| 1 | <?xml version="1.0" encoding="UTF-8"?> |
| 2 | <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook MathML Module V1.1b1//EN" |
| 3 | "http://www.oasis-open.org/docbook/xml/mathml/1.1CR1/dbmathml.dtd"> |
| 4 | <refentry id="glStencilFunc"> |
| 5 | <refmeta> |
| 6 | <refmetainfo> |
| 7 | <copyright> |
| 8 | <year>1991-2006</year> |
| 9 | <holder>Silicon Graphics, Inc.</holder> |
| 10 | </copyright> |
| 11 | </refmetainfo> |
| 12 | <refentrytitle>glStencilFunc</refentrytitle> |
| 13 | <manvolnum>3G</manvolnum> |
| 14 | </refmeta> |
| 15 | <refnamediv> |
| 16 | <refname>glStencilFunc</refname> |
| 17 | <refpurpose>set front and back function and reference value for stencil testing</refpurpose> |
| 18 | </refnamediv> |
| 19 | <!-- eqn: ignoring delim $$ --> |
| 20 | <refsynopsisdiv><title>C Specification</title> |
| 21 | <funcsynopsis> |
| 22 | <funcprototype> |
| 23 | <funcdef>void <function>glStencilFunc</function></funcdef> |
| 24 | <paramdef>GLenum <parameter>func</parameter></paramdef> |
| 25 | <paramdef>GLint <parameter>ref</parameter></paramdef> |
| 26 | <paramdef>GLuint <parameter>mask</parameter></paramdef> |
| 27 | </funcprototype> |
| 28 | </funcsynopsis> |
| 29 | </refsynopsisdiv> |
| 30 | <refsect1 id="parameters"><title>Parameters</title> |
| 31 | <variablelist> |
| 32 | <varlistentry> |
| 33 | <term><parameter>func</parameter></term> |
| 34 | <listitem> |
| 35 | <para> |
| 36 | Specifies the test function. |
| 37 | Eight symbolic constants are valid: |
| 38 | <constant>GL_NEVER</constant>, |
| 39 | <constant>GL_LESS</constant>, |
| 40 | <constant>GL_LEQUAL</constant>, |
| 41 | <constant>GL_GREATER</constant>, |
| 42 | <constant>GL_GEQUAL</constant>, |
| 43 | <constant>GL_EQUAL</constant>, |
| 44 | <constant>GL_NOTEQUAL</constant>, and |
| 45 | <constant>GL_ALWAYS</constant>. The initial value is <constant>GL_ALWAYS</constant>. |
| 46 | </para> |
| 47 | </listitem> |
| 48 | </varlistentry> |
| 49 | <varlistentry> |
| 50 | <term><parameter>ref</parameter></term> |
| 51 | <listitem> |
| 52 | <para> |
| 53 | Specifies the reference value for the stencil test. |
| 54 | <parameter>ref</parameter> is clamped to the range |
| 55 | <inlineequation><mml:math> |
| 56 | <!-- eqn: [0, 2 sup n - 1]:--> |
| 57 | <mml:mfenced open="[" close="]"> |
| 58 | <mml:mn>0</mml:mn> |
| 59 | <mml:mrow> |
| 60 | <mml:msup><mml:mn>2</mml:mn> |
| 61 | <mml:mi mathvariant="italic">n</mml:mi> |
| 62 | </mml:msup> |
| 63 | <mml:mo>-</mml:mo> |
| 64 | <mml:mn>1</mml:mn> |
| 65 | </mml:mrow> |
| 66 | </mml:mfenced> |
| 67 | </mml:math></inlineequation>, |
| 68 | where |
| 69 | <inlineequation><mml:math><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation> |
| 70 | is the number of bitplanes in the stencil buffer. The |
| 71 | initial value is 0. |
| 72 | </para> |
| 73 | </listitem> |
| 74 | </varlistentry> |
| 75 | <varlistentry> |
| 76 | <term><parameter>mask</parameter></term> |
| 77 | <listitem> |
| 78 | <para> |
| 79 | Specifies a mask that is ANDed with both the reference value |
| 80 | and the stored stencil value when the test is done. The initial value |
| 81 | is all 1's. |
| 82 | </para> |
| 83 | </listitem> |
| 84 | </varlistentry> |
| 85 | </variablelist> |
| 86 | </refsect1> |
| 87 | <refsect1 id="description"><title>Description</title> |
| 88 | <para> |
| 89 | Stenciling, |
| 90 | like depth-buffering, |
| 91 | enables and disables drawing on a per-pixel basis. |
| 92 | Stencil planes are first drawn into using GL drawing primitives, then |
| 93 | geometry and images are rendered using the stencil planes to mask out |
| 94 | portions of the screen. |
| 95 | Stenciling is typically used in multipass rendering algorithms |
| 96 | to achieve special effects, |
| 97 | such as decals, |
| 98 | outlining, |
| 99 | and constructive solid geometry rendering. |
| 100 | </para> |
| 101 | <para> |
| 102 | The stencil test conditionally eliminates a pixel based on the outcome |
| 103 | of a comparison between the reference value |
| 104 | and the value in the stencil buffer. |
| 105 | To enable and disable the test, call <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry> |
| 106 | with argument <constant>GL_STENCIL_TEST</constant>. |
