitterOffsets.length );
				sampleWeight += roundingRange * uniformCenteredDistribution;

			}

			this.copyUniforms[ 'opacity' ].value = sampleWeight;
			renderer.setClearColor( this.clearColor, this.clearAlpha );
			renderer.setRenderTarget( this.sampleRenderTarget );
			renderer.clear();
			renderer.render( this.scene, this.camera );

			renderer.setRenderTarget( this.renderToScreen ? null : writeBuffer );

			if ( i === 0 ) {

				renderer.setClearColor( 0x000000, 0.0 );
				renderer.clear();

			}

			this.fsQuad.render( renderer );

		}

		if ( this.camera.setViewOffset && originalViewOffset.enabled ) {

			this.camera.setViewOffset(

				originalViewOffset.fullWidth, originalViewOffset.fullHeight,

				originalViewOffset.offsetX, originalViewOffset.offsetY,

				originalViewOffset.width, originalViewOffset.height

			);

		} else if ( this.camera.clearViewOffset ) {

			this.camera.clearViewOffset();

		}

		renderer.autoClear = autoClear;
		renderer.setClearColor( this._oldClearColor, oldClearAlpha );

	}

}


// These jitter vectors are specified in integers because it is easier.
// I am assuming a [-8,8) integer grid, but it needs to be mapped onto [-0.5,0.5)
// before being used, thus these integers need to be scaled by 1/16.
//
// Sample patterns reference: https://msdn.microsoft.com/en-us/library/windows/desktop/ff476218%28v=vs.85%29.aspx?f=255&MSPPError=-2147217396
const _JitterVectors = [
	[
		[ 0, 0 ]
	],
	[
		[ 4, 4 ], [ - 4, - 4 ]
	],
	[
		[ - 2, - 6 ], [ 6, - 2 ], [ - 6, 2 ], [ 2, 6 ]
	],
	[
		[ 1, - 3 ], [ - 1, 3 ], [ 5, 1 ], [ - 3, - 5 ],
		[ - 5, 5 ], [ - 7, - 1 ], [ 3, 7 ], [ 7, - 7 ]
	],
	[
		[ 1, 1 ], [ - 1, - 3 ], [ - 3, 2 ], [ 4, - 1 ],
		[ - 5, - 2 ], [ 2, 5 ], [ 5, 3 ], [ 3, - 5 ],
		[ - 2, 6 ], [ 0, - 7 ], [ - 4, - 6 ], [ - 6, 4 ],
		[ - 8, 0 ], [ 7, - 4 ], [ 6, 7 ], [ - 7, - 8 ]
	],
	[
		[ - 4, - 7 ], [ - 7, - 5 ], [ - 3, - 5 ], [ - 5, - 4 ],
		[ - 1, - 4 ], [ - 2, - 2 ], [ - 6, - 1 ], [ - 4, 0 ],
		[ - 7, 1 ], [ - 1, 2 ], [ - 6, 3 ], [ - 3, 3 ],
		[ - 7, 6 ], [ - 3, 6 ], [ - 5, 7 ], [ - 1, 7 ],
		[ 5, - 7 ], [ 1, - 6 ], [ 6, - 5 ], [ 4, - 4 ],
		[ 2, - 3 ], [ 7, - 2 ], [ 1, - 1 ], [ 4, - 1 ],
		[ 2, 1 ], [ 6, 2 ], [ 0, 4 ], [ 4, 4 ],
		[ 2, 5 ], [ 7, 5 ], [ 5, 6 ], [ 3, 7 ]
	]
];

export { SSAARenderPass };