Adjust black borders to different aspect ratios in same movie, HELP?

[Mermade]

Maybe its in the wrong section. But...I have searched the net and this forum without any luck.

I have Blurays of Transformers, Dark Knight and Armin Only Mirage.

Problem is that certain parts of the movies are filmed with IMAX and have bigger resolution and aspect ratio.
I dont know how to remove black borders from the rest of the scenes to keep everything as is.
If i use (auto)crop the IMAX scenes will also be cut not only the black borders...

Is there any settings or plugins to use to automatically detect and crop only the black borders?
I use avisynth and have meGUI, RipBot, Xvid4psp etc aswell.
All help is appreciated.

[skampy]

I don't there is any decoder/encoder that supports midstream AR switches.

Anyway, what is the problem with keeping the black bars? It barely hurts compression (<.5%) to keep them.

[-TiLT-]

There is no way to be 100% sure what is a black bar and what is just a really dark scene, because black is relative. Pure black, really dark black, greyish black, etc. If the decoder does not use the right colorspace, black becomes greyish.
That's why automatic crop algorithms analyze a distributed portion of frames (with the hope to hit at least one frame where the black borders can be detected correctly).

The only way to optimize the situation is to apply crop by range. See applyrange() in avisynth. Then one has to align the visual area by macroblocks and add the black borders again -> addborders().

This reduces the impact in compression, as the encoder does not need to distribute bitrate to semi-black macroblocks (a transition between black and visual area is considered a sharp contour an needs extra bitrate)