Orthographic projections in RealityCapture

There is a possibility in RealityCapture to create arbitrary ortho projections. This tutorial walks you through the process of creating such projection from a model, through setting parameters of an orthographic projection up to exporting a projection and a digital surface model in various formats.

As an example, we will use a model of a gravel pit created for purposes of the previous tutorial named Quality assessment. ( couldn't find such a tutorial)

Ortho Projection tool

Once a model has been created, the Ortho Projection tool becomes active. To create a region for the ortho projection, go to MESH MODEL -> Set Reconstruction Region (the same procedure as for creating a reconstruction region when calculating a 3D model). From the drop-down menu, you can choose the type (see the picture below).

If the reconstruction region is not specified, the Ortho Projection tool will automatically create a reasonable region.

There are 2 ways how to activate the tool:

1. WORKFLOW -> Process -> Ortho Projection
2. SCENE 3D -> Tools -> Mesh model -> Ortho Projection

You can then use the BOX WIDGET to modify the position, size, and rotation of the region.

Inside the region, there is a triaxial widget. Left-click and drag a colored arrow in order to move the region in its direction. Right-click and drag the arrow to move the region in a direction perpendicular to the selected arrow.

Left-click and drag colored quarter circles to rotate the region in a respective way.

Outside the reconstruction region, there are little circles filled with different colors. Left-click and drag blue circles to change the width of the region, green circles to change the length of the region, and magenta circles to change the height of the region.

By selecting a specific grey square in the corner of the region, you can change the plane of a projection. The highlighted orange square then defines the top-left corner of the rendered orthographic projection. The red arrows show the direction of a projection.

You can also use SCENE 3D -> Tools -> Mesh model -> Lasso tool to specify an exact region from which you want to create an orthophoto. 

To automatically fit the project region to the selected triangles, go to SCENE 3D -> Tools -> Mesh model -> Set Reconstruction Region -> Set Region Automatically. Once you have a selection of triangles, the Ortho Projection tool works just with this selection. This means that only selected triangles will be projected to the chosen side. To activate the Ortho Projection tool, go to SCENE 3D -> Tools -> Mesh model -> Ortho Projection.

Setting the parameters of an ortho-projection

Once you activate the tool, the Ortho Projection Tool table shows up in the 1Ds view (see the picture below).

In the Type line, you can specify the type of an ortho projection. The arbitrary type is selected automatically.

NOTE: The Type item is available only for georeferenced projects, otherwise the type of projection is always Arbitrary and the coordinate system is Local Euclidean.

• Arbitrary means there are no restrictions on the position of the region or in a direction of the projection.

  

• Selecting it to the Top ensures that your projection is aligned with the chosen coordinate system. This type can be used when you need to make a geo-referenced orthophoto or a digital surface model, and you need to export it to a GIS application. When selected, rotation parameters 'Yaw' and 'Pitch' are locked.

  

• Side type means that the projection plane is orthogonal to the ground plane of the selected coordinate system (often used in CAD project documentation). When selected, rotation parameters 'Yaw' and 'Pitch' are locked.

  

• Use the Map type when exporting an orthophoto to KML or KMZ format. In this case, the coordinate system will be set to GPS (WGS 84) and all rotations will be locked to 0. 

  

Info group defines parameters (Coordinate system, Centre, Rotation, Size) of the reconstruction region.

• Coordinate system – for georeferenced scenes, you can choose the coordinate system in which an ortho-projection will be rendered.

In the Rendering method, you can specify the type: True ortho from a colored/textured model or Image mosaicing (general or aerial). Image mosaicing enables users to create ortho projections from images, unlike another method which is using colorized dense mesh model. The difference between Aerial and General is in how your cameras are distributed. With the Aerial method, it is recommended to have cameras in the same plane, or in parallel planes, and it is faster than General. The general method is used for projects where cameras are distributed around the object, and it is recommended to be used with terrestrial projects.

 

Width and Height define the resolution of the rendered projection. When changing one of these parameters, the other parameter and the Ortho pixel size will be recomputed accordingly to preserve the aspect ratio defined by the reconstruction region.

Ortho pixel size is a size of a pixel in the units of a chosen coordinate system.

If you are not sure how to define the resolution of an orthographic projection, RealityCapture will calculate the optimal parameters with respect to the properties of a selected model using the Estimate optimal resolution function. 

You can also adjust the background of an orthophoto by changing the parameters Backface color type, Backface color, and Backface color transparency.

