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OpenGL ES 1.0 and EGL 1.0 reference manual and man pages (written by
Claude Knaus, based on OpenGL SI man pages).

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<refentry id="glBlendFunc">
  <refmeta>
    <refentrytitle>glBlendFunc</refentrytitle>
    <manvolnum>3G</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>glBlendFunc</refname>
    <refpurpose>specify pixel arithmetic</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <title>C Specification</title>

    <funcsynopsis>
      <funcprototype>
        <funcdef>void <function>glBlendFunc</function></funcdef>
        <paramdef>GLenum <parameter>sfactor</parameter></paramdef>
        <paramdef>GLenum <parameter>dfactor</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Parameters</title>

    <variablelist>
      <varlistentry>
        <term>
          <parameter>sfactor</parameter>
        </term>

        <listitem>
          <para>Specifies how the red, green, blue, and alpha
          source blending factors are computed. The following
          symbolic constants are accepted: 
          <constant>GL_ZERO</constant>, 
          <constant>GL_ONE</constant>, 
          <constant>GL_DST_COLOR</constant>, 
          <constant>GL_ONE_MINUS_DST_COLOR</constant>, 
          <constant>GL_SRC_ALPHA</constant>, 
          <constant>GL_ONE_MINUS_SRC_ALPHA</constant>, 
          <constant>GL_DST_ALPHA</constant>, 
          <constant>GL_ONE_MINUS_DST_ALPHA</constant>, and 
          <constant>GL_SRC_ALPHA_SATURATE</constant>. The initial value is 
          <constant>GL_ONE</constant>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <parameter>dfactor</parameter>
        </term>

        <listitem>
          <para>Specifies how the red, green, blue, and alpha
          destination blending factors are computed. Eight symbolic
          constants are accepted: 
          <constant>GL_ZERO</constant>, 
          <constant>GL_ONE</constant>, 
          <constant>GL_SRC_COLOR</constant>, 
          <constant>GL_ONE_MINUS_SRC_COLOR</constant>, 
          <constant>GL_SRC_ALPHA</constant>, 
          <constant>GL_ONE_MINUS_SRC_ALPHA</constant>, 
          <constant>GL_DST_ALPHA</constant>, and 
          <constant>GL_ONE_MINUS_DST_ALPHA</constant>. The initial value is 
          <constant>GL_ZERO</constant>.</para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>Pixels can be drawn using a function that blends the
    incoming (source) values with the values that are already in
    the color buffer (the destination values). Use 
    <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry> and 
    <citerefentry><refentrytitle>glDisable</refentrytitle></citerefentry>
    with argument <constant>GL_BLEND</constant>
    to enable and disable blending.
    Blending is initially disabled.
    </para>

    <para>
    <function>glBlendFunc</function>
    defines the operation of blending when it is enabled. 
    <parameter>sfactor</parameter>
    specifies which of nine methods is used to scale the source
    color components. 
    <parameter>dfactor</parameter>
    specifies which of eight methods is used to scale the
    destination color components. The eleven possible methods are
    described in the following table. Each method defines four
    scale factors, one each for red, green, blue, and alpha.</para>

    <para>In the table and in subsequent equations, source and
    destination color components are referred to as 
    <inlineequation><math><mrow><mo>(</mo>
      <msub><mi>R</mi><mi>s</mi></msub><mo>,</mo>
      <msub><mi>G</mi><mi>s</mi></msub><mo>,</mo>
      <msub><mi>B</mi><mi>s</mi></msub><mo>,</mo>
      <msub><mi>A</mi><mi>s</mi></msub>
    <mo>)</mo></mrow></math></inlineequation>
    and 
    <inlineequation><math><mrow><mo>(</mo>
      <msub><mi>R</mi><mi>d</mi></msub><mo>,</mo>
      <msub><mi>G</mi><mi>d</mi></msub><mo>,</mo>
      <msub><mi>B</mi><mi>d</mi></msub><mo>,</mo>
      <msub><mi>A</mi><mi>d</mi></msub>
    <mo>)</mo></mrow></math></inlineequation>.
    They are understood to have integer values between 0 and 
    <inlineequation><math><mrow><mo>(</mo>
      <msub><mi>k</mi><mi>R</mi></msub><mo>,</mo>
      <msub><mi>k</mi><mi>G</mi></msub><mo>,</mo>
      <msub><mi>k</mi><mi>B</mi></msub><mo>,</mo>
      <msub><mi>k</mi><mi>A</mi></msub>
    <mo>)</mo></mrow></math></inlineequation>,
    where</para>

