php-doc-en/reference/image/functions/imagecolorallocatealpha.xml

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<refentry xml:id="function.imagecolorallocatealpha" xmlns="http://docbook.org/ns/docbook">
 <refnamediv>
  <refname>imagecolorallocatealpha</refname>
  <refpurpose>Allocate a color for an image</refpurpose>
 </refnamediv>
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type class="union"><type>int</type><type>false</type></type><methodname>imagecolorallocatealpha</methodname>
   <methodparam><type>GdImage</type><parameter>image</parameter></methodparam>
   <methodparam><type>int</type><parameter>red</parameter></methodparam>
   <methodparam><type>int</type><parameter>green</parameter></methodparam>
   <methodparam><type>int</type><parameter>blue</parameter></methodparam>
   <methodparam><type>int</type><parameter>alpha</parameter></methodparam>
  </methodsynopsis>
  <para>
   <function>imagecolorallocatealpha</function> behaves identically to
   <function>imagecolorallocate</function> with the addition of the transparency
   parameter <parameter>alpha</parameter>.
  </para>
 </refsect1>
 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    &gd.image.description;
    <varlistentry>
     <term><parameter>red</parameter></term>
     <listitem>
      <para>&gd.value.red;</para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>green</parameter></term>
     <listitem>
      <para>&gd.value.green;</para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>blue</parameter></term>
     <listitem>
      <para>&gd.value.blue;</para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>alpha</parameter></term>
     <listitem>
      <para>
       A value between <literal>0</literal> and <literal>127</literal>.
       <literal>0</literal> indicates completely opaque while 
       <literal>127</literal> indicates completely transparent.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
   The <parameter>red</parameter>, <parameter>green</parameter>
   and <parameter>blue</parameter> parameters are integers
   between 0 and 255 or hexadecimals between 0x00 and 0xFF.
  </para>
 </refsect1>
 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   A color identifier or &false; if the allocation failed.
  </para>
  &return.falseproblem;
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <informaltable>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>&Version;</entry>
      <entry>&Description;</entry>
     </row>
    </thead>
    <tbody>
     &gd.changelog.image-param;
    </tbody>
   </tgroup>
  </informaltable>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <example>
   <title>Example of using <function>imagecolorallocatealpha</function></title>
   <programlisting role="php">
<![CDATA[
<?php
$size = 300;
$image=imagecreatetruecolor($size, $size);

// something to get a white background with black border
$back = imagecolorallocate($image, 255, 255, 255);
$border = imagecolorallocate($image, 0, 0, 0);
imagefilledrectangle($image, 0, 0, $size - 1, $size - 1, $back);
imagerectangle($image, 0, 0, $size - 1, $size - 1, $border);

$yellow_x = 100;
$yellow_y = 75;
$red_x    = 120;
$red_y    = 165;
$blue_x   = 187;
$blue_y   = 125;
$radius   = 150;

// allocate colors with alpha values
$yellow = imagecolorallocatealpha($image, 255, 255, 0, 75);
$red    = imagecolorallocatealpha($image, 255, 0, 0, 75);
$blue   = imagecolorallocatealpha($image, 0, 0, 255, 75);

// drawing 3 overlapped circle
imagefilledellipse($image, $yellow_x, $yellow_y, $radius, $radius, $yellow);
imagefilledellipse($image, $red_x, $red_y, $radius, $radius, $red);
imagefilledellipse($image, $blue_x, $blue_y, $radius, $radius, $blue);

// don't forget to output a correct header!
header('Content-Type: image/png');

// and finally, output the result
imagepng($image);
imagedestroy($image);
?>
]]>
   </programlisting>
   &example.outputs.similar;
   <mediaobject>
    <alt>Output of example : Example of using imagecolorallocatealpha()</alt>
    <imageobject>
     <imagedata fileref="en/reference/image/figures/imagecolorallocatealpha.png"/>
    </imageobject>
   </mediaobject>
  </example>
  <example xml:id="imagecolorallocatealpha.example.convert">
   <title>Convert typical alpha values for use with <function>imagecolorallocatealpha</function></title>
   <para>
    Usually alpha values of <literal>0</literal> designate fully transparent pixels,
    and the alpha channel has 8 bits. To convert such alpha values to be compatible
    with <function>imagecolorallocatealpha</function>, some simple arithmetic is
    sufficient:
   </para>
   <programlisting role="php">
<![CDATA[
<?php
$alpha8 = 0; // fully transparent
var_dump(127 - ($alpha8 >> 1));
$alpha8 = 255; // fully opaque
var_dump(127 - ($alpha8 >> 1));
?>
]]>
   </programlisting>
   &example.outputs;
   <screen>
<![CDATA[
int(127)
int(0)
]]>
   </screen>
  </example>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <simplelist>
   <member><function>imagecolorallocate</function></member>
   <member><function>imagecolordeallocate</function></member>
  </simplelist>
 </refsect1>
</refentry>
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