I defined some glossy (Glossy BSDF shader) planes by specifying the four vertices and vertex order for each. They are separate meshes, and there are small cylinders separating them.
I rendered them in Cycles under an outdoor HDRI lighting.
Strangely, some adjacent planes are rendered in very different brightness, giving me this unpleasant discontinuity.
I checked the two surface normals -- they are very close.
Why do I still have this shading difference? How may I fix it?