Fix NiUVController UV offset calculations (bug #5995)

CHANGELOG.md

@@ -118,6 +118,7 @@
     Bug #5923: Clicking on empty spaces between journal entries might show random topics
     Bug #5934: AddItem command doesn't accept negative values
     Bug #5975: NIF controllers from sheath meshes are used
+    Bug #5995: NiUVController doesn't calculate the UV offset properly
     Feature #390: 3rd person look "over the shoulder"
     Feature #832: OpenMW-CS: Handle deleted references
     Feature #1536: Show more information about level on menu

components/nifosg/controller.cpp

@@ -282,14 +282,17 @@ void UVController::apply(osg::StateSet* stateset, osg::NodeVisitor* nv)
         float uScale = mUScale.interpKey(value);
         float vScale = mVScale.interpKey(value);
 
-        osg::Matrix flipMat;
-        flipMat.preMultTranslate(osg::Vec3f(0,1,0));
-        flipMat.preMultScale(osg::Vec3f(1,-1,1));
-
+        // First scale the UV relative to (0,0),
+        // then offset the UV to change the scaling origin to its center.
         osg::Matrixf mat = osg::Matrixf::scale(uScale, vScale, 1);
-        mat.setTrans(uTrans, vTrans, 0);
-
-        mat = flipMat * mat * flipMat;
+        float uOffset = 0.5f * (1.f - uScale);
+        float vOffset = 0.5f * (1.f - vScale);
+        // Apply the original offsets:
+        // U offset is supposed to be subtracted regardless of the graphics library,
+        // while V offset is made negative to account for OpenGL's Y axis convention.
+        uOffset -= uTrans;
+        vOffset -= vTrans;
+        mat.setTrans(uOffset, vOffset, 0);
 
         // setting once is enough because all other texture units share the same TexMat (see setDefaults).
         if (!mTextureUnits.empty())