Hey @MMayrh,

Thank you for reaching out to us. This is not how this parameter works. The parameter DOCUMENT_PREVIEW_IMAGE is of data type BitmapButtonStruct, not of BaseBitmap.

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The documentation is a bit misleading here. This is not the document preview bitmap, but sort of the preview bitmap delegate. I.e., an entity that is used to retrieve a preview bitmap (for a document in this case). A BitmapButtonStruct wraps a node, an ID, and a dirty flag.

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This parameter does not make too much sense in the Python API, here is what the C++ API does when the parameter is access for a document:

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I.e., it returns itself (the doc), the ID of the parameter (DOCUMENT_PREVIEW_IMAGE) and the dirty state of its internal current preview bitmap as the BitmapButtonStruct bbs. What you could technically try, is implement a node, e.g., an object, and then set that object as the preview provider for a document. Your node would for that have to implement MSG_DESCRIPTION_GETBITMAP, because that is what effectively will be called. But that is all very theoretical, DOCUMENT_PREVIEW_IMAGE is largely unused in our code base, and I do not see any implemnation for the SetParameter part. So, the document will likely just ignore you trying to overwrite its preview provider. There could be some base implemenation kicking in, but I doubt it.

But what you definitely cannot do, is just set there a bitmap to overwrite the preview image of the document (assuming that was what you wanted to do). That would also not make too much sense since a document is constantly recalculating its preview image. So, on the next update that image would be gone (if it would work like that).

Cheers,
Ferdinand

Check C4D version for using Fields