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74 lines
4.7 KiB
HTML
74 lines
4.7 KiB
HTML
6 years ago
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<!DOCTYPE html>
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Additions to NewGRF Specifications</title>
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<style type="text/css">
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span.abuse { font-family: "Courier New", Courier, mono; background-color: rgb(255, 58, 31); }
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span.option{ font-family: "Courier New", Courier, mono; background-color: rgb(255,255, 30); }
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span.free { font-family: "Courier New", Courier, mono; background-color: rgb(30, 178, 54); }
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span.used { font-family: "Courier New", Courier, mono; }
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span.used_p{ font-family: "Courier New", Courier, mono; background-color: cyan; }
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td.bits { white-space: nowrap; text-align: center; font-family: "Courier New", Courier, mono; }
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td.caption { white-space: nowrap; text-align: left; }
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td li { white-space: nowrap; text-align: left; }
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th { white-space: nowrap; text-align: center; }
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</style>
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</head>
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<body>
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<h2>Additions to NewGRF Specifications in thi branch</h2>
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<p>This document describes non-standard additions to the <a href="https://newgrf-specs.tt-wiki.net/wiki/Main_Page">Official OpenTTD NewGRF Specifications</a> which are present in this branch.
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<p>This additions MAY also be present in other branches/repositories/etc. They MAY be removed or moved in future, if necessary.</p>
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<p>NewGRFs which use any of these features SHOULD use the <a href="#feature-test">feature testing</a> mechanism described below to check whether individual added features are supported.</p>
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<h3 id="feature-test">Action 14 - Feature Tests</h3>
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<p>See <a href="https://newgrf-specs.tt-wiki.net/wiki/Action14">Action 14 Specification</a> for background information.</p>
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<h4>Feature Test: C "FTST"</h4>
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<p>Each FTST chunk (type C) describes an individual feature test.<br />
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Sub-chunks within each FTST chunk may appear in any order, however each sub-chunk SHOULD only appear ONCE within an individual FTST chunk.</p>
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<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,
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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>
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<h4>Feature Name: C "FTST" -> T "NAME"</h4>
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<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 />
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If the named feature is not present, or if this field is omitted, the version is 0.<br />
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If the named feature is present, the version number will be at least 1.</p>
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<p>The feature testing mechanism itself has the feature name: <font face="monospace">feature_test</font>, this document describes version 1.</p>
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<h4>Feature Minimum Version: C "FTST" -> B "MINV"</h4>
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<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 />
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The default value is 1.</p>
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<h4>Feature Maximum Version: C "FTST" -> B "MAXV"</h4>
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<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 />
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The default value is 0xFFFF (no maximum).</p>
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<h4>Feature Set Global Variable 0x9D Bit: C "FTST" -> B "SETP"</h4>
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<p>Within an FTST chunk, the SETP binary (type B) field contains the bit number to set/clear in
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<a href="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 />
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If the test is successful, the bit is set (to 1), otherwise the bit is cleared (to 0).<br />
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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 />
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Global variable 0x9D can then be tested by using a standard <a href="https://newgrf-specs.tt-wiki.net/wiki/Action7">Action 7 or 9</a>, or a standard <a href="https://newgrf-specs.tt-wiki.net/wiki/VariationalAction2">Variational Action 2</a>.<br />
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If this field is omitted, no bit is set or cleared, and the test is not observable.
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</p>
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<br />
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<h4>Example:</h4>
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<pre>
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// Set bit 4 of global variable 0x9D if sample_feature_1 is present with a version of at least 4
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// Set bit 5 of global variable 0x9D if sample_feature_2 is present with a version of at least 5 and at most 6
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-1 * -1 14
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"C" "FTST"
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"T" "NAME" 00 "sample_feature_1" 00
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"B" "MINV" \w2 \w4
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"B" "SETP" \w1 04
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00
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"C" "FTST"
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"T" "NAME" 00 "sample_feature_2" 00
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"B" "MINV" \w2 \w5
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"B" "MAXV" \w2 \w6
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"B" "SETP" \w1 05
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00
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00
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....
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// Skip 1 sprite if bit 4 of global variable 0x9D is not set (indicating that sample_feature_1 with a version of at least 4 is NOT present)
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-1 * -1 07 9D 01 \70 04 01
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</pre>
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</body>
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</html>
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