| 107 | To specify actions based on the outcome of the stencil test, call |
| 108 | <citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry> or |
| 109 | <citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry>. |
| 110 | </para> |
| 111 | <para> |
| 112 | There can be two separate sets of <parameter>func</parameter>, <parameter>ref</parameter>, and |
| 113 | <parameter>mask</parameter> parameters; one affects back-facing polygons, and the other |
| 114 | affects front-facing polygons as well as other non-polygon primitives. |
| 115 | <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry> sets both front |
| 116 | and back stencil state to the same values. Use <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry> |
| 117 | to set front and back stencil state to different values. |
| 118 | </para> |
| 119 | <para> |
| 120 | <parameter>func</parameter> is a symbolic constant that determines the stencil comparison function. |
| 121 | It accepts one of eight values, |
| 122 | shown in the following list. |
| 123 | <parameter>ref</parameter> is an integer reference value that is used in the stencil comparison. |
| 124 | It is clamped to the range |
| 125 | <inlineequation><mml:math> |
| 126 | <!-- eqn: [0, 2 sup n - 1]:--> |
| 127 | <mml:mfenced open="[" close="]"> |
| 128 | <mml:mn>0</mml:mn> |
| 129 | <mml:mrow> |
| 130 | <mml:msup><mml:mn>2</mml:mn> |
| 131 | <mml:mi mathvariant="italic">n</mml:mi> |
| 132 | </mml:msup> |
| 133 | <mml:mo>-</mml:mo> |
| 134 | <mml:mn>1</mml:mn> |
| 135 | </mml:mrow> |
| 136 | </mml:mfenced> |
| 137 | </mml:math></inlineequation>, |
| 138 | where |
| 139 | <inlineequation><mml:math><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation> |
| 140 | is the number of bitplanes in the stencil buffer. |
| 141 | <parameter>mask</parameter> is bitwise ANDed with both the reference value |
| 142 | and the stored stencil value, |
| 143 | with the ANDed values participating in the comparison. |
| 144 | </para> |
| 145 | <para> |
| 146 | If <emphasis>stencil</emphasis> represents the value stored in the corresponding |
| 147 | stencil buffer location, |
| 148 | the following list shows the effect of each comparison function |
| 149 | that can be specified by <parameter>func</parameter>. |
| 150 | Only if the comparison succeeds is the pixel passed through |
| 151 | to the next stage in the rasterization process |
| 152 | (see <citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>). |
| 153 | All tests treat <emphasis>stencil</emphasis> values as unsigned integers in the range |
| 154 | <inlineequation><mml:math> |
| 155 | <!-- eqn: [0, 2 sup n - 1]:--> |
| 156 | <mml:mfenced open="[" close="]"> |
| 157 | <mml:mn>0</mml:mn> |
| 158 | <mml:mrow> |
| 159 | <mml:msup><mml:mn>2</mml:mn> |
| 160 | <mml:mi mathvariant="italic">n</mml:mi> |
| 161 | </mml:msup> |
| 162 | <mml:mo>-</mml:mo> |
| 163 | <mml:mn>1</mml:mn> |
| 164 | </mml:mrow> |
| 165 | </mml:mfenced> |
| 166 | </mml:math></inlineequation>, |
| 167 | where |
| 168 | <inlineequation><mml:math><mml:mi mathvariant="italic">n</mml:mi></mml:math></inlineequation> |
| 169 | is the number of bitplanes in the stencil buffer. |
| 170 | </para> |
| 171 | <para> |
| 172 | The following values are accepted by <parameter>func</parameter>: |
| 173 | </para> |
| 174 | <variablelist> |
| 175 | <varlistentry> |
| 176 | <term><constant>GL_NEVER</constant></term> |
| 177 | <listitem> |
| 178 | <para> |
| 179 | Always fails. |
| 180 | </para> |
| 181 | </listitem> |
| 182 | </varlistentry> |
| 183 | <varlistentry> |
| 184 | <term><constant>GL_LESS</constant></term> |
| 185 | <listitem> |
| 186 | <para> |
| 187 | Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) < ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ). |
| 188 | </para> |
| 189 | </listitem> |
| 190 | </varlistentry> |
| 191 | <varlistentry> |
| 192 | <term><constant>GL_LEQUAL</constant></term> |
| 193 | <listitem> |
| 194 | <para> |
| 195 | Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) <= ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ). |
| 196 | </para> |
| 197 | </listitem> |
| 198 | </varlistentry> |
| 199 | <varlistentry> |
| 200 | <term><constant>GL_GREATER</constant></term> |
