<div class="refentry" lang="en" xml:lang="en"><a id="glFeedbackBuffer"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>glFeedbackBuffer — controls feedback mode</p></div><div class="refsynopsisdiv"><h2>C Specification</h2><div class="funcsynopsis"><table><tr><td><code class="funcdef">void <b class="fsfunc">glFeedbackBuffer</b>(</code></td><td>GLsizei  </td><td><var class="pdparam">size</var>, </td></tr><tr><td> </td><td>GLenum  </td><td><var class="pdparam">type</var>, </td></tr><tr><td> </td><td>GLfloat *  </td><td><var class="pdparam">buffer</var><code>)</code>;</td></tr></table></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="parameters"></a><h2>Parameters</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>size</code></em></span></dt><dd><p>

Specifies the maximum number of values that can be written into <em class="parameter"><code>buffer</code></em>.

</p></dd><dt><span class="term"><em class="parameter"><code>type</code></em></span></dt><dd><p>

Specifies a symbolic constant that describes the information

that will be returned for each vertex.

<code class="constant">GL_2D</code>,

<code class="constant">GL_3D</code>,

<code class="constant">GL_3D_COLOR</code>,

<code class="constant">GL_3D_COLOR_TEXTURE</code>, and

<code class="constant">GL_4D_COLOR_TEXTURE</code> are accepted.

</p></dd><dt><span class="term"><em class="parameter"><code>buffer</code></em></span></dt><dd><p>

Returns the feedback data.

</p></dd></dl></div></div><div class="refsect1" lang="en" xml:lang="en"><a id="description"></a><h2>Description</h2><p>

The <code class="function">glFeedbackBuffer</code> function controls feedback.

Feedback, like selection, is a GL mode.

The mode is selected by calling

<a class="citerefentry" href="glRenderMode"><span class="citerefentry"><span class="refentrytitle">glRenderMode</span></span></a> with <code class="constant">GL_FEEDBACK</code>.

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.

</p><p>

<code class="function">glFeedbackBuffer</code> has three arguments:

<em class="parameter"><code>buffer</code></em> is a pointer to an array of floating-point values

into which feedback information is placed.

<em class="parameter"><code>size</code></em> indicates the size of the array.

<em class="parameter"><code>type</code></em> is a symbolic constant describing the information

that is fed back for each vertex.

<code class="function">glFeedbackBuffer</code> must be issued before feedback mode is enabled

(by calling <a class="citerefentry" href="glRenderMode"><span class="citerefentry"><span class="refentrytitle">glRenderMode</span></span></a> with argument <code class="constant">GL_FEEDBACK</code>).

Setting <code class="constant">GL_FEEDBACK</code> without establishing the feedback buffer,

or calling <code class="function">glFeedbackBuffer</code> while the GL is in feedback mode,

is an error.

</p><p>

When <a class="citerefentry" href="glRenderMode"><span class="citerefentry"><span class="refentrytitle">glRenderMode</span></span></a> 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 <em class="parameter"><code>size</code></em>. If the feedback

data required more room than was available in <em class="parameter"><code>buffer</code></em>,

<a class="citerefentry" href="glRenderMode"><span class="citerefentry"><span class="refentrytitle">glRenderMode</span></span></a> returns a negative value.

To take the GL out of feedback mode, call

<a class="citerefentry" href="glRenderMode"><span class="citerefentry"><span class="refentrytitle">glRenderMode</span></span></a> with a parameter value other than <code class="constant">GL_FEEDBACK</code>.

</p><p>

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 <a class="citerefentry" href="glPolygonMode"><span class="citerefentry"><span class="refentrytitle">glPolygonMode</span></span></a> 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.

</p><p>

The <a class="citerefentry" href="glPassThrough"><span class="citerefentry"><span class="refentrytitle">glPassThrough</span></span></a> command can be used to insert a marker

into the feedback buffer.

See <a class="citerefentry" href="glPassThrough"><span class="citerefentry"><span class="refentrytitle">glPassThrough</span></span></a>.

</p><p>

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 <em class="parameter"><code>type</code></em>.

Colors are fed back as four values in RGBA mode and one value

in color index mode.

