I have sent you a MAX2010_64/scanline file showing some of these problems.
OK, this is a very vexing problem. I net-render an animation and when playing back the resulting video, the tree shadows 'change' in the animation. What is changing about them? Several things.
1. Sometimes 'some' tree shadows within a frame completely disappear for a single frame... see the above images. First I must track down the specific frame number that this happens on and inspect the frame compared to the other frames (very time consuming). Sure enough... 'some' of the tree shadows are missing for just this one frame. I have tried re-rendering from the main workstation just these frames and sometimes they are fixed and sometimes not.
2. Sometimes 'some' tree shadows within a frame become 50% more transparent for a single frame. If there is a single tree and it's casting only a single shadow... how does it become 50% transparent for only one frame then returns to 50% more opaque? The materials used are 2-sided... is this a problem? Not for all the other trees whose opacity doesn't change per frame.
3. Sometimes the tree shadow positions move for just one frame, then return to their original positions. This effect is a little bit less visible than the others but still annoying as I can see it happening. The shadows sort of wiggle with motion. This could be related to the scene precision as my scenes are usually large and rounding errors could be positioning the shadows in slightly different positions for each frame. Still, I would love to know a way to fix it.
Here is what is happening in the scene I sent. An aircraft flys over a mountainous matte terrain (the aircraft is not in the scene). A camera is attached to the moving aircraft and is therefore watching the aircraft fly along. There are FP trees on the ground below. The FP trees are simple 3 plane trees with simple materials that are 100% self-illuminated. There is a special light attached to the moving aircraft and the only job of this light is to cast XShadows from the trees onto the matte terrain. The light does not shine on all the trees... only the trees near the aircraft. The XShadow map is 10240x10240 so should be plenty big for casting decent shadows and does cast decent shadows on almost all the frames except the problem ones.
In the scene, go the Render dialog and type in a range of 7729 to 7729 and render to a file with alpha. This will render a single frame. Then type in a range of 7730 to 7730 and render to a file. Compare the files in an image viewer that will let you see alpha channel (such as XnView). By switching between the two images, you can clearly see that shadows are missing in frame 7730. They will come back in frame 7731. No matter what I do, I can not make the shadows in frame 7730 render properly like all the other frames.
Changing the XShadow map size to 20000x20000 does nothing to fix this problem so it's not the shadow map, and all the other trees are rendering their shadows just fine a 10240x10240.
Thanks for any ideas!