AssimpImporter: don't assume Assimp gives us reasonable attenuation.

And patch it if it's not what's expected by LightData for directional
and ambient lights. This worked with COLLADA but breaks with Blender
where Assimp encodes the max distance into it for some reason.

src/MagnumPlugins/AssimpImporter/AssimpImporter.cpp

@@ -889,10 +889,19 @@ Containers::Optional<LightData> AssimpImporter::doLight(UnsignedInt id) {
         return {};
     }
 
-    return LightData{lightType, color, 1.0f,
-        /* For a DIRECTIONAL and AMBIENT light this is (1, 0, 0), which is
-           exactly what we expect (yay!) */
-        {l->mAttenuationConstant, l->mAttenuationLinear, l->mAttenuationQuadratic},
+    /* In case of COLLADA, for a DIRECTIONAL and AMBIENT light this is (1, 0,
+       0), which is exactly what we expect (yay). In case of Blend files
+       however, this is (1, 0.16, 0.0064) or some other value, so we have to
+       patch it to avoid an assert in LightData later. */
+    /** @todo for blender the values are calculated from max distance, which
+        could be used to restore the `range` property: https://github.com/assimp/assimp/blob/985a5b9667b25390a00e217ee2086882a101d74a/code/AssetLib/Blender/BlenderLoader.cpp#L1251-L1262 */
+    Vector3 attenuation{l->mAttenuationConstant, l->mAttenuationLinear, l->mAttenuationQuadratic};
+    if((lightType == LightData::Type::Directional || lightType == LightData::Type::Ambient) && attenuation != Vector3{1.0f, 0.0f, 0.0f}) {
+        Warning{} << "Trade::AssimpImporter::light(): patching attenuation" << attenuation << "to" << Vector3{1.0f, 0.0f, 0.0f} << "for" << lightType;
+        attenuation = {1.0f, 0.0f, 0.0f};
+    }
+
+    return LightData{lightType, color, 1.0f, attenuation,
         Rad{l->mAngleInnerCone}, Rad{l->mAngleOuterCone},
         l};
 }

src/MagnumPlugins/AssimpImporter/AssimpImporter.h

@@ -351,6 +351,10 @@ verbosity levels in each instance.
     -   Specular color
     -   Custom light orientation vectors --- the orientation is always only
         inherited from the node containing the light
+-   @ref LightData::Type::Directional and @ref LightData::Type::Ambient expect
+    the attenuation to be constant, but Assimp doesn't follow that for certain
+    file formats (such as Blender). In that case the attenuation value from
+    Assimp is ignored.
 -   Area lights are not supported
 -   For certain file formats (such as COLLADA), if a light isn't referenced by
     any node, it gets ignored during import

src/MagnumPlugins/AssimpImporter/Test/AssimpImporterTest.cpp

@@ -110,6 +110,7 @@ struct AssimpImporterTest: TestSuite::Tester {
     void camera();
     void cameraOrthographic();
     void light();
+    void lightDirectionalBlender();
     void lightUnsupported();
     void cameraLightReferencedByTwoNodes();
 
@@ -249,6 +250,7 @@ AssimpImporterTest::AssimpImporterTest() {
               &AssimpImporterTest::camera,
               &AssimpImporterTest::cameraOrthographic,
               &AssimpImporterTest::light,
+              &AssimpImporterTest::lightDirectionalBlender,
               &AssimpImporterTest::lightUnsupported,
               &AssimpImporterTest::cameraLightReferencedByTwoNodes,
 
@@ -1706,6 +1708,29 @@ void AssimpImporterTest::light() {
     }), TestSuite::Compare::Container);
 }
 
+void AssimpImporterTest::lightDirectionalBlender() {
+    /* Versions before 5.1.3 say "BLEND: Expected at least one object with no
+       parent". */
+    if(_assimpVersion < 513)
+        CORRADE_SKIP("Blender 2.8+ files are supported only since Assimp 5.1.3.");
+
+    Containers::Pointer<AbstractImporter> importer = _manager.instantiate("AssimpImporter");
+    CORRADE_VERIFY(importer->openFile(Utility::Directory::join(ASSIMPIMPORTER_TEST_DIR, "light-directional.blend")));
+
+    /* While COLLADA files (the light() test above) have the attenuation as
+       expected, Assimp's Blender importer encodes max distance into it, which
+       has to be patched away */
+    std::ostringstream out;
+    Warning redirectWarning{&out};
+    Containers::Optional<LightData> light = importer->light("Sun");
+    CORRADE_VERIFY(light);
+    CORRADE_COMPARE(light->type(), LightData::Type::Directional);
+    CORRADE_COMPARE(light->color(), (Color3{0.3f, 0.4f, 0.5f}));
+    CORRADE_COMPARE(light->intensity(), 1.0f);
+    CORRADE_COMPARE(light->attenuation(), (Vector3{1.0f, 0.0f, 0.0f}));
+    CORRADE_COMPARE(out.str(), "Trade::AssimpImporter::light(): patching attenuation Vector(1, 0.16, 0.0064) to Vector(1, 0, 0) for Trade::LightData::Type::Directional\n");
+}
+
 void AssimpImporterTest::lightUnsupported() {
     Containers::Pointer<AbstractImporter> importer = _manager.instantiate("AssimpImporter");