| 201 | <listitem> |
| 202 | <para> |
| 203 | Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) > ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ). |
| 204 | </para> |
| 205 | </listitem> |
| 206 | </varlistentry> |
| 207 | <varlistentry> |
| 208 | <term><constant>GL_GEQUAL</constant></term> |
| 209 | <listitem> |
| 210 | <para> |
| 211 | Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) >= ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ). |
| 212 | </para> |
| 213 | </listitem> |
| 214 | </varlistentry> |
| 215 | <varlistentry> |
| 216 | <term><constant>GL_EQUAL</constant></term> |
| 217 | <listitem> |
| 218 | <para> |
| 219 | Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) = ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ). |
| 220 | </para> |
| 221 | </listitem> |
| 222 | </varlistentry> |
| 223 | <varlistentry> |
| 224 | <term><constant>GL_NOTEQUAL</constant></term> |
| 225 | <listitem> |
| 226 | <para> |
| 227 | Passes if ( <parameter>ref</parameter> & <parameter>mask</parameter> ) != ( <emphasis>stencil</emphasis> & <parameter>mask</parameter> ). |
| 228 | </para> |
| 229 | </listitem> |
| 230 | </varlistentry> |
| 231 | <varlistentry> |
| 232 | <term><constant>GL_ALWAYS</constant></term> |
| 233 | <listitem> |
| 234 | <para> |
| 235 | Always passes. |
| 236 | </para> |
| 237 | </listitem> |
| 238 | </varlistentry> |
| 239 | </variablelist> |
| 240 | </refsect1> |
| 241 | <refsect1 id="notes"><title>Notes</title> |
| 242 | <para> |
| 243 | Initially, the stencil test is disabled. |
| 244 | If there is no stencil buffer, |
| 245 | no stencil modification can occur and it is as if |
| 246 | the stencil test always passes. |
| 247 | </para> |
| 248 | <para> |
| 249 | <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry> is the same as |
| 250 | calling <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry> |
| 251 | with <parameter>face</parameter> set to <constant>GL_FRONT_AND_BACK</constant>. |
| 252 | </para> |
| 253 | </refsect1> |
| 254 | <refsect1 id="errors"><title>Errors</title> |
| 255 | <para> |
| 256 | <constant>GL_INVALID_ENUM</constant> is generated if <parameter>func</parameter> is not one of the eight |
| 257 | accepted values. |
| 258 | </para> |
| 259 | </refsect1> |
| 260 | <refsect1 id="associatedgets"><title>Associated Gets</title> |
| 261 | <para> |
| 262 | <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument |
| 263 | <constant>GL_STENCIL_FUNC</constant>, <constant>GL_STENCIL_VALUE_MASK</constant>, |
| 264 | <constant>GL_STENCIL_REF</constant>, <constant>GL_STENCIL_BACK_FUNC</constant>, |
| 265 | <constant>GL_STENCIL_BACK_VALUE_MASK</constant>, <constant>GL_STENCIL_BACK_REF</constant>, |
| 266 | or <constant>GL_STENCIL_BITS</constant> |
| 267 | </para> |
| 268 | <para> |
| 269 | <citerefentry><refentrytitle>glIsEnabled</refentrytitle></citerefentry> with argument <constant>GL_STENCIL_TEST</constant> |
| 270 | </para> |
| 271 | </refsect1> |
| 272 | <refsect1 id="seealso"><title>See Also</title> |
| 273 | <para> |
| 274 | <citerefentry><refentrytitle>glBlendFunc</refentrytitle></citerefentry>, |
| 275 | <citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>, |
| 276 | <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>, |
| 277 | <citerefentry><refentrytitle>glLogicOp</refentrytitle></citerefentry>, |
| 278 | <citerefentry><refentrytitle>glStencilFuncSeparate</refentrytitle></citerefentry>, |
| 279 | <citerefentry><refentrytitle>glStencilMask</refentrytitle></citerefentry>, |
| 280 | <citerefentry><refentrytitle>glStencilMaskSeparate</refentrytitle></citerefentry>, |
| 281 | <citerefentry><refentrytitle>glStencilOp</refentrytitle></citerefentry>, |
| 282 | <citerefentry><refentrytitle>glStencilOpSeparate</refentrytitle></citerefentry> |
| 283 | </para> |
| 284 | </refsect1> |
| 285 | <refsect1 id="Copyright"><title>Copyright</title> |
| 286 | <para> |
| 287 | Copyright <trademark class="copyright"></trademark> 1991-2006 |
| 288 | Silicon Graphics, Inc. This document is licensed under the SGI |
| 289 | Free Software B License. For details, see |
| 290 | <ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>. |
| 291 | </para> |
| 292 | </refsect1> |
| 293 | </refentry> |