Attribute Generation and Analysis
Attributes are attached to both point cloud and surface mesh data. Many attribute functions are common to both point clouds and surface mesh data.
By default point clouds have Red, Green, and Blue (RGB) attributes, and intensity if the information exists in the imported data. Sometimes these attributes may be NULL (empty). VRGS requires at least 1 attribute in order to display a dataset. Red, Green, and Blue attributes are preferred. False colour RGB can be created based on elevation or intensity.
Meshes do not require any attributes to display them if they have associated textures.
Creating attributes
False Colour
Right + click a point cloud or surface mesh in the Data tree, then select Visualization → False Color → By Elevation to colour the object by height, or By Attribute to colour it by an attribute you have already generated.
WARNING: Creating a false colour RGB set will overwrite any existing RGB values.
Attributes common to point clouds and meshes
Tensor Analysis
Right + click the object in the Data tree and choose Attributes & Analysis → Tensor Analysis (for a mesh, Attributes & Analysis → Geometry → Tensor Analysis).
WARNING: Potentially a very long operation.
An approach developed by Fernández1 and developed further by García-Sellés et. al.2 uses a tensor analysis approach to analyse the topography of a point dataset to generate a variety of parameters. The dialog offers two ways of choosing the neighbourhood each point is analysed over:
- Sphere Radius — the volume of the model around each point that the tensor analysis uses. As a minimum this should be twice the de-cluster distance, or twice the data spacing. Larger values give a smoothed result; small values are more susceptible to noise in the data.
- K Nearest Neighbours — use a fixed number of nearest points instead of a fixed distance, which adapts to varying point density. Use Approximate Nearest Neighbours (fast) trades a little accuracy for a large speed gain on big clouds.
The Check All button selects all attributes for output.
The most useful attributes are:
Dip&Azimuth(of the outcrop model surface)Colinearity(how linear the points are)Coplanarity(how flat the points are)Observations(the point count — useful to check if the sphere radius is too small or too big)Grade(0 rejected, 1 good, 2 excellent — colour the model by this to see at a glance where an orientation measurement can be trusted)
Every attribute the dialog can produce is defined in the Attributes reference, including the Plane Fit Quality group that Grade belongs to.
Colour Conversion
Attributes & Analysis → Color Conversion converts the RGB values to a different colour space — greyscale, XYZ, LAB or HSV — which may give a different visual representation of the data. Meshes can also take their RGB straight from the texture.
Editing and Filtering Attributes
Colour Maps
By default, attributes are shown with a magma colourmap, chosen in the Properties Panel for the attribute. VRGS provides several other perceptually uniform (or linear) colourmaps. Perceptually uniform colourmaps minimise visualisation bias of the data and provide a universally readable dataset for people that may be colour-vision deficient3.
Perceptually uniform (sequential)
- magma, inferno, plasma, viridis, cividis
- batlow, lipari
Diverging and cyclic
- vik, roma, broc (diverging — for data with a meaningful midpoint)
- romaO, vikO, corkO, twilight (cyclic — for angular data such as azimuth, so that 0° and 360° share a colour)
Single-hue and elevation-specific
- Blues, Reds, Greys
- Ordnance Survey (OS) Elevation
- USGS Elevation
Attributes can be transformed, edited, and displayed with different colourmaps. Current transforms are Log, Square Root, Clip, Box-Cox, Normalise, and Exponential.
The Clip attribute operation allows high and low values to be truncated, effectively performing a contrast stretch on the attribute. This allows the full colour scale to be used effectively in the display. The red and yellow dotted lines can be dragged to change the degree of clipping.
Filters
The attribute properties also provide a means to filter by attributes, this allows us to, for example, show only the point where the dip attribute is > 45 degrees. Filters can be set on each attribute and the results of all the filters are combined to give the final display.
In the attributes property page (see above) check the Use Filter option. Set the filter minimum and filter maximum values to match the range of values you wish to use. Then in the parent point clouds properties, check the Use Attribute Filters box. This allows multiple filters to be switched off easily.
RGB Blended Display
False colour displays can be created by using attributes in place of the colour channels in the RGB view. A display can be generated, for example, using Dip for the Red channel, Azimuth for the Green channel and co-planarity for the blue channel. In order to do this, show the attributes for the point cloud, then in the Properties Panel select the attributes to replace the Red, Green and Blue colour channels with.
See also
- Attributes — the full definition of every vertex and triangle attribute VRGS can compute.
- Charts, Histograms and Filtering — plotting an attribute's distribution and dragging the histogram to filter the 3D view.
- Texture Attributes — attributes taken from a mesh's texture rather than its geometry.