
<refentry id="glFeedbackBuffer">
    <refmeta>
        <refmetainfo>
            <copyright>
                <year>1991-2006</year>
                <holder>Silicon Graphics, Inc.</holder>
            </copyright>
        </refmetainfo>
        <refentrytitle>glFeedbackBuffer</refentrytitle>
        <manvolnum>3G</manvolnum>
    </refmeta>
    <refnamediv>
        <refname>glFeedbackBuffer</refname>
        <refpurpose>controls feedback mode</refpurpose>
    </refnamediv>
    <refsynopsisdiv><title>C Specification</title>
        <funcsynopsis>
            <funcprototype>
                <funcdef>void <function>glFeedbackBuffer</function></funcdef>
                <paramdef>GLsizei <parameter>size</parameter></paramdef>
                <paramdef>GLenum <parameter>type</parameter></paramdef>
                <paramdef>GLfloat * <parameter>buffer</parameter></paramdef>
            </funcprototype>
        </funcsynopsis>
    </refsynopsisdiv>
    <!-- eqn: ignoring delim $$ -->
    <refsect1 id="parameters"><title>Parameters</title>
        <variablelist>
        <varlistentry>
            <term><parameter>size</parameter></term>
            <listitem>
                <para>
                    Specifies the maximum number of values that can be written into <parameter>buffer</parameter>.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>type</parameter></term>
            <listitem>
                <para>
                    Specifies a symbolic constant that describes the information
                    that will be returned for each vertex.
                    <constant>GL_2D</constant>,
                    <constant>GL_3D</constant>,
                    <constant>GL_3D_COLOR</constant>,
                    <constant>GL_3D_COLOR_TEXTURE</constant>, and
                    <constant>GL_4D_COLOR_TEXTURE</constant> are accepted.
                </para>
            </listitem>
        </varlistentry>
        <varlistentry>
            <term><parameter>buffer</parameter></term>
            <listitem>
                <para>
                    Returns the feedback data.
                </para>
            </listitem>
        </varlistentry>
        </variablelist>
    </refsect1>
    <refsect1 id="description"><title>Description</title>
        <para>
            The <function>glFeedbackBuffer</function> function controls feedback.
            Feedback, like selection, is a GL mode.
            The mode is selected by calling
            <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry> with <constant>GL_FEEDBACK</constant>.
            When the GL is in feedback mode,
            no pixels are produced by rasterization.
            Instead, information about primitives that would have been
            rasterized is fed back to the application using the GL.
        </para>
        <para>
            <function>glFeedbackBuffer</function> has three arguments:
            <parameter>buffer</parameter> is a pointer to an array of floating-point values
            into which feedback information is placed.
            <parameter>size</parameter> indicates the size of the array.
            <parameter>type</parameter> is a symbolic constant describing the information
            that is fed back for each vertex.
            <function>glFeedbackBuffer</function> must be issued before feedback mode is enabled
            (by calling <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry> with argument <constant>GL_FEEDBACK</constant>).
            Setting <constant>GL_FEEDBACK</constant> without establishing the feedback buffer,
            or calling <function>glFeedbackBuffer</function> while the GL is in feedback mode,
            is an error.
        </para>
        <para>
            When <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry> is called while in feedback mode, it returns the number of entries
            placed in the feedback array and resets the feedback array pointer to the base
            of the feedback buffer. The returned value never exceeds <parameter>size</parameter>. If the feedback
            data required more room than was available in <parameter>buffer</parameter>,
            <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry> returns a negative value.
            To take the GL out of feedback mode, call
            <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry> with a parameter value other than <constant>GL_FEEDBACK</constant>.
        </para>
        <para>
            While in feedback mode,
            each primitive, bitmap, or pixel rectangle that would be rasterized
            generates a block of values that are copied into the feedback array.
            If doing so would cause the number of entries to exceed the maximum,
            the block is partially written so as to fill the array
            (if there is any room left at all),
            and an overflow flag is set.
            Each block begins with a code indicating the primitive type,
            followed by values that describe the primitive's vertices and
            associated data.
            Entries are also written for bitmaps and pixel rectangles.
            Feedback occurs after polygon culling and <citerefentry><refentrytitle>glPolygonMode</refentrytitle></citerefentry> interpretation
            of polygons has taken place,
            so polygons that are culled are not returned in the feedback buffer.
            It can also occur after polygons with more than three edges are broken up
            into triangles,
            if the GL implementation renders polygons by performing this decomposition.
        </para>
        <para>
            The <citerefentry><refentrytitle>glPassThrough</refentrytitle></citerefentry> command can be used to insert a marker
            into the feedback buffer.
            See <citerefentry><refentrytitle>glPassThrough</refentrytitle></citerefentry>.
        </para>
        <para>
            Following is the grammar for the blocks of values written
            into the feedback buffer.
