Fix T97761: incorrect mixing of integers

Sometimes integers are mixed using float weights. In those cases
the mixed result has to be converted from into float again.
Previously, this was done using a simple cast, which was unexpected
because e.g. 14.999 would be cast to 14 instead of 15.
Now, the values are rounded properly.

This can affect existing files unfortunately without a good option
for versioning. Gladly, very few files seem to depend on the details
of the old behavior.

Differential Revision: https://developer.blender.org/D14892

1 files changed, 10 insertions, 2 deletions

diff --git a/source/blender/blenlib/BLI_math_base.hh b/source/blender/blenlib/BLI_math_base.hh
index 81f5343056e..034c6968c94 100644
--- a/source/blender/blenlib/BLI_math_base.hh
+++ b/source/blender/blenlib/BLI_math_base.hh
@@ -108,12 +108,20 @@ template<typename T,
          BLI_ENABLE_IF((is_math_float_type<FactorT>))>
 inline T interpolate(const T &a, const T &b, const FactorT &t)
 {
-  return a * (1 - t) + b * t;
+  auto result = a * (1 - t) + b * t;
+  if constexpr (std::is_integral_v<T> && std::is_floating_point_v<FactorT>) {
+    result = std::round(result);
+  }
+  return result;
 }
 
 template<typename T> inline T midpoint(const T &a, const T &b)
 {
-  return (a + b) * T(0.5);
+  auto result = (a + b) * T(0.5);
+  if constexpr (std::is_integral_v<T>) {
+    result = std::round(result);
+  }
+  return result;
 }
 
 }  // namespace blender::math