[Mesa-dev] [PATCH v3] mesa: rotation of 0-vector

[Sergii Romantsov]

Specification doesn't define behaviour for rotation of 0-vector.
In https://github.com/KhronosGroup/OpenGL-API/issues/41 said that
behaviour is undefined and agreed that it would be fine for
implementation to do something useful for this.
Windows and Nvidia drivers have a workaround for that.
For compatibility proposed optimized version of computations.
Specification defines a formula of rotation (see "OpenGL 4.6
(Compatibility Profile) - May 14, 2018", paragraph "12.1.1 Matrices").

Optimized formula will look so:

               R = cos(angle) * I

That is equavalent to logic that magnitude of (0,0,0)-vector is 1.0.

-v2: logic moved to _math_matrix_rotate

-v3: general optimization of computations

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100960
Signed-off-by: Sergii Romantsov <sergii.romantsov at globallogic.com>
---
 src/mesa/math/m_matrix.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/mesa/math/m_matrix.c b/src/mesa/math/m_matrix.c
index 57a4953..3eef122 100644
--- a/src/mesa/math/m_matrix.c
+++ b/src/mesa/math/m_matrix.c
@@ -824,6 +824,18 @@ _math_matrix_rotate( GLmatrix *mat,
                M(1,0) = s;
             }
          }
+         else {
+            /* https://bugs.freedesktop.org/show_bug.cgi?id=100960
+             * https://github.com/KhronosGroup/OpenGL-API/issues/41
+             * Here we will treat magnitude as 1.0 if it really 0.0.
+             * So that is kind of workaround for empty-vectors to have
+             * compatibility with Windows and Nvidia drivers.
+             */
+            optimized = GL_TRUE;
+            M(0,0) = c;
+            M(1,1) = c;
+            M(2,2) = c;
+         }
       }
       else if (z == 0.0F) {
          optimized = GL_TRUE;
-- 
2.7.4