            Each primitive is indicated with a unique identifying value
            followed by some number of vertices.
            Polygon entries include an integer value indicating how many vertices follow.
            A vertex is fed back as some number of floating-point values,
            as determined by <parameter>type</parameter>.
            Colors are fed back as four values in RGBA mode and one value
            in color index mode.
        </para>

        <para>
            <para>
                feedbackList <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> feedbackItem feedbackList | feedbackItem
            </para>
            <para>
                feedbackItem <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> point | lineSegment | polygon | bitmap | pixelRectangle | passThru
            </para>
            <para>
                point <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> <constant>GL_POINT_TOKEN</constant> vertex
            </para>
            <para>
                lineSegment <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> <constant>GL_LINE_TOKEN</constant> vertex vertex | <constant>GL_LINE_RESET_TOKEN</constant> vertex vertex
            </para>
            <para>
                polygon <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> <constant>GL_POLYGON_TOKEN</constant> n polySpec
            </para>
            <para>
                polySpec <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> polySpec vertex | vertex vertex vertex
            </para>
            <para>
                bitmap <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> <constant>GL_BITMAP_TOKEN</constant> vertex
            </para>
            <para>
                pixelRectangle <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> <constant>GL_DRAW_PIXEL_TOKEN</constant> vertex | <constant>GL_COPY_PIXEL_TOKEN</constant> vertex
            </para>
            <para>
                passThru <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> <constant>GL_PASS_THROUGH_TOKEN</constant> value
            </para>
            <para>
                vertex <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> 2d | 3d | 3dColor | 3dColorTexture | 4dColorTexture
            </para>
            <para>
                2d <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value value
            </para>
            <para>
                3d <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value value value
            </para>
            <para>
                3dColor <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value value value color
            </para>
            <para>
                3dColorTexture <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value value value color tex
            </para>
            <para>
                4dColorTexture <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value value value value color tex
            </para>
            <para>
                color <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> rgba | index
            </para>
            <para>
                rgba <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value value value value
            </para>
            <para>
                index <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value
            </para>
            <para>
                tex <inlineequation><mml:math><mml:mrow><mml:mn>&#x2190;</mml:mn></mml:mrow></mml:math></inlineequation> value value value value
            </para>
            <para>
            </para>
        </para>
        <para>
            <emphasis>value</emphasis>
            is a floating-point number,
            and
            <emphasis>n</emphasis>
            is a floating-point integer giving the number of vertices in the polygon.
            <constant>GL_POINT_TOKEN</constant>,
            <constant>GL_LINE_TOKEN</constant>,
            <constant>GL_LINE_RESET_TOKEN</constant>,
            <constant>GL_POLYGON_TOKEN</constant>,
            <constant>GL_BITMAP_TOKEN</constant>,
            <constant>GL_DRAW_PIXEL_TOKEN</constant>,
            <constant>GL_COPY_PIXEL_TOKEN</constant> and
            <constant>GL_PASS_THROUGH_TOKEN</constant> are symbolic floating-point constants.
            <constant>GL_LINE_RESET_TOKEN</constant> is returned whenever the line stipple pattern
            is reset.
            The data returned as a vertex depends on the feedback <parameter>type</parameter>.
        </para>
        <para>
            The following table gives the correspondence between <parameter>type</parameter>
            and the number of values per vertex.
            <emphasis>k</emphasis> is 1 in color index mode and 4 in RGBA mode.
        </para>
        <para>
        </para>
        <informaltable frame="topbot">
            <tgroup cols="5" align="left">
                <colspec colwidth="2.7*" />
                <colspec colwidth="1.5*" />
                <colspec colwidth="1*" align="center"/>
                <colspec colwidth="1*" align="center"/>
                <colspec colwidth="2.5*" align="center"/>
                <thead>
                    <row>
                        <entry rowsep="1" align="left"><emphasis role="bold">
                        Type
                        </emphasis></entry>
                        <entry rowsep="1" align="left"><emphasis role="bold">
                        Coordinates
                        </emphasis></entry>
                        <entry rowsep="1" align="center"><emphasis role="bold">
                        Color
                        </emphasis></entry>
                        <entry rowsep="1" align="center"><emphasis role="bold">
                        Texture
                        </emphasis></entry>
                        <entry rowsep="1" align="center"><emphasis role="bold">
                        Total Number of Values
                        </emphasis></entry>
                    </row>
                </thead>
                <tbody>
                    <row>
                        <entry align="left">
                        <constant>GL_2D</constant>
                        </entry>
                        <entry align="left">
                        <emphasis>x</emphasis>, <emphasis>y</emphasis>
                        </entry>
                        <entry align="center">
                        </entry>
                        <entry align="center">
                        </entry>
