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Q1:

In the log you have info" error: conversion from ‘vision_msgs::msg::Detection2DArray_<std::allocator<void> >::_detections_type’ to non-scalar type ‘vision_msgs::msg::Detection2D’

I would iterate over the array of Detection2D (detections) in Detection2DArray.msg to get both results and bbox from Detection2D each iteration.

Detection2DArray.msg is of the form:

# A list of 2D detections, for a multi-object 2D detector.

std_msgs/Header header

# A list of the detected proposals. A multi-proposal detector might generate
#   this list with many candidate detections generated from a single input.
Detection2D[] detections

Detection2D.msg is of the form:

# Defines a 2D detection result.
#
# This is similar to a 2D classification, but includes position information,
#   allowing a classification result for a specific crop or image point to
#   to be located in the larger image.

Header header

# Class probabilities
ObjectHypothesisWithPose[] results

# 2D bounding box surrounding the object.
BoundingBox2D bbox

# The 2D data that generated these results (i.e. region proposal cropped out of
#   the image). Not required for all use cases, so it may be empty.
sensor_msgs/Image source_img

Try to write MWE code with just raw messages of both types to see how to get each part of the message you want.

Q2:

I think C++11 range-based for loops could maybe help you there?

"C++11 extends the syntax of the for statement to allow for easy iteration over a range of elements."