Wow, that makes it even more impressive what you lot have done if you've even done it without documentation! :O  hats off.
 
The only doc I found on developer.garmin.com was called the device interface sdk, unfortunately it was from 2006 and the only mention of Vista was a sidenote about version 2.12 of the software (my Vista is running version 3.60).  Given that garble understands my Vista but not my Vista HCx I'm assuming that the interfaces have changed since May 2006.
I can have another look at that pdf though if you're saying you got accurate information from there.  It looks like I need to dig into the Protocol Capability Protocol as a starter, because this is where it starts to look funny.  Shame the pdf doesn't even _mention_ the screenshot function though :(
 
About the other gpsbabel formats, that's interesting, but not at all easy to find.  At least it confused me so much originally that I assumed gpsbabel couldn't do it.  It would be great if the list of formats included a one-line example, that way it would be much easier to spot which of the pagefuls of formats to experiment with.  I'll try the ones you suggested, thanks.  The program I use to post-process the tracks is not too fussy about field order, delimiters etc so that should be no problem.
 
About the screendumps yes I agree it doesn't fit too beautifully into gpsbabel, and I've no idea how much fun the integration would be, making it fit with the other functions.  And I have no clue how many models support such a feature or how standard their behaviours are - I just know that garble generates 4-colour bitmap files from my etrex Vista and I assume it worked for the author's receivers (I tweaked the code a little but didn't write it myself).  It's certainly much much less than a couple hundred lines of code by itself but maybe by the time it's made generic and multi-receiver then it would grow...
 
All I know for sure is that the output from the HCx model is different and the values which garble expect to mean height and width of the image, don't (at all).  I mean, I know the format of the data must be different because the bit depth is higher but still.
 


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