be a bit more strict about negative values in textures/environment maps

src/emitters/envmap.cpp

@@ -181,6 +181,10 @@ public:
 				std::numeric_limits<Float>::infinity(), Spectrum::EIlluminant);
 		}
 
+		if (m_mipmap->getMinimum().min() < 0)
+			Log(EError, "\"%s\": The environment map contains negative pixel values! This is not allowed.",
+				m_filename.string().c_str());
+
 		if (props.hasProperty("intensityScale"))
 			Log(EError, "The 'intensityScale' parameter has been deprecated and is now called scale.");
 

src/textures/bitmap.cpp

@@ -277,6 +277,11 @@ public:
 					rfilter, m_wrapModeU, m_wrapModeV, m_filterType, m_maxAnisotropy, 
 					createCache ? cacheFile : fs::path(), timestamp);
 		}
+
+		if ((m_mipmap1 && m_mipmap1->getMinimum().min() < 0) ||
+		    (m_mipmap3 && m_mipmap3->getMinimum().min() < 0))
+			Log(EError, "\"%s\": The texture map contains negative pixel values! This is not allowed.",
+				m_filename.string().c_str());
 	}
 
 	inline ReconstructionFilter::EBoundaryCondition parseWrapMode(const std::string &wrapMode) {