Dahua NVR52-4KS2 Series User Manual page 189

There are three installation modes: ceiling mount, wall mount, and ground mount.
The different installations modes have different de-warp modes.
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Some models support de-warp of 180° fisheye camera. 180° fisheye camera supports de-warp in
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wall mount mode only.
Figure 5-55 Fisheye
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User's Manual