compare_histogram

Computes the color histogram of the image (cv::calcHist()) and compares it with a reference histogram (cv::compareHist()), e.g. to find the scene or the object of the reference.

opencv_apps	3.0.0 and later
ROS	ROS One, jazzy, kilted, lyrical and later
OpenCV	any version of the distributions
Group	Histogram comparison, the nodes of which publish `~/distance` (Float64)

Usage

# ROS 2
ros2 launch opencv_apps compare_histogram.launch.xml image:=/camera/image_raw debug_view:=false
# ROS 1
roslaunch opencv_apps compare_histogram.launch image:=/camera/image_raw debug_view:=false

Try it with the demo video

Download the input video of the demo above, publish it as image with image_publisher, and run the node on it, in the terminals:

wget https://ros-perception.github.io/opencv_apps/media/clips/street.mp4
# terminal 1: publish the video as image in a loop
ros2 run image_publisher image_publisher_node street.mp4 --ros-args -p publish_rate:=10.0 -r image_raw:=image
# terminal 2: run the node with the parameters of the demo
H=$(python3 -c "import cv2; img = cv2.VideoCapture('street.mp4').read()[1]; print([round(float(v), 6) for c in range(3) for v in cv2.normalize(cv2.calcHist([img], [c], None, [32], [0, 256]), None, 0, 1, cv2.NORM_MINMAX).flatten()])")  # the reference histogram of the first frame
ros2 launch opencv_apps compare_histogram.launch.xml image:=image histogram_comparison_type:=3 histogram_size:=32 node_name:=compare_histogram_color reference_histogram:="$H"
# terminal 3: view the result
ros2 run rqt_image_view rqt_image_view /compare_histogram_color/image

The reference histogram is that of the first frame of the clip, as test/ros2/test_compare_histogram.py.

See Credits of the demo videos for the license of the video.

Subscribed topics

Topic	Type
`image`	`sensor_msgs/msg/Image`

Published topics

Topic	Type
`~/channel_distances`	`std_msgs/msg/Float64MultiArray`
`~/distance`	`std_msgs/msg/Float64`
`~/histogram`	`std_msgs/msg/Float64MultiArray`
`~/image`	`sensor_msgs/msg/Image`

Parameters

See also the common parameters.

Name	Type	Default (range)	Changed at run time	Description
`accumulate`	bool	false	yes	Accumulation flag. If it is set, the histogram is not cleared between images
`histogram_comparison_type`	int	0 (0 - 3)	yes	Histogram Comparison method, 0: Correlation, 1: Chi_Square, 2: Intersection, 3: Bhattacharyya
`histogram_size`	int	256 (1 - 1024)	yes	Number of bins of the histogram of each channel
`queue_size`	int	3	no, restart the node	Specify queue_size of input image subscribers
`reference_histogram`	double[]	[]	yes	Reference of the histogram comparison, channels x histogram_size values in the layout of ~/histogram

Source

src/nodelet/compare_histogram_nodelet.cpp, launch/compare_histogram.launch.xml