Cycles: Adding Hilbert Spiral as a tile order for rendering

This patch adds the "Hilbert Spiral", a custom-designed continuous space-filling curve, as a tile order for rendering in Cycles.
It essentially works by dividing the tiles into tile blocks which are processed in a spiral outwards from the center. Inside each
block, the tiles are processed in a regular Hilbert curve pattern. By rotating that pattern according to the spiral direction,
a continuous curve is obtained, which helps with cache coherency and therefore rendering speed.

The curve is a compromise between the faster-rendering Bottom-to-Top etc. orders and the Center order, which is a bit slower,
but starts with the more important areas. The Hilbert Spiral also starts in the center (unless huge tiles are used) and is still
marginally slower than Bottom-to-Top, but noticeably faster than Center.

Reviewers: sergey, #cycles, dingto

Reviewed By: #cycles, dingto

Subscribers: iscream, gregzaal, sergey, mib2berlin

Differential Revision: https://developer.blender.org/D1166

1 files changed, 2 insertions, 1 deletions

diff --git a/intern/cycles/render/tile.h b/intern/cycles/render/tile.h
index 09e1b25dda7..700e00c9e0a 100644
--- a/intern/cycles/render/tile.h
+++ b/intern/cycles/render/tile.h
@@ -47,7 +47,8 @@ enum TileOrder {
 	TILE_RIGHT_TO_LEFT = 1,
 	TILE_LEFT_TO_RIGHT = 2,
 	TILE_TOP_TO_BOTTOM = 3,
-	TILE_BOTTOM_TO_TOP = 4
+	TILE_BOTTOM_TO_TOP = 4,
+	TILE_HILBERT_SPIRAL = 5,
 };
 
 /* Tile Manager */