After adjusting ortho-projection settings, you can save them by clicking on a small icon Save settings in the bottom-left corner of the Ortho Projection Tool panel (the lower icon in the red box in the picture below). Accordingly, you can reload any previously saved settings by clicking the upper icon Load settings in the red box in the picture below.

You can create an orthographic projection by clicking Render, or create an empty ortho projection and render multiple ones at once later by clicking Add to batch. Rendered ortho projection will appear in the 1Ds view (Ortho projection 1 in the picture below), an ortho projection added to the batch appears in the 1Ds view with the note 'unprocessed' (Ortho projection 2 in the picture below). The symbol in the green box in the picture below indicates that the ortho projection is georeferenced. 

When selecting a specific region with the Lasso tool, only this region is visible in the rendered orthophoto. You can also see the reconstruction region used when rendering the selected ortho-photo in the 3D view (see the picture below).

Parameters of a rendered ortho projection

You can see all created projections in the 1Ds view and access detailed information on each of them by clicking on it, then the Selected ortho photo(s) menu will show up (see the picture below). The Width and Height define the size of an orthophoto in pixels (the ones previously defined in the Ortho Projection tool), and Units per column/row pixel define ortho pixel size in coordinate system units (in our project 0.031 m, which is 31 mm).

You can also see the calculated surface and volume here. Cut volume is the volume between the visible surface and the bottom side of the reconstruction region. Fill volume is the volume between the visible surface and the top side of the reconstruction region. Area 2D is the area of the projection of the visible surface to the bottom side of the reconstruction region. Area 3D is the area of the visible surface. 

In the Coordinate system part, you can see the specifications of the coordinate system and the projection type used to create an ortho projection.

You can render unprocessed ortho projections individually or together. Select the desired unprocessed ortho projection (or more of them) in the 1Ds view. The table Selected ortho photo(s) will show up defining the parameters of the selected ortho projection(s). If there are more orthographic projections selected and their parameters mismatch, the sign '< Different values >' shows up (see the picture below). To render all selected ortho projections, click Render unprocessed ortho projection(s). The processed orthophotos will be added to the project tree in the 1Ds view. To delete all selected unprocessed ortho projections, simply select Delete unprocessed ortho projection(s). 

In the 2D view, the selected ortho projection can be visualized. Click the ORTHO 2D context menu to see all the available options for orthographic projections. In the 2D view, you can see a graphic scale and the current coordinates of a mouse cursor in the WGS84 coordinate system and in the coordinate system of rendered ortho projection (green boxes in the picture below).

 ORTHO 2D context menu

 Now we will take a look at all the options the context menu ORTHO 2D offers.

Source

In the picture below, we have visualized three layers (from left to right – Image, DSM Depth, DSM Altitude) of the same orthophoto. You can also adjust the color scale of the Depth and Altitude layers by moving the top and bottom ends of the color scale.  

Display

In the Display part of the ORTHO context menu, you can enable or disable the display of some selected features on the orthophoto (see the picture below).

In our example, we have enabled the display of the ground control points, the reconstruction region, and the pivot. You can also fit the image to the view by selecting Reset view.  

Tools

In all layers of the selected orthophoto, there is an option to measure distance. You can select 2 points by moving the endpoints of the displayed line after the Measure Distance from the Tools box was activated. The 2D distance (distance in the image) and the 3D distance will appear in the units of the orthophoto’s coordinate system.

You can also use the Sample Points tool to create a sampling grid for better orientation in the orthographic projection. You can define the sampling parameters in the Ortho Sampling Tool tab (red box in the picture below). You can define the span of the grid points by defining the Distance between samples in the units of the orthophoto coordinate system, by setting the Number of points of the generated grid, or by limiting the maximum Raw size, which is the estimated size of the exported sampled points in the memory. The other two parameters will be recalculated accordingly.

If the sampling is too dense, RealityCapture changes the grid step for viewing and the red notification in the bottom-right corner shows up (see the picture below).

Display in the Map view

To see whether your component and also your ortho projection are georeferenced correctly, you can display orthophoto(s) in a Map view. You can change a view to the Map view by dragging the white button in the top-right corner of the layout cell area. In the Map view context tab, you can choose from several map providers. We have chosen OpenStreet Topo.

To display one or more ortho projections, just select them in the 1Ds view holding the Ctrl key, and drag and drop them into the Map view. 

You can use the button Frame control points in the Map view context windows to fit the area of control points into the view or the button Frame Images to fit the selected ortho projections into the view. To reset the zoom and position in the map to its default state press Reset view. 

