Ya, you are right the problem/bug lies in this line.
I changed this parameter to
if (material instanceof OBJMaterial
&& ((OBJMaterial)material).isOpticalDensitySet()) {
this.sunflow.parameter("eta", 2.5);
} else { ...Then I got the result : glass material render properly
Then I go search about OpticalDensity and come up page OBJLoader.java
I think in this page you are not described the vales&limitation.(I dont know java programing)
ie, Ni , min value = 0.001 max value = 10. (Internet search result )
& this will create the improper value Error.
glass & mirror OpticalDensity strength difference.
I hope this will help you to fix the bug
Thank you.
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