    <informalequation>
    <math>
      <msub><mi>k</mi><mi>c</mi></msub>
      <mo>=</mo>
      <msup><mn>2</mn><msub><mi>m</mi><mi>c</mi></msub></msup>
      <mo>-</mo>
      <mn>1</mn>
    </math>
    </informalequation>

    <para> and 
    <inlineequation><math><mrow><mo>(</mo>
      <msub><mi>m</mi><mi>R</mi></msub><mo>,</mo>
      <msub><mi>m</mi><mi>G</mi></msub><mo>,</mo>
      <msub><mi>m</mi><mi>B</mi></msub><mo>,</mo>
      <msub><mi>m</mi><mi>A</mi></msub>
    <mo>)</mo></mrow></math></inlineequation>
    is the number of red, green, blue, and alpha bitplanes.</para>

    <para>Source and destination scale factors are referred to as 
    <inlineequation><math><mrow><mo>(</mo>
      <msub><mi>s</mi><mi>R</mi></msub><mo>,</mo>
      <msub><mi>s</mi><mi>G</mi></msub><mo>,</mo>
      <msub><mi>s</mi><mi>B</mi></msub><mo>,</mo>
      <msub><mi>s</mi><mi>A</mi></msub>
    <mo>)</mo></mrow></math></inlineequation>
    and 
    <inlineequation><math><mrow><mo>(</mo>
      <msub><mi>d</mi><mi>R</mi></msub><mo>,</mo>
      <msub><mi>d</mi><mi>G</mi></msub><mo>,</mo>
      <msub><mi>d</mi><mi>B</mi></msub><mo>,</mo>
      <msub><mi>d</mi><mi>A</mi></msub>
    <mo>)</mo></mrow></math></inlineequation>.

    The scale factors described in the table, denoted 
    <inlineequation><math><mrow><mo>(</mo>
      <msub><mi>f</mi><mi>R</mi></msub><mo>,</mo>
      <msub><mi>f</mi><mi>G</mi></msub><mo>,</mo>
      <msub><mi>f</mi><mi>B</mi></msub><mo>,</mo>
      <msub><mi>f</mi><mi>A</mi></msub>
    <mo>)</mo></mrow></math></inlineequation>,
    represent either source or destination factors. All scale
    factors have range [0, 1].</para>

    <informaltable pgwide='0' frame='none'>
      <tgroup cols='2' align='center'>
        <colspec align="left"/>
        <colspec align="center"/>

        <thead>
          <row rowsep='1'>
            <entry>Parameter</entry>
            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <msub><mi>f</mi><mi>R</mi></msub><mo>,</mo>
                <msub><mi>f</mi><mi>G</mi></msub><mo>,</mo>
                <msub><mi>f</mi><mi>B</mi></msub><mo>,</mo>
                <msub><mi>f</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>
        </thead>

