all right. thanks for your answer Dr.Sassi.

@Dr-Sassi
Perhaps I can obtain the relevant functions from the Cinema 4D SDK to launch a .c4d scene in the background and capture a screenshot. However, I believe this approach might be too resource-intensive. Does the .c4d file already contain a pre-generated thumbnail, and how can I retrieve it?

@Dr-Sassi
I apologize for the confusion caused by my previous response. My goal is to create software that can display thumbnails of .c4d files, regardless of whether the user has Cinema 4D installed. Therefore, my question is how to correctly retrieve the thumbnail of a .c4d file in an environment without Cinema 4D installed.

I want to extract the thumbnail image from a .c4d file without having Cinema 4D installed on my system. I noticed that there is a win_thumbnail.dll file located at Maxon Cinema 4D Rxx/resource/libs/win64/. Can I use this DLL to achieve my goal?
Also I'm trying to use tools like WinHex to open the file header of a .c4d file and find relevant information, but I don't know the detailed structure of the file. So, where is the thumbnail of a c4d file stored?
it will be great if someone give me a hand.:)