Applications for each 3D steps

This section describes all possible configuration of CARS applications.

CARS applications are defined and called by their name in applications configuration section:

"applications":{
    "application_name": {
        "method": "application_dependent",
        "parameter1": 3,
        "parameter2": 0.3
    }
},

Be careful with these parameters: no mechanism ensures consistency between applications for now. And some parameters can degrade performance and DSM quality heavily. The default parameters have been set as a robust and consistent end to end configuration for the whole pipeline.

Grid generation

Name: “grid_generation”

Description

From sensors image, compute the stereo-rectification grids

Configuration

Name	Description	Type	Default value	Required
method	Method for grid generation	string	epipolar	Yes
epi_step	Step of the deformation grid in nb. of pixels	int	30	No
save_grids	Save the generated grids (not available yet)	boolean	false	No
geometry_loader	Geometry external library	string	“otb”	No

"applications": {
    "grid_generation": {
        "method": "epipolar",
        "epi_step": 35
    }
},

Resampling

Name: “resampling”

Description

Input images are resampled with grids.

Configuration

Name	Description	Type	Default value	Required
method	Method for resampling	string	bicubic	Yes
epi_tile_size	size in pixels of tile	int	500	No
save_epipolar_image	Save the generated images in output folder	boolean	false	No
save_epipolar_color	Save the generated images (only if color is available)	boolean	false	No

"applications": {
    "resampling": {
        "method": "bicubic",
        "epi_tile_size": 600
    }
},

Sparse matching

Name: “sparse_matching”

Description

Compute keypoints matches on pair images

Configuration

Name	Description	Type	available value	Default value	Required
method	Method for sparse matching	string	“sift”	“sift”	Yes
disparity_margin	Add a margin to min and max disparity as percent of the disparity range.	float		0.02	No
elevation_delta_lower_bound	Expected lower bound for elevation delta with respect to input low resolution DTM in meters	int, float		-1000	No
elevation_delta_upper_bound	Expected upper bound for elevation delta with respect to input low resolution DTM in meters	int, float		1000	No
epipolar_error_upper_bound	Expected upper bound for epipolar error in pixels	float		10.0	No
epipolar_error_maximum_bias	Maximum bias for epipolar error in pixels	float		0.0	No
disparity_outliers_rejection_percent		float		0.1	No
sift_matching_threshold		float		0.6	No
sift_n_octave		int		8	No
sift_n_scale_per_octave		int		3	No
sift_dog_threshold		float		20.0	No
sift_edge_threshold		float		-5.0	No
sift_magnification		float		2.0	No
sift_back_matching		Boolean		true	No
save_matches	Save matches	Boolean		false	No

A lot of information about parameters can be found on VLFEAT SIFT documentation.

"applications": {
    "sparse_matching": {
        "method": "sift",
        "disparity_margin": 0.01
    }
},

Dense matching

Name: “dense_matching”

Description

Compute disparity map from stereo-rectified pair images

Configuration

Name	Description	Type	available value	Default value	Required
method	Method for dense matching	string	“census_sgm” or “mccnn_sgm”	“census_sgm”	Yes
loader	external library use to compute dense matching	string	“pandora”	“pandora”	No
loader_conf	Configuration associated with loader	dict			No
min_elevation_offset	Override minimum disparity from prepare step with this offset in meters	int		None	No
max_elevation_offset	Override maximum disparity from prepare step with this offset in meters	int		None	No
use_sec_disp	Compute secondary disparity map	boolean		false	No
min_epi_tile_size		int		300	No
max_epi_tile_size		int		300	No
epipolar_tile_margin_in_percent		int		60	No
save_disparity_map	Save disparity map	boolean		false	No

See Pandora documentation for more information.

"applications": {
    "dense_matching": {
        "method": "census_sgm",
        "loader": "pandora",
        "loader_conf": "path_to_user_pandora_configuration"
    }
},

Triangulation

Name: “triangulation”

Description

Triangulating the sights and get for each point of the reference image a latitude, longitude, altitude point

Configuration

Name	Description	Type	available value	Default value	Required
method	Method for triangulation	string	“line_of_sight_intersection”	“line_of_sight_intersection”	Yes
geometry_loader	Geometry external library	string	“otb”	“otb”	No
use_geoid_alt	Use geoid grid as altimetric reference.	boolean		false	No
snap_to_img1	if all pairs share the same left image, modify lines of sights of secondary images to cross those of the ref image	boolean		false	No
add_msk_info		boolean		true	No
save_points_cloud	save points_cloud	boolean		false	No

"applications": {
    "triangulation": {
        "method": "line_of_sight_intersection",
        "use_geoid_alt": true
    }
},

Point Cloud fusion

Name: “point_cloud_fusion”

Description

Merge points clouds coming from each pair

Only one method is available for now: “mapping_to_terrain_tiles”

Configuration

Name	Description	Type	available value	Default value	Required
method	Method for fusion	string	“mapping_to_terrain_tiles”	“mapping_to_terrain_tiles”	Yes
resolution	Resolution of the dsm	float	should be > 0	0.5	No
terrain_tile_size		int		None	No
save_points_cloud	Save points clouds	boolean		false	No

Example

"applications": {
    "point_cloud_fusion": {
        "method": "mapping_to_terrain_tiles",
        "resolution": 0.5,
        "save_points_cloud": true
    }
},

Warning

Be careful with resolution to be consistent with resolution in rasterization. No mechanism ensures consistency between applications for now.

Point Cloud outliers removing

Name: “point_cloud_outliers_removing”

Description

Point cloud outliers removing

Configuration

Name	Description	Type	available value	Default value	Required
method	Method for point cloud outliers removing	string	“statistical”, “small_components”	“statistical”	Yes
save_points_cloud	Save points clouds	boolean		false	No

If method is statistical:

Name	Description	Type	available value	Default value	Required
activated		boolean		true	No
k		int	should be > 0	50	No
std_dev_factor		float		5.0	No

If method is small_components

Name	Description	Type	available value	Default value	Required
activated		boolean		true	No
on_ground_margin		int		10	No
connection_distance		float		3.0	No
nb_points_threshold		int		50	No
clusters_distance_threshold		float		None	No

Example

"applications": {
    "point_cloud_outliers_removing": {
        "method": "small_components",
        "on_ground_margin": 10,
        "save_points_cloud": true
    }
},

Point Cloud Rasterization

Name: “point_cloud_rasterization”

Description

Project altitudes on regular grid.

Only one simple gaussian method is available for now.

Configuration

Name	Description	Type	available value	Default value	Required
method		string	simple_gaussian	simple_gaussian	Yes
dsm_radius		float, int		1.0	No
sigma		float		None	No
grid_points_division_factor		int		None	No
resolution	altitude grid step (dsm)	float		0.5	No
dsm_no_data		int		-32768
color_no_data		int		0
color_dtype		string		“uint16”
msk_no_data		int		65535
write_color	Save color ortho-image	boolean		false	No
write_stats		boolean		false	No
write_msk		boolean		false	No
write_dsm	Save dsm	boolean		true	No

Example

"applications": {
    "point_cloud_rasterization": {
        "method": "simple_gaussian",
        "dsm_radius": 1.5
    }
},

Warning

Be careful with resolution to be consistent with resolution in rasterization. No mechanism ensures consistency between applications for now.