(workshop-meshing)=

# Creating surface meshes

```{index} ground, classification, filtering
```

This exercise uses PDAL to create surface meshes. PDAL is able to use a number
of meshing filters: <https://pdal.io/en/latest/stages/filters.html>. Three of these
are 'in the box', without needing plugins compiled. These are 2D Delaunay
triangulation, Greedy projection meshing and Poisson surface meshing.

In this exercise we'll create a Poisson surface mesh - a watertight isosurface
\- from our input point cloud.

## Exercise

We will create mesh models of a building and its surrounds using an entwine
data input source.

After running each command, the output `.ply` file can be viewed in Meshlab or
[CloudCompare].

```{note}
Since QGIS doesn't support `.ply` files we can't use it for this exercise.
In order to use QGIS you can convert the files to OBJ or `.copc` using PDAL.
```

```{seealso}
PDAL implements Mischa Kazhdan's Poisson surface reconstruction algorithm.
For details see `[Kazhdan2006]_`.
```

```{note}
This example will be using the output of using {ref}`workshop-clipping`
exercise, specifically using `stadium.copc.laz` file.  Please produce that
file by following that exercise before starting this one.
```

```{note}
`writers.ply` will write out mesh vertices by default. In this exercise we
set the attribute `faces="true"`. Try using the ply writer without it. Also,
if you're using a machine with a lot of processing power, try increasing the
`depth` parameter for a more detailed mesh.
```

### Mesh Command

Invoke the following command, substituting accordingly, in your `Conda Shell`:

```console
$ pdal translate -i ./exercises/analysis/clipping/stadium.copc.laz \
-o ./exercises/analysis/meshing/first-mesh.ply \
poisson --filters.poisson.depth=16 \
--verbose 4
```

```doscon
> pdal translate -i ./exercises/analysis/clipping/stadium.copc.laz ^
-o ./exercises/analysis/meshing/first-mesh.ply ^
poisson --filters.poisson.depth=16 ^
--verbose 4
```

```console
(PDAL Debug) Debugging...
(pdal translate Debug) Executing pipeline in standard mode.
(pdal translate filters.poisson Debug) Computing normal vectors
```

You can view the mesh in Cloud Compare, you should see something similar to:

```{image} ../../images/first-mesh.png
:target: ../../../_images/first-mesh.png
```

To colorize the points, select `first-mesh` in the left window. Under
`Properties` set `Colors` to `RGB`. You should see the following:

```{image} ../../images/first-mesh-rgb.png
:target: ../../../_images/first-mesh-rgb.png
```

### Filtering

If we want to just mesh a building, or just terrain, or both we can apply a
{ref}`filters.expression` filter based on point classification. These data have ground labeled as
class 2, and buildings as 6.

In this exercise we will create a poisson mesh surface of a building and the
ground surrounding it, using the same data subset as above and adding a
{ref}`filters.expression` stage to limit the set of points used in mesh creation.

### Command

Invoke the following command, substituting accordingly, in your `Conda Shell`:

```console
$ pdal translate -i ./exercises/analysis/clipping/stadium.copc.laz \
-o ./exercises/analysis/meshing/building-exercise.ply \
expression poisson \
--filters.expression.expression="Classification == 2 || Classification == 6" \
--filters.poisson.depth=16 \
--verbose 4
```

```doscon
> pdal translate -i ./exercises/analysis/clipping/stadium.copc.laz ^
-o ./exercises/analysis/meshing/building-exercise.ply ^
expression poisson ^
--filters.expression.limits="Classification == 2 || Classification == 6" ^
--filters.poisson.depth=16 ^
--verbose 4
```

```console
(PDAL Debug) Debugging...
(pdal translate Debug) Executing pipeline in standard mode.
(pdal translate filters.poisson Debug) Computing normal vectors
```

[cloudcompare]: https://www.danielgm.net/cc/
