threejs/examples/js/lights/LightProbeGenerator.js

( function () {

	class LightProbeGenerator {

		// https://www.ppsloan.org/publications/StupidSH36.pdf
		static fromCubeTexture( cubeTexture ) {

			let totalWeight = 0;
			const coord = new THREE.Vector3();
			const dir = new THREE.Vector3();
			const color = new THREE.Color();
			const shBasis = [ 0, 0, 0, 0, 0, 0, 0, 0, 0 ];
			const sh = new THREE.SphericalHarmonics3();
			const shCoefficients = sh.coefficients;
			for ( let faceIndex = 0; faceIndex < 6; faceIndex ++ ) {

				const image = cubeTexture.image[ faceIndex ];
				const width = image.width;
				const height = image.height;
				const canvas = document.createElement( 'canvas' );
				canvas.width = width;
				canvas.height = height;
				const context = canvas.getContext( '2d' );
				context.drawImage( image, 0, 0, width, height );
				const imageData = context.getImageData( 0, 0, width, height );
				const data = imageData.data;
				const imageWidth = imageData.width; // assumed to be square

				const pixelSize = 2 / imageWidth;
				for ( let i = 0, il = data.length; i < il; i += 4 ) {

					// RGBA assumed

					// pixel color
					color.setRGB( data[ i ] / 255, data[ i + 1 ] / 255, data[ i + 2 ] / 255 );

					// convert to linear color space
					convertColorToLinear( color, cubeTexture.encoding );

					// pixel coordinate on unit cube

					const pixelIndex = i / 4;
					const col = - 1 + ( pixelIndex % imageWidth + 0.5 ) * pixelSize;
					const row = 1 - ( Math.floor( pixelIndex / imageWidth ) + 0.5 ) * pixelSize;
					switch ( faceIndex ) {

						case 0:
							coord.set( - 1, row, - col );
							break;
						case 1:
							coord.set( 1, row, col );
							break;
						case 2:
							coord.set( - col, 1, - row );
							break;
						case 3:
							coord.set( - col, - 1, row );
							break;
						case 4:
							coord.set( - col, row, 1 );
							break;
						case 5:
							coord.set( col, row, - 1 );
							break;

					}

					// weight assigned to this pixel

					const lengthSq = coord.lengthSq();
					const weight = 4 / ( Math.sqrt( lengthSq ) * lengthSq );
					totalWeight += weight;

					// direction vector to this pixel
					dir.copy( coord ).normalize();

					// evaluate SH basis functions in direction dir
					THREE.SphericalHarmonics3.getBasisAt( dir, shBasis );

					// accummuulate
					for ( let j = 0; j < 9; j ++ ) {

						shCoefficients[ j ].x += shBasis[ j ] * color.r * weight;
						shCoefficients[ j ].y += shBasis[ j ] * color.g * weight;
						shCoefficients[ j ].z += shBasis[ j ] * color.b * weight;

					}

				}

			}

			// normalize
			const norm = 4 * Math.PI / totalWeight;
			for ( let j = 0; j < 9; j ++ ) {

				shCoefficients[ j ].x *= norm;
				shCoefficients[ j ].y *= norm;
				shCoefficients[ j ].z *= norm;

			}

			return new THREE.LightProbe( sh );

		}
		static fromCubeRenderTarget( renderer, cubeRenderTarget ) {

			// The renderTarget must be set to RGBA in order to make readRenderTargetPixels works
			let totalWeight = 0;
			const coord = new THREE.Vector3();
			const dir = new THREE.Vector3();
			const color = new THREE.Color();
			const shBasis = [ 0, 0, 0, 0, 0, 0, 0, 0, 0 ];
			const sh = new THREE.SphericalHarmonics3();
			const shCoefficients = sh.coefficients;
			for ( let faceIndex = 0; faceIndex < 6; faceIndex ++ ) {

				const imageWidth = cubeRenderTarget.width; // assumed to be square
				const data = new Uint8Array( imageWidth * imageWidth * 4 );
				renderer.readRenderTargetPixels( cubeRenderTarget, 0, 0, imageWidth, imageWidth, data, faceIndex );
				const pixelSize = 2 / imageWidth;
				for ( let i = 0, il = data.length; i < il; i += 4 ) {

					// RGBA assumed

					// pixel color
					color.setRGB( data[ i ] / 255, data[ i + 1 ] / 255, data[ i + 2 ] / 255 );

					// convert to linear color space
					convertColorToLinear( color, cubeRenderTarget.texture.encoding );

					// pixel coordinate on unit cube

					const pixelIndex = i / 4;
					const col = - 1 + ( pixelIndex % imageWidth + 0.5 ) * pixelSize;
					const row = 1 - ( Math.floor( pixelIndex / imageWidth ) + 0.5 ) * pixelSize;
					switch ( faceIndex ) {

						case 0:
							coord.set( 1, row, - col );
							break;
						case 1:
							coord.set( - 1, row, col );
							break;
						case 2:
							coord.set( col, 1, - row );
							break;
						case 3:
							coord.set( col, - 1, row );
							break;
						case 4:
							coord.set( col, row, 1 );
							break;
						case 5:
							coord.set( - col, row, - 1 );
							break;

					}

					// weight assigned to this pixel

					const lengthSq = coord.lengthSq();
					const weight = 4 / ( Math.sqrt( lengthSq ) * lengthSq );
					totalWeight += weight;

					// direction vector to this pixel
					dir.copy( coord ).normalize();

					// evaluate SH basis functions in direction dir
					THREE.SphericalHarmonics3.getBasisAt( dir, shBasis );

					// accummuulate
					for ( let j = 0; j < 9; j ++ ) {

						shCoefficients[ j ].x += shBasis[ j ] * color.r * weight;
						shCoefficients[ j ].y += shBasis[ j ] * color.g * weight;
						shCoefficients[ j ].z += shBasis[ j ] * color.b * weight;

					}

				}

			}

			// normalize
			const norm = 4 * Math.PI / totalWeight;
			for ( let j = 0; j < 9; j ++ ) {

				shCoefficients[ j ].x *= norm;
				shCoefficients[ j ].y *= norm;
				shCoefficients[ j ].z *= norm;

			}

			return new THREE.LightProbe( sh );

		}

	}
	function convertColorToLinear( color, encoding ) {

		switch ( encoding ) {

			case THREE.sRGBEncoding:
				color.convertSRGBToLinear();
				break;
			case THREE.LinearEncoding:
				break;
			default:
				console.warn( 'WARNING: LightProbeGenerator convertColorToLinear() encountered an unsupported encoding.' );
				break;

		}

		return color;

	}

	THREE.LightProbeGenerator = LightProbeGenerator;

} )();