It is also possible to display orthophotos rendered in different coordinate systems in the Map view at the same time. 

You can select the specific ortho projection also from Map view, Selected ortho photo(s) table will show up in the 1Ds menu. In the Map View Objects context menu, you can bring the selected object(s) one level forward by the tool Bring to Front or send one level back by the tool Send to back. You can also automatically change the order of the object according to the z coordinate by Auto Arrange. To remove the selected object(s) from the Map view click Remove or Remove all if you wish to remove all objects from the Map view. 

In the section Display, you can enable or disable to display of all aligned cameras by the checkbox Cameras, residuals for each image by the checkbox Residuals, and all ground control points by the check box Control Points.

Export

Exporting an Ortho Projection

The Image layer of the rendered orthophoto can be exported through:

1. WORKFLOW -> Export -> Ortho Photo
2. ORTHO 2D context menu -> Export -> Ortho Projection

If multiple ortho projections are selected, then all selected projections are exported. They can be exported in these formats:

After selecting a specific format (apart from KML and KMZ), the Export ortho projection table, the one in the picture below, shows up.

• If you want to export a world file, set Export world file to 'True'. A world file is a plain text file used by many geographic information systems (GIS) to geo-reference raster map images.
• In the World file coordinate system, 3 options are possible: 'Projected' – model is projected on a selected plane of the reconstruction box. 'Global' – use when creating a map or a top view. 'Image' – choose when creating elevations or sections.
• When Export projection parameters file is set to 'True', a text file containing specific parameters of the projection with the extension *.rcortho will be exported.
• In the Compression line, options 'None' and 'LZW' are available. Choose 'LZW' if you want to compress (reduce) the size of the exported file.
• Append name to file name set to 'True' adds the ortho projection’s name to the file name.
• Set Export as BigTIFF to 'True' if you want to save the file as a BigTIFF instead of the regular 32bit TIFF file.

When exporting to KML or KMZ format, a different dialog shows up: 

• In the Tile image file format, you can choose the format of the images related to KML/KMZ files, and in the Tile image resolution (pixels) you can specify the resolution of these images.
• All other options have been mentioned previously.

Only projections in the WGS 84 coordinate system with all rotations set to 0 can be exported to KMZ/KML format. Otherwise, the following error message shows up: (still ???)

Example:

Here we will show you an example of exporting a projection to KMZ format, and subsequent loading into Google Earth. Set the reconstruction region to the desired size. In the Ortho projection tool, set the Type of projection to 'Top', Coordinate system to 'epsg:4326 – GPS (WGS 84)', and all Rotation parameters to 0.

Or you can simply choose the projection type to 'Map' and the application will set the rest for you. Render the orthographic projection. In the 1Ds view, select the rendered ortho projection. Choose one of the export options mentioned above and select the KMZ format. In the Export ortho projection dialog, choose the desired parameters and click OK. In Google Earth, simply open the exported KMZ file.

Digital Surface Model export

The Depth and Altitude layer of the rendered orthophoto can be exported through

1. WORKFLOW -> Export -> Digital Surface Model
2. ORTHO 2D context menu -> Export -> Digital Surface Model

Digital surface model can be exported in the same formats as the Image layer of an ortho projection. After selecting one of the formats, this dialog window shows up:

The only difference from the table when exporting Image layer, is the Pixel type line. Here you can choose from Altitude and Depth(local), depending on which layer is to be exported.  

Example:

In this example, we have exported the Altitude layer of an ortho projection in the 'epsg:3016 – SWEREF99 20 15' coordinate system in the TIFF format.

In order to see whether the DSM has been exported correctly (with correct coordinate system), we have loaded the TIFF file to free open-source GIS software – QGIS.

We can compare the coordinates of a mouse cursor placed approximately in the same position in the RC and in QGIS, both in the same epsg:3016 coordinate system.

Sampled Points export

Sampled Points can be exported through the ORTHO 2D, Tools context menu in the export section, and can be exported in these formats:  

After selecting a specific format, the Export model dialog shows up.

Depending on the chosen save-as-type of a model, you will get one of the following export model options - except the *.partList, which needs no additional export settings. You can find all dialogs and explanations of the parameters in the application Help -> Export, Import, and Re-use Computations -> Model Export.

Example: 

In this example, we have exported the Altitude layer of a projection in the 'epsg:3016 – SWEREF99 20 15' coordinate system in the TIFF format, and Sampled projection in XYZ Point Cloud format, and visualized them using QGIS plugin Qgis2threejs.