From dce16d57dc928ead124dcde1d241727c4f2bbc24 Mon Sep 17 00:00:00 2001
From: Sergey Sharybin <sergey.vfx@gmail.com>
Date: Thu, 12 Mar 2015 17:51:01 +0500
Subject: [PATCH] Revert "Fix T43865: Cycles: Watertight rendering produces
 artifacts on a huge plane"

The fix was really flacky, in terms during speed benchmarks i had
abort() in the fallback block to be sure it never runs in production
scenes, but that affected on the optimization as well. Without this
abort there's quite bad slowdown of 5-7% on the renders even tho
the Pleucker fallback was never run.

This is all weird and for now reverting the change which affects on
all the production scenes and will look into alternative fixes for
the original issue with precision loss on huge planes.

This reverts commit 9489205c5c0b9b432d02be4a3d0d15fc62ee6cb9.
---
 .../kernel/geom/geom_triangle_intersect.h     | 40 +++----------------
 1 file changed, 5 insertions(+), 35 deletions(-)

diff --git a/intern/cycles/kernel/geom/geom_triangle_intersect.h b/intern/cycles/kernel/geom/geom_triangle_intersect.h
index 06917dc16d8..c9e30a451da 100644
--- a/intern/cycles/kernel/geom/geom_triangle_intersect.h
+++ b/intern/cycles/kernel/geom/geom_triangle_intersect.h
@@ -154,15 +154,10 @@ ccl_device_inline bool triangle_intersect(KernelGlobals *kg,
 	 * the hit distance.
 	 */
 	const float T = (U * A_kz + V * B_kz + W * C_kz) * Sz;
-
-	/* Perform "near clipping". */
-	const float abs_T = xor_signmast(T, sign_mask);
-	if(abs_T < 0.0f) {
-		return false;
-	}
-	/* Perform "far clipping". */
-	const float abs_det = xor_signmast(det, sign_mask);
-	if(abs_T > isect->t * abs_det) {
+	const float sign_T = xor_signmast(T, sign_mask);
+	if ((sign_T < 0.0f) ||
+	    (sign_T > isect->t * xor_signmast(det, sign_mask)))
+	{
 		return false;
 	}
 
@@ -174,37 +169,12 @@ ccl_device_inline bool triangle_intersect(KernelGlobals *kg,
 	{
 		/* Normalize U, V, W, and T. */
 		const float inv_det = 1.0f / det;
-		const float t = T * inv_det;
-
-#ifdef __INTERSECTION_REFINE__
-		/* TODO(sergey): When intersection refine is enabled ray is being
-		 * pushed by quite small epsilon from the surface, which causes
-		 * numerical issues of watertight Woop intersection check with
-		 * huge triangles.
-		 *
-		 * Here we're working this around by checking distance in Pleucker
-		 * coordinates if intersection is suspiciously close to the point
-		 * in order to eliminate self-shadowing.
-		 *
-		 * Ideally we need to solve this in Woop intersection code but
-		 * it's quite tricky.
-		 */
-		if(UNLIKELY(abs_det > 100000.0f && t < 1e-3f)) {
-			const float3 Ng = cross(A - B, C - A);
-			const float pleucker_den = dot(Ng, dir);
-			const float pleucker_T = dot(A, Ng);
-			if(UNLIKELY(pleucker_T * pleucker_den < 0.0f)) {
-				return false;
-			}
-		}
-#endif
-
 		isect->prim = triAddr;
 		isect->object = object;
 		isect->type = PRIMITIVE_TRIANGLE;
 		isect->u = U * inv_det;
 		isect->v = V * inv_det;
-		isect->t = t;
+		isect->t = T * inv_det;
 		return true;
 	}
 	return false;