How to decode the ROS compressed image's data by Qt

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I want use UDP to send the snapshot of my camera. To reduce the network usage，I transformed it to sensor_msgs/compressed first, which format is jpg. and then I send it by UDP to my pc. I use QT to receive it and use Qimage
to save it to show. But it show blank. By comparing it with the jpg's specification , I found it might not conformance to specifications. It really like a mystery.