AP_Mount: fix object tracking command for ViewPro

[eppravitra]

The command should also take negative values

[eppravitra]

The change was tested with CubeOrangePlus connected to ViewPro A10T

[rmackay9]

Hi @eppravitra,

Thanks for your work on fixing the ViewPro driver. Are you sure this works correctly? It seems there could be two problems:

1. send_tracking_command2 uses the htobe16() method which I think takes a uint16_t meaning that param1 and param2 are just going to be cast to be unsigned anyway
2. The existing code that converts p1 and p2 to values between -960~+960 and -540 ~ +540 seems incorrect. Of course I wrote this code originally and you haven't touched it as part of this PR but I was unable to test this properly.

Shouldn't this line:
send_tracking_command2(TrackingCommand2::SET_POINT, (p1.x - 0.5) * 960, (p1.y - 0.5) * 540));

.. actually be ..

send_tracking_command2(TrackingCommand2::SET_POINT, (p1.x - 0.5) * 2.0 * 960, (p1.y - 0.5) * 2.0 * 540));

[eppravitra]

Hi @rmackay9,

The code works. I tested it with actual hardware. I very much agree with your concerns though. I am willing to help make changes, to make things clearer.

1. I could not find an equivalent function of htobe16() for int16 though. Do you have any suggestions? That is the reason why I have not changed that line.

2. I very much want to make this change.

send_tracking_command2(TrackingCommand2::SET_POINT, (p1.x - 0.5) * 2.0 * 960, (p1.y - 0.5) * 2.0 * 540));

With your permission, could I make the change? Of course, it will break someone's code that might be using this function, but the change will make it comply with mavlink documentation.

[rmackay9]

@eppravitra,

Thanks, as long as we think it is correct then I think the 2nd change should be fine. I suspect nobody has used the tracking function yet so it's likely safe. It looks like I have added a command to test this to our MAVLink Interfaces Camera wiki page but I only tested the very center so this is likely why I didn't notice the problem. Once we get this fixed, I'll add some more tests to that page.

Ideally when doing testing we should confirm that it's incorrect before and correct aftwards. So two tests if possible.

[eppravitra]

The scaling change has been made @rmackay9 .
I already test it with CubeOrangePlus and ViewPro A10T. It works.

I think this one is ready for a merge.

[rmackay9]

tested and it works well, thanks!