</p><p>

</p><p>

feedbackList <math overflow="scroll"><mrow><mn>←</mn></mrow></math> feedbackItem feedbackList | feedbackItem

</p><p>

</p><p>

feedbackItem <math overflow="scroll"><mrow><mn>←</mn></mrow></math> point | lineSegment | polygon | bitmap | pixelRectangle | passThru

</p><p>

</p><p>

point <math overflow="scroll"><mrow><mn>←</mn></mrow></math> <code class="constant">GL_POINT_TOKEN</code> vertex

</p><p>

</p><p>

lineSegment <math overflow="scroll"><mrow><mn>←</mn></mrow></math> <code class="constant">GL_LINE_TOKEN</code> vertex vertex | <code class="constant">GL_LINE_RESET_TOKEN</code> vertex vertex

</p><p>

</p><p>

polygon <math overflow="scroll"><mrow><mn>←</mn></mrow></math> <code class="constant">GL_POLYGON_TOKEN</code> n polySpec

</p><p>

</p><p>

polySpec <math overflow="scroll"><mrow><mn>←</mn></mrow></math> polySpec vertex | vertex vertex vertex

</p><p>

</p><p>

bitmap <math overflow="scroll"><mrow><mn>←</mn></mrow></math> <code class="constant">GL_BITMAP_TOKEN</code> vertex

</p><p>

</p><p>

pixelRectangle <math overflow="scroll"><mrow><mn>←</mn></mrow></math> <code class="constant">GL_DRAW_PIXEL_TOKEN</code> vertex | <code class="constant">GL_COPY_PIXEL_TOKEN</code> vertex

</p><p>

</p><p>

passThru <math overflow="scroll"><mrow><mn>←</mn></mrow></math> <code class="constant">GL_PASS_THROUGH_TOKEN</code> value

</p><p>

</p><p>

vertex <math overflow="scroll"><mrow><mn>←</mn></mrow></math> 2d | 3d | 3dColor | 3dColorTexture | 4dColorTexture

</p><p>

</p><p>

2d <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value value

</p><p>

</p><p>

3d <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value value value

</p><p>

</p><p>

3dColor <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value value value color

</p><p>

</p><p>

3dColorTexture <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value value value color tex

</p><p>

</p><p>

4dColorTexture <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value value value value color tex

</p><p>

</p><p>

color <math overflow="scroll"><mrow><mn>←</mn></mrow></math> rgba | index

</p><p>

</p><p>

rgba <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value value value value

</p><p>

</p><p>

index <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value

</p><p>

</p><p>

tex <math overflow="scroll"><mrow><mn>←</mn></mrow></math> value value value value

</p><p>

</p><p>

</p><p>

</p><p>

<span class="emphasis"><em>value</em></span>

is a floating-point number,

and

<span class="emphasis"><em>n</em></span>

is a floating-point integer giving the number of vertices in the polygon.

<code class="constant">GL_POINT_TOKEN</code>,

<code class="constant">GL_LINE_TOKEN</code>,

<code class="constant">GL_LINE_RESET_TOKEN</code>,

<code class="constant">GL_POLYGON_TOKEN</code>,

<code class="constant">GL_BITMAP_TOKEN</code>,

<code class="constant">GL_DRAW_PIXEL_TOKEN</code>,

<code class="constant">GL_COPY_PIXEL_TOKEN</code> and

<code class="constant">GL_PASS_THROUGH_TOKEN</code> are symbolic floating-point constants.

<code class="constant">GL_LINE_RESET_TOKEN</code> is returned whenever the line stipple pattern

is reset.

The data returned as a vertex depends on the feedback <em class="parameter"><code>type</code></em>.

</p><p>

The following table gives the correspondence between <em class="parameter"><code>type</code></em>

and the number of values per vertex.

<span class="emphasis"><em>k</em></span> is 1 in color index mode and 4 in RGBA mode.

</p><p>

</p><div class="informaltable"><table><colgroup><col /><col /><col /><col /><col /></colgroup><thead><tr><th><span class="bold"><strong>

Type

</strong></span></th><th><span class="bold"><strong>

Coordinates

</strong></span></th><th class="center"><span class="bold"><strong>

Color

</strong></span></th><th class="center"><span class="bold"><strong>

Texture

</strong></span></th><th class="center"><span class="bold"><strong>

Total Number of Values

</strong></span></th></tr></thead><tbody><tr><td>

<code class="constant">GL_2D</code>

</td><td>

<span class="emphasis"><em>x</em></span>, <span class="emphasis"><em>y</em></span>

</td><td class="center">

</td><td class="center">

</td><td class="center">

2

</td></tr><tr><td>

<code class="constant">GL_3D</code>

</td><td>

<span class="emphasis"><em>x</em></span>, <span class="emphasis"><em>y</em></span>, <span class="emphasis"><em>z</em></span>

</td><td class="center">

</td><td class="center">

</td><td class="center">

3

</td></tr><tr><td>

<code class="constant">GL_3D_COLOR</code>

</td><td>

<span class="emphasis"><em>x</em></span>, <span class="emphasis"><em>y</em></span>, <span class="emphasis"><em>z</em></span>

</td><td class="center">

<math overflow="scroll"><mi mathvariant="italic">k</mi></math>

</td><td class="center">

</td><td class="center">

<math overflow="scroll">

<mrow>

<mn>3</mn>

<mo>+</mo>

<mi mathvariant="italic">k</mi>

</mrow>

</math>

</td></tr><tr><td>

<code class="constant">GL_3D_COLOR_TEXTURE</code>

</td><td>

<span class="emphasis"><em>x</em></span>, <span class="emphasis"><em>y</em></span>, <span class="emphasis"><em>z</em></span>

