EXRLoader: Added lossyDctChannelDecode (#31482)

* EXRLoader: Added lossyDctChannelDecode.

* Clean up.

* Clean up.

* Clean up.

* Improved comment.

examples/jsm/loaders/EXRLoader.js

@@ -1022,6 +1022,81 @@ class EXRLoader extends DataTextureLoader {
 
 		}
 
+		function lossyDctChannelDecode( channelIndex, rowPtrs, channelData, acBuffer, dcBuffer, outBuffer ) {
+
+			const dataView = new DataView( outBuffer.buffer );
+			const cd = channelData[ channelIndex ];
+			const width = cd.width;
+			const height = cd.height;
+
+			const numBlocksX = Math.ceil( width / 8.0 );
+			const numBlocksY = Math.ceil( height / 8.0 );
+			const numFullBlocksX = Math.floor( width / 8.0 );
+			const leftoverX = width - ( numBlocksX - 1 ) * 8;
+			const leftoverY = height - ( numBlocksY - 1 ) * 8;
+
+			const currAcComp = { value: 0 };
+			let currDcComp = 0;
+			const dctData = new Float32Array( 64 );
+			const halfZigBlock = new Uint16Array( 64 );
+			const rowBlock = new Uint16Array( numBlocksX * 64 );
+
+			for ( let blocky = 0; blocky < numBlocksY; ++ blocky ) {
+
+				let maxY = 8;
+
+				if ( blocky == numBlocksY - 1 ) maxY = leftoverY;
+
+				for ( let blockx = 0; blockx < numBlocksX; ++ blockx ) {
+
+					halfZigBlock.fill( 0 );
+					halfZigBlock[ 0 ] = dcBuffer[ currDcComp ++ ];
+					unRleAC( currAcComp, acBuffer, halfZigBlock );
+					unZigZag( halfZigBlock, dctData );
+					dctInverse( dctData );
+					convertToHalf( dctData, rowBlock, blockx * 64 );
+
+				}
+
+				// Write decoded data to output buffer
+				for ( let y = 8 * blocky; y < 8 * blocky + maxY; ++ y ) {
+
+					let offset = rowPtrs[ channelIndex ][ y ];
+
+					for ( let blockx = 0; blockx < numFullBlocksX; ++ blockx ) {
+
+						const src = blockx * 64 + ( ( y & 0x7 ) * 8 );
+
+						for ( let x = 0; x < 8; ++ x ) {
+
+							dataView.setUint16( offset + x * INT16_SIZE * cd.type, rowBlock[ src + x ], true );
+
+						}
+
+						offset += 8 * INT16_SIZE * cd.type;
+
+					}
+
+					if ( numBlocksX != numFullBlocksX ) {
+
+						const src = numFullBlocksX * 64 + ( ( y & 0x7 ) * 8 );
+
+						for ( let x = 0; x < leftoverX; ++ x ) {
+
+							dataView.setUint16( offset + x * INT16_SIZE * cd.type, rowBlock[ src + x ], true );
+
+						}
+
+					}
+
+				}
+
+			}
+
+			cd.decoded = true;
+
+		}
+
 		function unRleAC( currAcComp, acBuffer, halfZigBlock ) {
 
 			let acValue;
@@ -1634,8 +1709,12 @@ class EXRLoader extends DataTextureLoader {
 
 			}
 
-			// Lossy DCT decode RGB channels
-			lossyDctDecode( cscSet, rowOffsets, channelData, acBuffer, dcBuffer, outBuffer );
+			// Decode lossy DCT data if we have a valid color space conversion set with the first RGB channel present
+			if ( cscSet.idx[ 0 ] !== undefined && channelData[ cscSet.idx[ 0 ] ] ) {
+
+				lossyDctDecode( cscSet, rowOffsets, channelData, acBuffer, dcBuffer, outBuffer );
+
+			}
 
 			// Decode other channels
 			for ( let i = 0; i < channelData.length; ++ i ) {
@@ -1673,7 +1752,11 @@ class EXRLoader extends DataTextureLoader {
 
 						break;
 
-					case LOSSY_DCT: // skip
+					case LOSSY_DCT:
+
+						lossyDctChannelDecode( i, rowOffsets, channelData, acBuffer, dcBuffer, outBuffer );
+
+						break;
 
 					default:
 						throw new Error( 'EXRLoader.parse: unsupported channel compression' );