GDC Introduction

Overview

The geometric distortion correction (GDC) subsystem implements fisheye correction (including 360° panoramic, 180° panoramic, and normal correction modes) and lens distortion correction (LDC). For details about the APIs and parameters related to the GDC, see the API description in "Media Data Processing V2".

hi_mpi_vi_set_chn_ldc_attr
hi_mpi_vi_get_chn_ldc_attr
hi_mpi_vpss_set_grp_fisheye_cfg
hi_mpi_vpss_get_grp_fisheye_cfg
hi_mpi_vpss_set_chn_fisheye
hi_mpi_vpss_get_chn_fisheye

The following figures show the correction process.

Figure 1 Fisheye correction

Figure 2 LDC

Mount and Correction Modes of Fisheye Correction

The fisheye camera supports three installation modes: floor mount, ceiling mount, and wall mount. The floor mount mode applies to the look-up scenario where the camera is installed on the desk or floor. The ceiling mount mode applies to the look-down scenario where the camera is installed on the ceiling. The wall mount mode applies to the scenario where the camera is installed on vertical planes such as the wall.

Fisheye correction supports 360° panoramic, 180° panoramic, and normal correction modes.

You can select the appropriate correction mode based on the installation scenario to achieve the optimal effect. The fisheye selects the correction region by setting the pan-tilt-zoom (PTZ) parameters in each correction mode, implementing the electronic PTZ function. The following table describes the PTZ parameters and correction models in each correction mode in typical scenarios.

**Table 1** Parameters in typical fisheye correction scenarios
Correction Mode	Typical Scenario	Parameter Description
360° panoramic correction	Ceiling mount and floor mount	pan: start position of the correction region tilt: movement of the correction region relative to the original image in the radius direction hor_zoom: range of the correction region (amplitude) ver_zoom: height of the correction region
180° panoramic correction	Wall mount	pan: whether the field of view (FoV) rotates leftward or rightward tilt: whether the FoV rotates upward or downward hor_zoom: horizontal FoV ver_zoom: vertical FoV
Normal correction	Wall mount, ceiling mount, and floor mount	pan: whether the FoV rotates leftward or rightward tilt: whether the FoV rotates upward or downward hor_zoom: horizontal and vertical FoV ver_zoom: invalid parameter

Wall mount in 360° panoramic correction mode is not supported. Ceiling mount and floor mount in 180° panoramic correction mode are not supported.
The outside radius parameter (out_radius) of the fisheye is used to limit the boundary of the original image content for the fisheye. The parts that exceed out_radius are considered to be invalid (black). out_radius should be set to the actual radius of the original image for the fisheye. The inner radius parameter (in_radius) is valid only in 360° panoramic correction mode. This parameter and out_radius determine the range of the correction region in the radius direction. out_radius must be set to the radius of the original image regardless of the correction mode, and the correction region is determined by the PTZ parameters.

LDC Modes

LDC supports two correction modes: barrel distortion correction and pincushion distortion correction, as shown below.

Figure 3 Barrel distortion correction mode

Figure 4 Pincushion distortion correction mode

**Table 2** LDC_V1 parameter configuration
Parameter	Configuration Scope	Description
center_x_offset	–511 to +511	Horizontal offset of the image center point relative to the physical center point
center_y_offset	–511 to +511	Vertical offset of the image center point relative to the physical center point
distortion_ratio	[-300, 500]	Correction strength. A negative value indicates the pincushion distortion correction mode, and a positive value indicates the barrel distortion correction mode.
aspect	bool	Whether to reserve the aspect ratio during FoV adjustment
xy_ratio	0 to 100	FoV parameter, which is valid only when aspect is set to 1.
x_ratio	0 to 100	Horizontal FoV parameter, which is valid only when aspect is set to 0.
y_ratio	0 to 100	Vertical FoV parameter, which is valid only when aspect is set to 0.

You are advised to use the checkerboard for correction and calibration.

FoV Introduction

Figure 5 Diagram of the horizontal and vertical FoV

Figure 6 Rotation diagram of the horizontal FoV

Figure 7 Rotation diagram of the vertical FoV

Parent topic: GDC Tuning