At first, got soft shadow. Because the number of triangle of area light source is much more than the one box scene, we use only 1 shadow ray per area light source patch.

Use transparent object.

Use photon mapping, we got the caustic.

Due to large patch number in area light source, when use 10 samples/pixel, we got smooth shadow. One big different between the cube object and sphere object is: There are multiple reflection ray in the cube and it is important to the image. If we don't trace the reflection ray further enought, we will got a black surface. (like in this image)

We implement the Fresnel Equations. (Schlick approximation). We can see the different on reflection on cylinder. We also add the reflection caustic. It is only one small step to got global photon mapping. I think I will do it later. Another pity is: shadow is not smooth enough, photon number is not enough. That is because the patch (triangle) number is much more than box scene. It make this scene is much more time consumming. Current image already takes 5 hours on a 1.8G P4 computer.