What is the real meaning of SOURCE BPM and how to set it in script?

[VladaK]

Hi,
I have strat to use Reaper befor few days and I'm doing a script, which prepares fade in/out and tempo gradual transitions between media items to help me to make smooth transitions (image in the attachement). But I
- don't understand the relation between playback rate, source bpm and tempo/time signature.
- don't know, how to set the source bpm in the script

The core steps of the script are:
1. for the selected item media - find all items in any track, which overlaps the selected one.
2. set the fades in/out for the selected item (it depends on overlapped area and works with multiple overlapping and partial/complete overlapping)
3. ask user for the tempo of the selected item
4. set the source bpm of the selected item to the tempo (I havent found the function to set the source bpm yet, which I can use in the script).
5. set the selected item timebase to "Beats (auto-stretch at tempo changes)"
6. Add 4 tempo/time signatures markers:
beginning and end of the item = actual tempo
after fade in and before fade out = the tempo of the selected item

The internal time signatures and the source bpm are equal, so I suppose, the media plays in correct tempo. But it doesn't.
I have tryied to correct the tempo
- time/tempo signature for the whole item = 121 bpm
- source bpm = 121
- playback rate = 1
- different item timebase
But it seems, that the relation between source bpm and playback rate is different, than I suppose and I was not able to correct it.

I have found other solution for my script, but a little bit more complicated. But I would like to understand:
- What is the relation between source bpm, item playback rate and time/tempo marker?
- Is the source bpm a real value for the source bpm and its original length?
- Is there a function to set the source bpm?

Thank you for your answeres, ideas, questions!

Vlada

[schwa]

The source BPM that is displayed when you set the playback rate in the item properties dialog is a calculation based on the item length, item playback rate, and project tempo envelope. When you input a new value, the item length and playback rate are adjusted based on the number. The source BPM doesn't persist as an attribute of the media item.

[amagalma]

@Schwa: could you give us the math formula, or better expose it as a reascript function?

Something like:
CalculateTakePlayRateFromBPM( take, source_bpm )

Edit: This function does the job -> GetTempoMatchPlayRate