1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
|
<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html><html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>gl_FrontFacing - OpenGL 4 Reference Pages</title>
<link rel="stylesheet" type="text/css" href="opengl-man.css"/>
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1"/>
<script type="text/x-mathjax-config">
MathJax.Hub.Config({
MathML: {
extensions: ["content-mathml.js"]
},
tex2jax: {
inlineMath: [['$','$'], ['\\(','\\)']]
}
});
</script>
<script type="text/javascript" src="https://c328740.ssl.cf1.rackcdn.com/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"/>
</head>
<body>
<header/>
<div class="refentry" id="gl_FrontFacing">
<div class="titlepage"/>
<div class="refnamediv">
<h2>Name</h2>
<p>gl_FrontFacing — indicates whether a primitive is front or back facing</p>
</div>
<div class="refsynopsisdiv">
<h2>Declaration</h2>
<code class="fieldsynopsis"><span class="modifier">in </span><span class="type">bool </span><span class="varname">gl_FrontFacing </span>;</code>
</div>
<div class="refsect1" id="description">
<h2>Description</h2>
<p>
Available only in the fragment language, <code class="varname">gl_FrontFacing</code> is an input variable
whose value is <code class="function">true</code> if the fragment belongs to a front-facing primitive and
false otherwise. The determination of whether a triangle primitive is front-facing is made by
examining the sign of the area of the triangle, including a possible reversal of this sign
as controlled by <code class="varname">glFrontFace</code>. One way to compute this area is:
</p>
<p>
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:mi>a</mml:mi><mml:mo>=</mml:mo><mml:mfrac><mml:mn>1</mml:mn><mml:mn>2</mml:mn></mml:mfrac>
<mml:msubsup>
<mml:mo>∑</mml:mo>
<mml:mrow>
<mml:mi>j</mml:mi><mml:mo>=</mml:mo><mml:mn>0</mml:mn>
</mml:mrow>
<mml:mrow>
<mml:mi>n</mml:mi><mml:mo>-</mml:mo><mml:mn>1</mml:mn>
</mml:mrow>
</mml:msubsup>
<mml:msubsup>
<mml:mi>x</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
<mml:msubsup>
<mml:mi>y</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow>
</mml:msubsup>
<mml:mo>-</mml:mo>
<mml:msubsup>
<mml:mi>x</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi><mml:mo>+</mml:mo><mml:mn>1</mml:mn></mml:mrow>
</mml:msubsup>
<mml:msubsup>
<mml:mi>y</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
</mml:math>
</p>
<p>
where
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:msubsup>
<mml:mi>x</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
</mml:math> and
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll">
<mml:msubsup>
<mml:mi>y</mml:mi>
<mml:mrow><mml:mi>w</mml:mi></mml:mrow>
<mml:mrow><mml:mi>i</mml:mi></mml:mrow>
</mml:msubsup>
</mml:math> are
the x and y window coordinates of the <span class="emphasis"><em>i</em></span>th vertex of the n-vertex polygon.
</p>
<p>
The sign of this computation is negated when the value of
<code class="constant">GL_CLIP_ORIGIN</code> (the clip volume origin, set
with
<a class="citerefentry" href="glClipControl.xhtml"><span class="citerefentry"><span class="refentrytitle">glClipControl</span></span></a>)
is <code class="constant">GL_UPPER_LEFT</code>.
</p>
</div>
<div class="refsect1" id="versions">
<h2>Version Support</h2>
<div class="informaltable">
<table style="border-collapse: collapse; border-top: 2px solid ; border-bottom: 2px solid ; border-left: 2px solid ; border-right: 2px solid ; ">
<colgroup>
<col style="text-align: left; "/>
<col style="text-align: center; " class="firstvers"/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; "/>
<col style="text-align: center; " class="lastvers"/>
</colgroup>
<thead>
<tr>
<th style="text-align: left; border-right: 2px solid ; ">
</th>
<th style="text-align: center; border-bottom: 2px solid ; " colspan="12">
<span class="bold"><strong>OpenGL Shading Language Version</strong></span>
</th>
</tr>
<tr>
<th style="text-align: left; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>Variable Name</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>1.10</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>1.20</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>1.30</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>1.40</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>1.50</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>3.30</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>4.00</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>4.10</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>4.20</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>4.30</strong></span>
</th>
<th style="text-align: center; border-right: 2px solid ; border-bottom: 2px solid ; ">
<span class="bold"><strong>4.40</strong></span>
</th>
<th style="text-align: center; border-bottom: 2px solid ; ">
<span class="bold"><strong>4.50</strong></span>
</th>
</tr>
</thead>
<tbody>
<tr>
<td style="text-align: left; border-right: 2px solid ; ">gl_FrontFacing</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; border-right: 2px solid ; ">✔</td>
<td style="text-align: center; ">✔</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="refsect1" id="seealso">
<h2>See Also</h2>
<p>
<a class="citerefentry" href="gl_FragCoord.xhtml"><span class="citerefentry"><span class="refentrytitle">gl_FragCoord</span></span></a>,
<a class="citerefentry" href="glClipControl.xhtml"><span class="citerefentry"><span class="refentrytitle">glClipControl</span></span></a>
</p>
</div>
<div class="refsect1" id="Copyright">
<h2>Copyright</h2>
<p>
Copyright <span class="trademark"/>© 2011-2014 Khronos Group.
This material may be distributed subject to the terms and conditions set forth in
the Open Publication License, v 1.0, 8 June 1999.
<a class="link" href="http://opencontent.org/openpub/" target="_top">http://opencontent.org/openpub/</a>.
</p>
</div>
</div>
<footer/>
</body>
</html>
|
