As Tilt Brush and Quill are not voxel based, a new VR-based voxel painting system with large (40km^3) and detailed (0.3mm^3) canvas would be interesting. We develop an array of octree of depth 24, using 5 indices per cell: parent, child, and 3-neighbors to accelerate ray traversal. We adaptively refine or coarsen the octree in CPU and sync it with GPU, and then ray cast front to back. To accelerate, we develop a foveated rendering algorithm. We design a quadtree render target whose resolution is dynamically adjusted to heat map, traverse ray, and then interpolate the color in screen space. We traverse ray through upper-level cells as the ray cone widens. We analyze floating point error propagations to thoroughly understand precision problems in deep cells and ray intersections.