I don\'t think the tree transparency bug is really a bug. It has something to do with how a 3D engine calculates what to display when transforming from the 3-dimensional object space to the 2-dimensional view space.
Basically an algorithm called \"z-buffer\" is used. It tries to calculate, if an object is occluded by another object when looked at from the point of view YOU have when looking at it.
So when you take Luddrigos first screenshot the leftmost tree occludes the stone behind it. This happens because the tree consists of a flat image composed of both transparent and opaque parts. The transparent parts are meant to let you see through them so you should see the rock...
But the z-buffer algorithm calculated, that the rock is occluded and therefore it is not rendered.[1] So what happens when you have a transparent pixel and you decided not to draw whats lies behind? Indeed, you simply see the background which is allways rendered.[2]
This bug could only be solved by the PS team if they stopped using transparent parts that occlude what lies behind.
Or the people working on the crystal space engine have to change their z-buffer implementation to ignore transparent pixels when transforming from object to view space.
Hope this helped,
MC.
PS: Of course it is possible that I got this totally wrong. (--> signature)
I\'m not a PS dev so there could be tons of other reasons for this to happen

But to me this seems to be reasonable.
footnotes:
[1] In fact it
is rendered and removed or rather painted over afterwards when painting the 2-dimensional image map. The pixel resulting from the rock is painted over with the (transparent) pixel from the tree in front of it. It\'s done this way because \"occlusion culling\" is very complicated to do and takes much more computational power than simply rendering everything and painting the foremost point (the one with the smallest z-value which corresponds to the viewing distance of the object) in the view frustrum when flattening it to view space.
[2] As noted above, all objects are rendered, but the background is usually treated a bit different so its shown through the transparent pixels.