geom: fix encoder geometry scaling

This commit fixes an error in the geometry scaling, resulting in
an incorrect reconstructed point (used for attribute coding) when
qP > 10, and node-based quantisation is used.

tmc3/geometry_octree_decoder.cpp

@@ -896,12 +896,13 @@ Vec3<int32_t>
 invQuantPosition(int qp, Vec3<uint32_t> quantMasks, const Vec3<int32_t>& pos)
 {
   // pos represents the position within the coded tree as follows:
-  //   |pppppqqqqqq|00
+  //     |pppppqqqqqq|00
   //  - p = unquantised bit
   //  - q = quantised bit
   //  - 0 = bits that were not coded (MSBs of q)
   // The reconstruction is:
-  //   |ppppp00qqqqqq|
+  //   |ppppp00qqqqqq| <- just prior to scaling
+  //   |pppppssssssss| <  after scaling (s = scale(q))
 
   QuantizerGeom quantizer(qp);
   int shiftBits = (qp - 4) / 6;

tmc3/geometry_octree_encoder.cpp

@@ -887,8 +887,8 @@ geometryQuantization(
       int32_t quantPos = quantizer.quantize(pos & quantBitsMask);
       quantPos = PCCClip(quantPos, 0, clipMax);
 
-      // NB: this representation is: |ppppppqqq|00, which is the
-      // same as the decoder
+      // NB: this representation is: |ppppppqqq00|, which, except for
+      // the zero padding, is the same as the decoder.
       pointCloud[i][k] = (pos & ~quantBitsMask) | (quantPos << qpShift);
     }
   }
@@ -907,7 +907,7 @@ geometryScale(
     int quantBitsMask = (1 << quantNodeSizeLog2[k]) - 1;
     for (int i = node.start; i < node.end; i++) {
       int32_t pos = int32_t(pointCloud[i][k]);
-      int32_t quantPos = (pos >> qpShift) & quantBitsMask;
+      int32_t quantPos = (pos & quantBitsMask) >> qpShift;
       pointCloud[i][k] = (pos & ~quantBitsMask) | quantizer.scale(quantPos);
     }
   }