none the wiser about the cause, or a solution.
I've been modding this area (to automatically generate new nts and thm files if a video is edited off-box).
These bitmaps are left lying around because a failure is caught in ts mkbmp when exec'ing ffmeg to generate the bmp file. The catch command does not return any msg to help with diagnose the issue. The bitmap is generated perfectly.
Following such a failure, the thumbnail is not generated again from the decrypt path. Neither is it queued for generation by the daily catch-up scan whilst the failed queue entry persists, or if the Humax generates a thumbnail. In fact it's
never generated, even if you use the 'force' facility, as ffmpeg refuses to overwite the bmp file without the '-y' option.
This fail happens quite often - I've not seen any pattern to the recordings that suffer in this way - ffmeg returns the (meaningless) non-zero exit result because of some minor recoverable problem with the video stream and the thumbnail sequence gives up in response to this.
There are various ways round this - probably the simplest is to get rid of the error checking. In my case I put in code in the error path to check the bmp file is present and the right size - it always has been.
The diagnostics here ('puts' the message to stdout after the caught fail, which is redirected to null in the auto process) currently aren't effective. I've switched this to output to stderr with the usual log time stamp (stderr is redirected to the auto log for the auto process). However, I've never seen it give any information even when a fatal error is forced so maybe the puts is deliberate. I don't have much idea of what Jim-TCL is supposed to be doing with this message facility.
There are a other problems with the thumbnail generation. For example they are never generated if the Humax generates them before decrytion completes. Since the thumbnail is generated during shutdown after a recording, this is quite frequent for my usage. If this wasn't the case I suspect there would be a lot more of these bitmaps lying around.
I've corrected these problems and things are working as I expect (plus changes for me so if I indulge in off-box edits, the nts file is automatically regenerated, along with the thumbnail).
Mind you, thumbnails aren't very effective really, but at least they're a little more meaningful if they're well into the recording. And no more bitmap debris.