Passing data between two ros call back functions

General Support
,
1

Hi,
May I ask that how to make the data from one subscriber call back function available to another subscriber call back function. I tried to add Global variable for the call back function, but it do not seems to work. E.g

rospy.Subscriber("/camera/color/image_raw", Image, showImg, queue_size=1, buff_size=2**24)  # 2 power of 24
rospy.Subscriber("/camera/aligned_depth_to_color/image_raw", Image, showDepth, queue_size=1, buff_size=2**24)

def showDepth(data2):
    global WidthPixel, HeightPixel, depth, Width, Height

def showImg(data):
    global xgOri, ygOri, xg, yg, wg, hg,count, countMax, xgOld, ygOld,DistanceMovX,DistanceMovY,prediction,xgN,ygN,velocityXN,velocityYN

I still getting the error message that “NameError: global name ‘HeightPixel’ is not defined”
May I know is there a way to solve this.
Thank you for reading this post.

2

You must return the value, use keyword ‘return’ at end of function along with the variables you want to return.
This link might be useful:using_return_in_functions
Also you must call the function again in the new function.

3

Hi @zhangmingcheng28,

Perhaps you are not using the keyword global correctly (not sure because I can’t see the complete code).

If you want any variable to be accessible to both functions, just define it outside both functions. It could be like this (just an example - do not copy :slight_smile: ):

# the variables below will be available in both functions
widthPixel = 0.0
depth = 0.0

def showDepth(data2):
    # because we need to modify `depth` we use the word global to reference it
    global depth
    depth = data2.depth
    
    # However because we are just reading `widthPixel` we don't need to "global" it
    if widthPixel > 0.0:
       whatever = depth/widthPixel

def showImg(data):
   # because we need to modify `widthPixel` we use the word global to reference it
   global widthPixel
   widthPixel = data.width
   
    # However because we are just reading `depth` we don't need to "global" it
   whatever = widthPixel * depth

The whatever variable defined in each function are local to the defining functions, but widthPixel and depth are available to both.

4

Thank you for the advice.

1 Like