<h2>Additions to NewGRF Specifications in JGR's Patchpack</h2>
<p>This document describes non-standard additions to the <ahref="https://newgrf-specs.tt-wiki.net/wiki/Main_Page">Official OpenTTD NewGRF Specifications</a> which are present in this patchpack.
<p>NewGRFs which use any of these features SHOULD use the <ahref="#feature-test">feature testing</a> mechanism described below to check whether individual added features are supported.</p>
<p>Feature tests can be safely used on implementations which do not implement the described feature test mechanism because unknown Action 14 blocks are ignored,
and the observable result (in global variable 0x9D) is equivalent to the case where all feature tests have failed, indicating that the feature is not present.</p>
<p>Within an FTST chunk, the NAME text (type T) field contains the name of the feature to test for. The value of the language ID byte is ignored.<br/>
If the named feature is not present, or if this field is omitted, the version is 0.<br/>
If the named feature is present, the version number will be at least 1.</p>
<p>The feature testing mechanism itself has the feature name: <fontface="monospace">feature_test</font>, this document describes version 1.</p>
<h4>Feature Minimum Version: C "FTST" -> B "MINV"</h4>
<p>Within an FTST chunk, the MINV binary (type B) field contains the minimum (inclusive) (≥) version to accept. This is a Word (2 bytes).<br/>
The default value is 1.</p>
<h4>Feature Maximum Version: C "FTST" -> B "MAXV"</h4>
<p>Within an FTST chunk, the MINV binary (type B) field contains the maximum (inclusive) (≤) version to accept. This is a Word (2 bytes).<br/>
The default value is 0xFFFF (no maximum).</p>
<h4>Feature Set Global Variable 0x9D Bit: C "FTST" -> B "SETP"</h4>
<p>Within an FTST chunk, the SETP binary (type B) field contains the bit number to set/clear in
<ahref="https://newgrf-specs.tt-wiki.net/wiki/GlobalVariables">global variable</a> 0x9D (TTD Platform) to store the result of the test. This is 1 byte.<br/>
If the test is successful, the bit is set (to 1), otherwise the bit is cleared (to 0).<br/>
The bit number MUST be in the range: 4 ≤ bit number ≤ 31. These bits can be assumed to be 0 on implementations which do not support this feature test mechanism.<br/>
Global variable 0x9D can then be tested by using a standard <ahref="https://newgrf-specs.tt-wiki.net/wiki/Action7">Action 7 or 9</a>, or a standard <ahref="https://newgrf-specs.tt-wiki.net/wiki/VariationalAction2">Variational Action 2</a>.<br/>
If this field is omitted, no bit is set or cleared, and the test is not observable.
<p>Each A0PM chunk (type C) describes an individual property mapping.<br/>
Sub-chunks within each A0PM chunk may appear in any order, however except where otherwise noted each sub-chunk SHOULD only appear ONCE within an individual A0PM chunk.</p>
<p>Property mapping can be safely used on implementations which do not implement the property mapping mechanism if Action 0 sprites which use mapped property IDs are skipped if one or more of:
<ul>
<li>The global variable 0x8D is checked to determine whether the property mapping operation was successful.</li>
<li>The feature name <fontface="monospace">property_mapping</font> is checked for.</li>
</ul>
Unknown Action 14 blocks are ignored, and do not need to be skipped.</p>
<h4>Property Name: C "A0PM" -> T "NAME"</h4>
<p>Within an A0PM chunk, the NAME text (type T) field contains the name of the property to map. The value of the language ID byte is ignored.</p>
<h4>Action 0 Feature ID: C "A0PM" -> B "FEAT"</h4>
<p>Within an A0PM chunk, the FEAT binary (type B) field contains the <ahref="https://newgrf-specs.tt-wiki.net/wiki/Action0#Feature">Action 0 feature ID</a>. This is 1 byte.</p>
<h4>Property ID: C "A0PM" -> B "PROP"</h4>
<p>Within an A0PM chunk, the PROP binary (type B) field contains the property ID to allocate to the named property, this value can used in Action 0 sprites. This is 1 byte.<br/>
It is possible to override existing properties, however this use is not recommended.</p>
<h4>Success Indicator Global Variable 0x8D Bit: C "A0PM" -> B "SETT"</h4>
<p>Within an A0PM chunk, the SETT binary (type B) field contains the bit number to set/clear in
<ahref="https://newgrf-specs.tt-wiki.net/wiki/GlobalVariables">global variable</a> 0x8D (TTD version) to store whether the mapping operation was successful. This is 1 byte.<br/>
If the operation is successful, the bit is set (to 1), otherwise the bit is cleared (to 0).<br/>
The bit number MUST be in the range: 4 ≤ bit number ≤ 31. These bits can be assumed to be 0 on implementations which do not support this property mapping mechanism.<br/>
Global variable 0x8D can then be tested by using a standard <ahref="https://newgrf-specs.tt-wiki.net/wiki/Action7">Action 7 or 9</a>, or a standard <ahref="https://newgrf-specs.tt-wiki.net/wiki/VariationalAction2">Variational Action 2</a>.<br/>
If this field is omitted, no bit is set or cleared.
</p>
<h4>Fallback Mode: C "A0PM" -> B "FLBK"</h4>
<p>Within an A0PM chunk, the FLBK binary (type B) field contains the fallback mode. This is 1 byte.<br/>
The fallback mode may take the following values:
<table>
<tr><th>Value</th><th>Behaviour</th></tr>
<tr><td>0</td><td>Attempts to map an unknown property name are ignored. Use of the mapped property in an Action 0 is ignored. This is the default.</td></tr>
<tr><td>1</td><td>Attempts to map an unknown property name are ignored. Use of the mapped property in an Action 0 is an error.</td></tr>
<tr><td>2</td><td>Attempting to map an unknown property name is an error.</td></tr>
</table>
Attempts to set a fallback mode other than those listed above are silently ignored. More fallback modes MAY be added in future versions of this mechanism.<br/>
This chunk MAY be specified more than once, in which case the last specified valid value is used.<br/>
Note that even when using fallback mode 0, above, if the property mapping feature is not present, then use of the mapped property ID in an Action 0 is an error.
If the size of the data does provided is not valid for the given property, the attempt to set the property MAY be ignored or partially applied.<br/>
Note that each use of the mapped property ID is followed by Num-info iterations of the size and data pair above. See: <ahref="https://newgrf-specs.tt-wiki.net/wiki/Action0">Action 0 Specification</a>.
