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Gray Frames in Video Stream

In the local video stream preview, you may see portions of the view periodically grayed-out and pixelated

Typically this behavior is due to dropped key frames. In an H264 encoding, the full image is only sent every N frames while intermediate frames contain the difference relative to the last full image (called the key frame or I-frame). When not much is changing in the field of view, the difference will be small hence the gray pixels. 

OpenALPR may show gray frames even when a third party application (i.e. a VMS/NVR client or VLC Media Player) does not. These other applications may have built-in logic to handle dropped frames more gracefully than OpenALPR (i.e. defaulting back to the last received key frame). This is done to create a nicer user experience, however our priority is using the realtime stream data. Because of this, we do not make any assumptions about the fidelity of old key frames and will always show the raw data received from the camera.

Dropped frames usually happen when the network bandwidth is exceeded. This could be due to 
  • Insufficient bandwidth allocation (i.e. on a cellular connection)
  • Pulling multiple copies of the camera stream (one for OpenALPR and another for your VMS)
  • Using an excessively high frame rate or resolution. See our recommendations based on traffic speed
To remedy the situation, we recommend 
  1. Using a bandwidth calculator to estimate the required network speed for your application. Then, you can speed test the existing network for comparison 
  2. Checking if your camera allows you to set the group of pictures (GOP) value. This determines how frequently a key frame (or I-frame) is sent, and by decreasing its value you can save on bandwidth. ACTi Cameras has a detailed report on this topic with helpful visuals and technical information. As mentioned in their report, make sure to use variable bit rate in conjunction with any GOP changes.

Addison is the author of this solution article.

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