DSP

Displacement (DSP) map primitive

Creating a DSP object from miscellaneous images

We will use two examples to illustrate the next few sections.

1. A black and white (gray scale) image (png format):

with properties:

$ file Ex1.png
Ex1.png: PNG image, 142 x 150, 8-bit gray+alpha, non-interlaced

2. A color png file:

with properties:

$ file Ex2.png
Ex2.png: PNG image, 152 x 150, 8-bit/color RGBA, non-interlaced

Preparing your height field data

The DSP takes unsigned short (16-bit) integer data. Our various command-line data converters can help bring data in from pretty much any existing format, including image data, via various processing commands. If the data was in png image format, for example (which is basically 3-channel 8-bit integer data), the data could be prepared with a combination of 'png-pix', 'pix-bw', and 'cv'.

If you type the 'in' command, it will prompt you for each parameter individually and that should help some. For the DSP, the main parameters are: the source of the height data, the width (number of points in the Y direction) and length (number of points in the X direction) of the input data, width/length/height scaling factors, and whether to smoothly interpolate between cells (0 = do not interpolate, 1 = interpolate).

See the 'dsp_add' tool for combining two existing DSP data files into one.

A DSP primitive is an array of cells initially defined by points in the XY plane as positive heights from Z = 0. The DSP can then be transformed to other orientations and positions. The number of cells is (numX * numY).

The data format for the DSP primitive is network-ordered unsigned short integers (nu16). BRL-CAD has a couple of dozen tools that you can use for converting existing data into that raw format, such as the 'cv' command or the 'bw-d' and 'd-u' commands among other similar tool chains. If you use the cv command, the output format is "nus" for network unsigned shorts.

Using the two examples to convert the data to dsp format.

Example 1.

Convert it to a bw file (one pixel is one unsigned char):

$ png-bw Ex1.png > Ex1.bw

View the result:

$ bw-fb -w142 -n150 Ex1.bw

Convert it to the format required for a dsp file (nu16):

$ cv huc nu16 Ex1.bw Ex1.dsp

Example 2.

Convert it to a pix file (one pixel is defined by three unsigned chars):

$ png-pix Ex2.png > Ex2.pix

View the result:

$ pix-fb -w152 -n150 Ex2.pix

Convert it to a bw file (one pixel is one unsigned char):

$ pix-bw Ex2.pix > Ex2.bw

View the result:

$ bw-fb -w152 -n150 Ex2.bw

Convert it to the format required for a dsp file:

$ cv huc nu16 Ex2.bw Ex2.dsp

Importing DSP data into a .g file

Example 1.

In mged create a dsp object:

mged> in dsp1.s dsp f Ex1.dsp 142 150 0 ad 1 0.005
mged> r dsp1.r u dsp1.s

Example 2.

In mged create a dsp object:

mged> in dsp2.s dsp f Ex2.dsp 152 150 0 ad 1 0.005
mged> r dsp2.r u dsp2.s

Rendering your DSP

Example 1.

mged> B dsp.s
mged> ae 45 45
mged> rt

You should see something like this:

You can play around with the scaling factors (the end pair: 1 - cell width, 0.005 - cell height) to improve the looks of the image.

But now let's invert the file so we get its negative:

$ bwmod -m-1 -a255 < Ex1.bw > Ex1neg.bw

And make another dsp:

Example 2.

mged> B dsp2.s
mged> ae 45 45
mged> rt

You should see something like this:

Again, you could play with various parameters to get the desired look.

Creating a DSP object from manual or programmatic generation of data

Now let's consider a more practical example and a real test of BRL-CAD. We can import topological data and produce a realistic ground surface. There are many free sources of such data. See this site for a start:

http://www.naturalgfx.com/free_topo_gis.htm

We started with shapefile data for the US from:

http://seamless.usgs.gov/data_availability.php?serviceid=Dataset_19

We downloaded the zip archive 'ned_19_arc.zip' and unzipped in its own directory:

$ mkdir ned_19_usa_arcgis_shapefile
$ mv ned_19_arc.zip ned_19_usa_arcgis_shapefile
$ cd ned_19_usa_arcgis_shapefile
$ unzip ned_19_arc.zip
Archive:  ned_19_arc.zip
  inflating: ned_19_arc.dbf          
  inflating: ned_19_arc.pdf          
  inflating: ned_19_arc.prj          
  inflating: ned_19_arc.sbn          
  inflating: ned_19_arc.sbx          
  inflating: ned_19_arc.shp          
  inflating: ned_19_arc.shp.xml      
  inflating: ned_19_arc.shx          
  inflating: ned_19_arc.txt          
  inflating: ned_19_arc.xml          
  inflating: NED_DataDictionary2006.pdf  

See these pages for details of the [ESRI] shapefile format:

http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf

http://en.wikipedia.org/wiki/Shapefile

We will use one of two Perl modules, Geo::Shapelib or Geo::ShapeFile, and write a Perl program to manipulate the shapefile data to produce the desired dsp object. The Perl modules are available on CPAN here:

http://search.cpan.org/dist/Geo-Shapelib/Shapelib.pm

http://search.cpan.org/~jasonk/Geo-ShapeFile-2.51/ShapeFile.pm

Our program (tentatively named 'manip-shapefile.pl') will be made available in the BRL-CAD package.

[TO BE CONTINUED]