[Bf-blender-cvs] [a58a8296d9d] blender-v2.93-release: Fix T85470: Simple deform modifier causes artifacts at low angles

[Henrik Dick]

Commit: a58a8296d9da785920575725bbb41fd5481b5d2d
Author: Henrik Dick
Date:   Fri Apr 30 23:38:31 2021 +1000
Branches: blender-v2.93-release
https://developer.blender.org/rBa58a8296d9da785920575725bbb41fd5481b5d2d

Fix T85470: Simple deform modifier causes artifacts at low angles

The formula used to compute the bend did subtraction of two big numbers
to get the position. Changed to find the delta and add that,
by rearranging the formula into a more numerically stable form.

Reviewed By: mano-wii, campbellbarton

Ref D11074

===================================================================

M	source/blender/modifiers/intern/MOD_simpledeform.c

===================================================================

diff --git a/source/blender/modifiers/intern/MOD_simpledeform.c b/source/blender/modifiers/intern/MOD_simpledeform.c
index ea31bdc6e31..951eacdecda 100644
--- a/source/blender/modifiers/intern/MOD_simpledeform.c
+++ b/source/blender/modifiers/intern/MOD_simpledeform.c
@@ -170,10 +170,12 @@ static void simpleDeform_bend(const float factor,
   sint = sinf(theta);
   cost = cosf(theta);
 
+  /* NOTE: the operations below a susceptible to float precision errors
+   * regarding the order of operations, take care when changing, see: T85470 */
   switch (axis) {
     case 0:
       r_co[0] = x;
-      r_co[1] = (y - 1.0f / factor) * cost + 1.0f / factor;
+      r_co[1] = y * cost + (1.0f - cost) / factor;
       r_co[2] = -(y - 1.0f / factor) * sint;
       {
         r_co[0] += dcut[0];
@@ -182,7 +184,7 @@ static void simpleDeform_bend(const float factor,
       }
       break;
     case 1:
-      r_co[0] = (x - 1.0f / factor) * cost + 1.0f / factor;
+      r_co[0] = x * cost + (1.0f - cost) / factor;
       r_co[1] = y;
       r_co[2] = -(x - 1.0f / factor) * sint;
       {
@@ -193,7 +195,7 @@ static void simpleDeform_bend(const float factor,
       break;
     default:
       r_co[0] = -(y - 1.0f / factor) * sint;
-      r_co[1] = (y - 1.0f / factor) * cost + 1.0f / factor;
+      r_co[1] = y * cost + (1.0f - cost) / factor;
       r_co[2] = z;
       {
         r_co[0] += cost * dcut[0];