        <tbody>
          <row>
            <entry>
              <constant>GL_ZERO</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <mn>0</mn><mo>,</mo>
                <mn>0</mn><mo>,</mo>
                <mn>0</mn><mo>,</mo>
                <mn>0</mn>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_ONE</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_SRC_COLOR</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <msub><mi>R</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>R</mi></msub><mo>,</mo>
                <msub><mi>G</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>G</mi></msub><mo>,</mo>
                <msub><mi>B</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>B</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_ONE_MINUS_SRC_COLOR</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow>
              <mo>(</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn>
              <mo>)</mo>
              <mo>-</mo>
              <mo>(</mo>
                <msub><mi>R</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>R</mi></msub><mo>,</mo>
                <msub><mi>G</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>G</mi></msub><mo>,</mo>
                <msub><mi>B</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>B</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_DST_COLOR</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <msub><mi>R</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>R</mi></msub><mo>,</mo>
                <msub><mi>G</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>G</mi></msub><mo>,</mo>
                <msub><mi>B</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>B</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_ONE_MINUS_DST_COLOR</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow>
              <mo>(</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn>
              <mo>)</mo>
              <mo>-</mo>
              <mo>(</mo>
                <msub><mi>R</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>R</mi></msub><mo>,</mo>
                <msub><mi>G</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>G</mi></msub><mo>,</mo>
                <msub><mi>B</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>B</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_SRC_ALPHA</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_ONE_MINUS_SRC_ALPHA</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow>
              <mo>(</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn>
              <mo>)</mo>
              <mo>-</mo>
              <mo>(</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>s</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_DST_ALPHA</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_ONE_MINUS_DST_ALPHA</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow>
              <mo>(</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn><mo>,</mo>
                <mn>1</mn>
              <mo>)</mo>
              <mo>-</mo>
              <mo>(</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
                <msub><mi>A</mi><mi>d</mi></msub><mo>/</mo><msub><mi>k</mi><mi>A</mi></msub>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>

          <row>
            <entry>
              <constant>GL_SRC_ALPHA_SATURATE</constant>
            </entry>

            <entry>
              <inlineequation><math><mrow><mo>(</mo>
                <mi>i</mi><mo>,</mo>
                <mi>i</mi><mo>,</mo>
                <mi>i</mi><mo>,</mo>
                <mn>1</mn>
              <mo>)</mo></mrow></math></inlineequation>
            </entry>
          </row>
        </tbody>
      </tgroup>
    </informaltable>

    <para>In the table,</para>

    <informalequation>
      <math><mrow>
        <mi>i</mi><mo>=</mo><mo>min</mo><mo>(</mo>
          <msub><mi>A</mi><mi>s</mi></msub><mo>,</mo>
          <msub><mi>k</mi><mi>A</mi></msub><mo>-</mo><msub><mi>A</mi><mi>d</mi></msub>
          <mo>)</mo>
          <mo>/</mo>
          <msub><mi>k</mi><mi>A</mi></msub>
      </mrow></math>
    </informalequation>

    <para>To determine the blended values of a pixel, the system
    uses the following equations:</para>

    <informalequation><math><mtable>
      <mtr><mtd>
        <msub><mi>R</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <mo>min</mo><mo>(</mo>
        <msub><mi>k</mi><mi>R</mi></msub><mo>,</mo>
        <msub><mi>R</mi><mi>s</mi></msub>
        <msub><mi>s</mi><mi>R</mi></msub><mo>+</mo>
        <msub><mi>R</mi><mi>d</mi></msub>
        <msub><mi>d</mi><mi>R</mi></msub>
        <mo>)</mo>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>G</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <mo>min</mo><mo>(</mo>
        <msub><mi>k</mi><mi>G</mi></msub><mo>,</mo>
        <msub><mi>G</mi><mi>s</mi></msub>
        <msub><mi>s</mi><mi>G</mi></msub><mo>+</mo>
        <msub><mi>G</mi><mi>d</mi></msub>
        <msub><mi>d</mi><mi>G</mi></msub>
        <mo>)</mo>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>B</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <mo>min</mo><mo>(</mo>
        <msub><mi>k</mi><mi>B</mi></msub><mo>,</mo>
        <msub><mi>B</mi><mi>s</mi></msub>
        <msub><mi>s</mi><mi>B</mi></msub><mo>+</mo>
        <msub><mi>B</mi><mi>d</mi></msub>
        <msub><mi>d</mi><mi>B</mi></msub>
        <mo>)</mo>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>A</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <mo>min</mo><mo>(</mo>
        <msub><mi>k</mi><mi>A</mi></msub><mo>,</mo>
        <msub><mi>A</mi><mi>s</mi></msub>
        <msub><mi>s</mi><mi>A</mi></msub><mo>+</mo>
        <msub><mi>A</mi><mi>d</mi></msub>
        <msub><mi>d</mi><mi>A</mi></msub>
        <mo>)</mo>
      </mtd></mtr>
    </mtable></math></informalequation>

