Compress images into custom message

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I want to build a frame filter to synchronize the image streams from an RGBD camera into one message. I am using a ApproximateTime Synchronizer to listen to the sensor_msgs/Image and sensor_msgs/CameraInfo streams and want to publish them in a single message (see blow).

How do I compress/decompress the images to put them into the message? Is it possible to build the messages manually by compressing/decompressing the images with OpenCV? Can I utilize image_transport for that to avoid high memory consumption across several nodes?

Frame.msg

Header header
sensor_msgs/CompressedImage rgb_image
sensor_msgs/CompressedImage depth_image
sensor_msgs/CameraInfo rgb_cam
sensor_msgs/CameraInfo depth_cam