</td><td class="center">

<math overflow="scroll"><mi mathvariant="italic">k</mi></math>

</td><td class="center">

4

</td><td class="center">

<math overflow="scroll">

<mrow>

<mn>7</mn>

<mo>+</mo>

<mi mathvariant="italic">k</mi>

</mrow>

</math>

</td></tr><tr><td>

<code class="constant">GL_4D_COLOR_TEXTURE</code>

</td><td>

<span class="emphasis"><em>x</em></span>, <span class="emphasis"><em>y</em></span>, <span class="emphasis"><em>z</em></span>, <span class="emphasis"><em>w</em></span>

</td><td class="center">

<math overflow="scroll"><mi mathvariant="italic">k</mi></math>

</td><td class="center">

4

</td><td class="center">

<math overflow="scroll">

<mrow>

<mn>8</mn>

<mo>+</mo>

<mi mathvariant="italic">k</mi>

</mrow>

</math>

</td></tr></tbody></table></div><p>

Feedback vertex coordinates are in window coordinates,

except <span class="emphasis"><em>w</em></span>,

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.

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="notes"></a><h2>Notes</h2><p>

<code class="function">glFeedbackBuffer</code>, when used in a display list, is not compiled into the display list

but is executed immediately.

</p><p>

<code class="function">glFeedbackBuffer</code> returns only the texture coordinate of texture unit <code class="constant">GL_TEXTURE0</code>.

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="errors"></a><h2>Errors</h2><p>

<code class="constant">GL_INVALID_ENUM</code> is generated if <em class="parameter"><code>type</code></em> is not an accepted value.

</p><p>

<code class="constant">GL_INVALID_VALUE</code> is generated if <em class="parameter"><code>size</code></em> is negative.

</p><p>

<code class="constant">GL_INVALID_OPERATION</code> is generated if <code class="function">glFeedbackBuffer</code> is called while the

render mode is <code class="constant">GL_FEEDBACK</code>,

or if <a class="citerefentry" href="glRenderMode"><span class="citerefentry"><span class="refentrytitle">glRenderMode</span></span></a> is called with argument <code class="constant">GL_FEEDBACK</code> before

<code class="function">glFeedbackBuffer</code> is called at least once.

</p><p>

<code class="constant">GL_INVALID_OPERATION</code> is generated if <code class="function">glFeedbackBuffer</code>

is executed between the execution of <a class="citerefentry" href="glBegin"><span class="citerefentry"><span class="refentrytitle">glBegin</span></span></a>

and the corresponding execution of <a class="citerefentry" href="glEnd"><span class="citerefentry"><span class="refentrytitle">glEnd</span></span></a>.

</p></div><div class="refsect1" lang="en" xml:lang="en"><a id="associatedgets"></a><h2>Associated Gets</h2><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_RENDER_MODE</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FEEDBACK_BUFFER_POINTER</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FEEDBACK_BUFFER_SIZE</code>

</p><p>

<a class="citerefentry" href="glGet"><span class="citerefentry"><span class="refentrytitle">glGet</span></span></a> with argument <code class="constant">GL_FEEDBACK_BUFFER_TYPE</code>

</p></div>

{$pipelinestall}{$examples}

<div class="refsect1" lang="en" xml:lang="en"><a id="seealso"></a><h2>See Also</h2><p>

<a class="citerefentry" href="glBegin"><span class="citerefentry"><span class="refentrytitle">glBegin</span></span></a>,

<a class="citerefentry" href="glLineStipple"><span class="citerefentry"><span class="refentrytitle">glLineStipple</span></span></a>,

<a class="citerefentry" href="glPassThrough"><span class="citerefentry"><span class="refentrytitle">glPassThrough</span></span></a>,

<a class="citerefentry" href="glPolygonMode"><span class="citerefentry"><span class="refentrytitle">glPolygonMode</span></span></a>,

<a class="citerefentry" href="glRenderMode"><span class="citerefentry"><span class="refentrytitle">glRenderMode</span></span></a>,

<a class="citerefentry" href="glSelectBuffer"><span class="citerefentry"><span class="refentrytitle">glSelectBuffer</span></span></a>

</p></div><div class="refsect1" lang="en" xml:lang="en"><div id="Copyright"><h2>Copyright</h2><p>

Copyright © 1991-2006

Silicon Graphics, Inc. This document is licensed under the SGI

Free Software B License. For details, see

<a class="ulink" href="https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/" target="_top">https://web.archive.org/web/20171022161616/http://oss.sgi.com/projects/FreeB/</a>.

</p></div></div></div>