                        <entry align="center">
                        2
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_3D</constant>
                        </entry>
                        <entry align="left">
                        <emphasis>x</emphasis>, <emphasis>y</emphasis>, <emphasis>z</emphasis>
                        </entry>
                        <entry align="center">
                        </entry>
                        <entry align="center">
                        </entry>
                        <entry align="center">
                        3
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_3D_COLOR</constant>
                        </entry>
                        <entry align="left">
                        <emphasis>x</emphasis>, <emphasis>y</emphasis>, <emphasis>z</emphasis>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math><mml:mi mathvariant="italic">k</mml:mi></mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 3  +  k:-->
                            <mml:mrow>
                                <mml:mn>3</mml:mn>
                                <mml:mo>+</mml:mo>
                                <mml:mi mathvariant="italic">k</mml:mi>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_3D_COLOR_TEXTURE</constant>
                        </entry>
                        <entry align="left">
                        <emphasis>x</emphasis>, <emphasis>y</emphasis>, <emphasis>z</emphasis>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math><mml:mi mathvariant="italic">k</mml:mi></mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        4
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 7  +  k:-->
                            <mml:mrow>
                                <mml:mn>7</mml:mn>
                                <mml:mo>+</mml:mo>
                                <mml:mi mathvariant="italic">k</mml:mi>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                    <row>
                        <entry align="left">
                        <constant>GL_4D_COLOR_TEXTURE</constant>
                        </entry>
                        <entry align="left">
                        <emphasis>x</emphasis>, <emphasis>y</emphasis>, <emphasis>z</emphasis>, <emphasis>w</emphasis>
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math><mml:mi mathvariant="italic">k</mml:mi></mml:math></inlineequation>
                        </entry>
                        <entry align="center">
                        4
                        </entry>
                        <entry align="center">
                        <inlineequation><mml:math>
                            <!-- eqn: 8  +  k:-->
                            <mml:mrow>
                                <mml:mn>8</mml:mn>
                                <mml:mo>+</mml:mo>
                                <mml:mi mathvariant="italic">k</mml:mi>
                            </mml:mrow>
                        </mml:math></inlineequation>
                        </entry>
                    </row>
                </tbody>
            </tgroup>
        </informaltable>
        <para>
            Feedback vertex coordinates are in window coordinates,
            except <emphasis>w</emphasis>,
            which is in clip coordinates.
            Feedback colors are lighted, if lighting is enabled.
            Feedback texture coordinates are generated,
            if texture coordinate generation is enabled.
            They are always transformed by the texture matrix.
        </para>
    </refsect1>
    <refsect1 id="notes"><title>Notes</title>
        <para>
            <function>glFeedbackBuffer</function>, when used in a display list, is not compiled into the display list
            but is executed immediately.
        </para>
        <para>
            <function>glFeedbackBuffer</function> returns only the texture coordinate of texture unit <constant>GL_TEXTURE0</constant>.
        </para>
    </refsect1>
    <refsect1 id="errors"><title>Errors</title>
        <para>
            <constant>GL_INVALID_ENUM</constant> is generated if <parameter>type</parameter> is not an accepted value.
        </para>
        <para>
            <constant>GL_INVALID_VALUE</constant> is generated if <parameter>size</parameter> is negative.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <function>glFeedbackBuffer</function> is called while the
            render mode is <constant>GL_FEEDBACK</constant>,
            or if <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry> is called with argument <constant>GL_FEEDBACK</constant> before
            <function>glFeedbackBuffer</function> is called at least once.
        </para>
        <para>
            <constant>GL_INVALID_OPERATION</constant> is generated if <function>glFeedbackBuffer</function>
            is executed between the execution of <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>
            and the corresponding execution of <citerefentry><refentrytitle>glEnd</refentrytitle></citerefentry>.
        </para>
    </refsect1>
    <refsect1 id="associatedgets"><title>Associated Gets</title>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_RENDER_MODE</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FEEDBACK_BUFFER_POINTER</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FEEDBACK_BUFFER_SIZE</constant>
        </para>
        <para>
            <citerefentry><refentrytitle>glGet</refentrytitle></citerefentry> with argument <constant>GL_FEEDBACK_BUFFER_TYPE</constant>
        </para>
    </refsect1>
    <refsect1 id="seealso"><title>See Also</title>
        <para>
            <citerefentry><refentrytitle>glBegin</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glLineStipple</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glPassThrough</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glPolygonMode</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glRenderMode</refentrytitle></citerefentry>,
            <citerefentry><refentrytitle>glSelectBuffer</refentrytitle></citerefentry>
        </para>
    </refsect1>
    <refsect1 id="Copyright"><title>Copyright</title>
        <para>
            Copyright <trademark class="copyright"></trademark> 1991-2006
            Silicon Graphics, Inc. This document is licensed under the SGI
            Free Software B License. For details, see
            <ulink url="http://oss.sgi.com/projects/FreeB/">http://oss.sgi.com/projects/FreeB/</ulink>.
        </para>
    </refsect1>
</refentry>
