You can force specific frametypes for chapters by using a qpfile. Create a text file with notepad.
The format of this file is simply (each with a space between the next):
Frame Number | Frame Type | Quantizer
500 K -1
600 K -1
900 K -1
Frame numbers should be in ascending order. K stands for keyframe. All Quantizers should be set to -1 (automatic).
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