A while ago I adjusted my Zoneminder setup to use ffmpeg as source type. In older ZM versions the source type was often set to "remote" but this is nowadays considered deprecated.
The biggest advantage of using ffmpeg as source type is the recording of real mp4 videos, instead of creating many single jpeg files for each recorded event.
However since that change I noticed that all recorded events now started with a grey "layer" before the actual video.
I considered this as annoying but still OK as I would only come across that grey content in the first 1-2 seconds of an event - as long as the real footage is still there.
However when sending e-mail alerts of a recorded event, the attached screenshot (using the %EI1% variable in the e-mail body) also showed the grey content:
That's not good. The screenshot should give a quick sight of the reason, why the motion detection was triggered.
After some research I came across a thread in the Zoneminder forums, mentioning similar issues with grey frames. Following the discussion, it turns out that the ffmpeg encoder settings need to be set and the defaults (or at least what looks like defaults) are not actually applied to ffmpeg while recording+encoding a video.
The ffmpeg encoder parameters can be found in the settings of a camera (monitor) under the "Storage" settings:
The current parameters text field by default shows the following content:
# Lines beginning with # are a comment
# For changing quality, use the crf option
# 1 is best, 51 is worst quality
#crf=23
Somehow one would assume that crf=23 is set by default, but it's actually not. According to the discussion mentioned above, the comments also could cause some problems, so it's best to fully remove the content and only set the actual parameter crf=23:
As soon as the next event was recorded on that camera/monitor, the resulting video now started correctly - without any grey frames at the beginning.
This also had a positive effect on the e-mail notification, which now shows a correctly attached screenshot:
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