mesa/src/amd/vulkan/bvh

GFX12

GFX12 introduces a new BVH encoding for the image_bvh_dual_intersect_ray and image_bvh8_intersect_ray instructions.

BVH8 box node

bitsize/range	name	description
32	internal_child_offset	Offset of child BVH8 box nodes in units of 8 bytes.
32	primitive_child_offset	Offset of child primitive nodes in units of 8 bytes.
32	unused	Used by amdvlk for storing the parent node ID.
32	origin_x	x-offset applied to all child AABBs.
32	origin_y	y-offset applied to all child AABBs.
32	origin_z	z-offset applied to all child AABBs.
8	exponent_x
8	exponent_y
8	exponent_z
4	unused
4	child_count_minus_one
32	obb_matrix_index	Selects a matrix for transforming the ray before performing intersection tests. 0x7F to disable OBB.
96x8	children[8]

children[8] element layout:

bitsize/range	name	description
12	min_x	Fixed point child AABB coordinate.
12	min_y
4	cull_flags
4	unused
12	min_z
12	max_x
8	cull_mask
12	max_y
4	node_type
4	node_size	Increment for the child offset in units of 128 bytes.

The coordinates of child AABBs are encoded as follows:

• min: floor((x - origin_x) / extent)
• max: ceil((x - origin_x) / extent) - 1

image_bvh8_intersect_ray will return the node IDs of the child nodes.

Primitive node

Highlevel layout:

bitsize/range	name	description
52	header	Misc information about this node.
	vertex_prefixes[3]
	data	Compressed vertex positions followed by primitive/geometry index data.
29xtriangle_pair_count	pair_desc[triangle_pair_count]	Misc information about a triangle pair.

header layout:

bitsize/range	name	description
5	x_vertex_bits_minus_one
5	y_vertex_bits_minus_one
5	z_vertex_bits_minus_one
5	trailing_zero_bits
4	geometry_index_base_bits_div_2
4	geometry_index_bits_div_2
3	triangle_pair_count_minus_one
1	vertex_type
5	primitive_index_base_bits
5	primitive_index_bits
10	indices_midpoint	Bit offset where the geometry and primitive indices start (geometry indices in negative direction, primitive indices in positive direction)

The data field is split in three sections:

1. Vertex data, this is a list of floats which share the same prefix and the same number of trailing zero bits. The decompressed value (for example the x component of a vertex) is (prefix << 32 - prefix_bits_x) | read(x_vertex_bits) << trailing_zero_bits where prefix_bits_x is derived from x_vertex_bits and trailing_zero_bits (32 - x_vertex_bits - trailing_zero_bits).
2. Geometry indices.
3. Primitive indices.

Geometry indices are encoded the same way with the only difference being that geometry indices are read/written in negative direction starting from indices_midpoint. The indices section starts with a *_index_base_bits-bit value *_index_base which is the index of the first triangle. Subsequent triangles use indices calculated based on a *_index_bits-bit value:

• *_index = read(*_index_bits) if *_index_bits >= *_index_base_bits
• *_index = read(*_index_bits) | (*_index_base & ~BITFIELD_MASK(*_index_bits)) otherwise.

pair_desc(s) layout:

bitsize/range	name	description
1	prim_range_stop
1	tri1_double_sided
1	tri1_opaque
4	tri1_v0_index	Indices into data, 0xF for procedural nodes.
4	tri1_v1_index	0xF for procedural nodes.
4	tri1_v2_index
tri0 has identical fields:
1	tri0_double_sided
1	tri0_opaque
4	tri0_v0_index
4	tri0_v1_index
4	tri0_v2_index

image_bvh8_intersect_ray will return the following data for triangle nodes:

VGPR index	value
0	t0
1	(procedural0 << 31) | u0
2	(opaque0 << 31) | v0
3	(primitive_index0 << 1) | backface0
4	t1
5	(procedural1 << 31) | u1
6	(opaque1 << 31) | v1
7	(primitive_index1 << 1) | backface1
8	(geometry_index0 << 2) | navigation_bits
9	(geometry_index1 << 2) | navigation_bits

image_bvh8_intersect_ray will return the following data for procedural nodes:

VGPR index	value
3	primitive_index0 << 1
8	(geometry_index0 << 2) | navigation_bits
9	(geometry_index1 << 2) | navigation_bits

navigation_bits is 0 if there are more triangle pairs to process, 1 if this was the last triangle pair and 3 if prim_range_stop is set.

Instance node

bitsize/range	name	description
32x3x4	world_to_object
62	bvh_addr	Units of 4 bytes.
1	aabbs	Does the BLAS (only) contain AABBs? Used for pointer flag based culling.
1	unused
32	unused
24	user_data	Returned by the intersect instruction for instance nodes.
8	cull_mask
The instance node can have up to 4 quantized child nodes:
32	origin_x	x-offset applied to all child AABBs.
32	origin_y	y-offset applied to all child AABBs.
32	origin_z	z-offset applied to all child AABBs.
8	exponent_x
8	exponent_y
8	exponent_z
4	unused
4	child_count_minus_one
96x4	children[4]

image_bvh8_intersect_ray will return:

VGPR index	value
2	BLAS addr lo
3	BLAS addr hi
6	user_data
7	(child_ids[0] & 0xFF) | ((child_ids[1] & 0xFF) << 8) | ((child_ids[2] & 0xFF) << 16) | ((child_ids[3] & 0xFF) << 24)