    <para>Despite the apparent precision of the above equations,
    blending arithmetic is not exactly specified, because blending
    operates with imprecise integer color values. However, a blend
    factor that should be equal to 1 is guaranteed not to modify
    its multiplicand, and a blend factor equal to 0 reduces its
    multiplicand to 0. For example, when <parameter>sfactor</parameter>
    is <constant>GL_SRC_ALPHA</constant>, 
    <parameter>dfactor</parameter> is 
    <constant>GL_ONE_MINUS_SRC_ALPHA</constant>, and 
    <inlineequation><math><msub><mi>A</mi><mi>s</mi></msub></math></inlineequation>
    is equal to 
    <inlineequation><math><msub><mi>k</mi><mi>A</mi></msub></math></inlineequation>,
    the equations reduce to simple replacement:</para>

    <informalequation><math><mtable>
      <mtr><mtd>
        <msub><mi>R</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <msub><mi>R</mi><mi>s</mi></msub>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>G</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <msub><mi>G</mi><mi>s</mi></msub>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>B</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <msub><mi>B</mi><mi>s</mi></msub>
      </mtd></mtr>
      <mtr><mtd>
        <msub><mi>A</mi><mi>d</mi></msub>
        <mgroupalign/>
        <mo>=</mo>
        <mgroupalign/>
        <msub><mi>A</mi><mi>s</mi></msub>
      </mtd></mtr>
    </mtable></math></informalequation>

    <para>
    <function>glBlendFunc</function> operates on all pixel write operations,
    including the scan conversion of points, lines, and polygons.
    <function>glBlendFunc</function> does not affect
    <citerefentry><refentrytitle>glClear</refentrytitle></citerefentry>.
    </para>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <para>Transparency is best implemented using
    <function>glBlendFunc</function>(<constant>GL_SRC_ALPHA</constant>, 
    <constant>GL_ONE_MINUS_SRC_ALPHA</constant>)
    with primitives sorted from farthest to nearest. Note that
    this transparency calculation does not require the presence of
    alpha bitplanes in the color buffer.</para>

    <para><function>glBlendFunc</function>(<constant>GL_SRC_ALPHA</constant>, 
    <constant>GL_ONE_MINUS_SRC_ALPHA</constant>)
    is also useful for rendering antialiased points and lines.
    </para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <para>Incoming (source) alpha is correctly thought of as a
    material opacity, ranging from 1.0
    (<inlineequation><math><msub><mi>k</mi><mi>A</mi></msub></math></inlineequation>),
    representing complete opacity, to 0.0 (0), representing
    complete transparency.</para>
  </refsect1>

  <refsect1>
    <title>Errors</title>

    <para><constant>GL_INVALID_ENUM</constant> is generated if either 
    <parameter>sfactor</parameter> or <parameter>dfactor</parameter>
    is not an accepted value.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>glAlphaFunc</refentrytitle></citerefentry>, 
      <citerefentry><refentrytitle>glClear</refentrytitle></citerefentry>, 
      <citerefentry><refentrytitle>glDepthFunc</refentrytitle></citerefentry>, 
      <citerefentry><refentrytitle>glEnable</refentrytitle></citerefentry>, 
      <citerefentry><refentrytitle>glLogicOp</refentrytitle></citerefentry>, 
      <citerefentry><refentrytitle>glStencilFunc</refentrytitle></citerefentry>
    </para>
  </refsect1>
</refentry>