Application Notes

Application Note 1: Instancing

In general, the term "instancing" refers to the re-use of data - usually geometry - by using references instead of copies. During model generation in PRT instancing can happen in two ways:

1. Asset Instancing: if an asset is only inserted (insert()) and not modified by CGA (e.g. material properties) then the corresponding prtx::Geometry object will carry its original URI through into the encoder and can be re-used accordingly by the encoder.
2. Geometry/Material Instancing: if the geometry of an asset is used multiple times and its materials are modified in the rules then the encoder author can make use of the EncodePreparator instance() flag. If this flag is set to true, the EncodePreparator will re-use the geometry objects (same reference/pointer) and return a separate list of finalized materials (prtx::EncodePreparator::FinalizedInstance::getMaterials()). If the intended target format supports this type of instancing the encoder can directly translate this one-geometry-to-multiple-materials mapping into the target format.

Application Note 2: Exporting ordered meta data per initial shape

All built-in encoders of the CE SDK which support writing initial shape attributes understand the reserved initial shape attribute key "/enc/metaData/". If any attributes are prefixed with "/enc/metaData" the encoder will attempt to output them accordingly.

Here is an example how to get the attributes "myItem1, myItem2, myItem3" to output in the order "myItem1, myItem3, myItem2":

prt::AttributeMapBuilder* builder = prt::AttributeMapBuilder::create();
builder->setInt(L"/enc/metaData/myItem1", 10);
builder->setInt(L"/enc/metaData/myItem2", 20);
builder->setInt(L"/enc/metaData/myItem3", 30);
builder->setInt(L"myItemForCGA", 99);

const prt::AttributeMap* attrMap = builder->createAttributeMap();
builder->destroy();

prt::InitialShapeBuilder* isBuilder = prt::InitialShapeBuilder::create();
isBuilder->setAttributes(..., attrMap, ...);
const prt::InitialShape* initialShape = isBuilder->createInitialShapeAndReset();
isBuilder->destroy();

prt::generate(...); // -> encoders will emit "myItem1, myItem3, myItem2"

initialShape->destroy();
attrMap->destroy();

See also

prt::InitialShapeBuilder::setAttributes

Application Note 3: Requirements for CityEngine custom exporters

A PRT encoder must meet a few requirements to be successfully loaded by CityEngine as a custom exporter:

1. CityEngine needs to have version 2014.0 or later
2. The encoder factory needs to specify id, name, description, type and file extension (see below for an example).
3. To make an encoder option appear in the CityEngine UI, the following annotations are mandatory: LABEL, ORDER, GROUP and DESCRIPTION
4. The boolean option "errorFallback" needs to be present. If true, the encoder is expected to encode the initial shape geometry. Usually, this option should be hidden (see prtx::EncodeOptionsAnnotator::flagAsHidden()).

Here is an example how to setup an encoder info object inside an encoder factory (the full sources are available in the "stlenc" example):

prtx::EncoderInfoBuilder encoderInfoBuilder;
encoderInfoBuilder.setID(...);
encoderInfoBuilder.setName(...);
encoderInfoBuilder.setDescription(...);
encoderInfoBuilder.setType(prt::CT_GEOMETRY);
encoderInfoBuilder.setExtension(L".ext");

See also

prtx::EncoderInfoBuilder

Application Note 4: How-To create a FILE URI from a file system path

All URIs in PRT follow the standard RFC3986: http://www.ietf.org/rfc/rfc3986.txt In practice this means:

1. the URI string must be UTF-8 encoded
2. all characters which are not allowed according to section 2 of RFC3986 must be percent-encoded (see prtx::URI::percentEncode).
3. "file" URIs must contain an absolute path, i.e. start with a slash after "file:". This is also required on windows, i.e. "file:/C:/path/".

Here is an example how to create such an URI from a file system path:

// linux/osx example (assuming UTF8 for native strings)
std::string fsp = "/home/johndoe/my/path/hello.txt";
std::string uri = prtx::URI::SCHEME_FILE + ":" + fsp; // results in 'file:/home/johndoe/my/path/hello.txt'
uri = prtx::URI::percentEncode(uri);
std::wstring wuri(uri.size(), L' ');
std::copy(uri.begin(), uri.end(), wuri.begin());
// now use wuri.c_str() for prt API calls...

// windows example (assuming Latin1 string encoding)
std::string fsp = "C:/some/path/file.txt"; // note: windows paths must be converted to forward slashes
std::string u8fsp = boost::locale::conv::to_utf<char>(fsp, "Latin1");
std::string uri = prtx::URI::SCHEME_FILE + ":/" + u8fsp; // note the slash before the drive letter
uri = prtx::URI::percentEncode(uri);
std::wstring wuri(uri.size(), L' ');
std::copy(uri.begin(), uri.end(), wuri.begin());
// now use wuri.c_str() for prt API calls...