What is T_Packet_Data.my ??


In terminal.c there is the following code:

// does the camera have a tracking solution?
if(T_Packet_Data.my == 0)

how exactly does T_Packet_Data.my give us whether the camera has found a centered target? Is this what we should be using to know if our camera has a target in sight and centered?



This is the y axis location of the “blob” (the green target) on the cameras view. If there is no blob, it is zero. Theres no specific variable saying if something is in view, but this works just as well.

What’s the best way to tell if the camera has a centered target?

Well, logically, if the blob was centered in both the x and y dimensions, it would be safe to assume that the camera was centered on the target.

Now how could you tell how where the blob is…

I’m not totally sure where Kevin found this information (maybe he can elaborate more) but in tracking.h I see

// parameters for CMUcam2 with OV7620 camera module
#define IMAGE_WIDTH 159
#define IMAGE_HEIGHT 239

So from this data you can figure that the middle point of the field of view is (79, 119) which, is also the value of the defines PAN_CENTER_PWM_DEFAULT and TILT_CENTER_PWM_DEFAULT.

So, what you want to do is get T_Packet_Data.my the closest you can to TILT_CENTER_PWM_DEFAULT and T_Packet_Data.mx to PAN_CENTER_PWM_DEFAULT.


That’s what I thought too, but in debugging, it looked to me like mx and my DO NOT get cleared when the target is lost. As I noted in another thread, I changed the if(T_Packet_Data.my == 0) tests in tracking.c to if(T_Packet_Data.confidence <= 1) to make the tracking more reliable.

thanks for all the help y’all.
Totally saving my butt.
Can you point me to that other thread you mention? I am not finding it.

Ehm, where did you get that from? They get cleared when there is new data from the camera, and if the data from the camera contains a y that does not exist, it will be set to 0, as in it can not find it.

That is at least what I got from the code, and the fact that each time the slow loop is run data is retrieved from the camera.

Seriously? I’ve never seen it do that, but I’ll look into it. I’m working on an updated tracking.c and I’ll incorporate the fix(s) when I understand the problem better. BTW, did Rob Steele know about this?


Use the error values for this.


Greg, are you saying that the CMUcam2 will occasionally send a t-packet with a non-zero value for .my when it does not have a tracking solution?


I don’t think he is saying that it sends a t-packet with a non-zero value for .my/.mx but rather that the .my/.mx that is already stored does not change it’s value when the image of a blob is lost.

It however does when it gets new information from the camera, so that would make no sense.

After reading what Greg found in the other thread, Saturday our programmer changed the terminal routine to dump out mx, my, and confidence and we ran tests. We saw non-zero mx, my values with confidence up to 6 or so with the target outside the camera’s field of view. When the target is in the field of view the confidence was as low as 140 but was mostly well over 200.

The room the tests were run in had the camera on a 3’ table, rows of white flourescent lights were 8’-9’ up, and the target was at 3’. We determined the camera was picking up the white lights and putting those in the mx, my values. When we put the target on the floor and limited the tilt up angle the camera no longer picked up the white lights.

Our programmer changed the code to use confidence and it worked perfectly even with white lights in the field of view.

No. Looking at mx, my, or even confidence does not mean the target is centered. Mx and my are undocumented but they are the target centroid in pixels in the sensor grid of IMAGE_WIDTH pixels by IMAGE_HEIGHT pixels which Mike reported. The tracking code which has modes of searching, tracking, and locked is separate. But the default tracking code does not make these modes available for use elseswhere. I modified our tracking code to make these available.

This isn’t possible. The .mx value is only tested when the camera has sent a new t-packet. From what Greg and Keith are saying, it seems as though the camera does spit-out funky t-packets under weird lighting conditions.


Correct. I believe I saw non-zero "my"s with very low confidence (possibly as low as 0.)

I’m not sure if Rob was around when I made the discovery.

What Keith reported corresponds very closely to my experience, although I didn’t attempt controlled experiments. I just noticed that sometimes the camera locked on to a spot well away from the target, and noted that the confidence was zero (or so.)

Okay, I’ll update the code to use the .confidence value as a discriminator instead of .my. Thanks for the information.


I’m altering the tracking code to use the t-packet confidence value as a discriminator. Do you have a better idea of what the default value should be for the code?