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<div class="textblock"> 
<h1>Simple Decoder</h1>

This is an example of a simple decoder loop. It takes an input file
containing the compressed data (in IVF format), passes it through the
decoder, and writes the decompressed frames to disk. Other decoder
examples build upon this one.

The details of the IVF format have been elided from this example for
simplicity of presentation, as IVF files will not generally be used by
your application. In general, an IVF file consists of a file header,
followed by a variable number of frames. Each frame consists of a frame
header followed by a variable length payload. The length of the payload
is specified in the first four bytes of the frame header. The payload is
the raw compressed data.

<h2>Standard Includes</h2>

For decoders, you only have to include <code>vpx_decoder.h</code> and then any
header files for the specific codecs you use. In this case, we&#8217;re using
vp8. The <code>VPX_CODEC_DISABLE_COMPAT</code> macro can be defined to ensure
strict compliance with the latest SDK by disabling some backwards
compatibility features. Defining this macro is encouraged.

<pre class="c">#define VPX_CODEC_DISABLE_COMPAT 1
#include &quot;vpx/vpx_decoder.h&quot;
#include &quot;vpx/vp9dx.h&quot;
#define interface (vpx_codec_vp8_dx())</pre>

<h2>Initializing The Codec</h2>

The decoder is initialized by the following code. This is an example for
the VP8 decoder, but the code is analogous for all algorithms. Replace
<code>vpx_codec_vp8_dx()</code> with a pointer to the interface exposed by the
algorithm you want to use. The <code>cfg</code> argument is left as NULL in this
example, because we want the algorithm to determine the stream
configuration (width/height) and allocate memory automatically. This
parameter is generally only used if you need to preallocate memory,
particularly in External Memory Allocation mode.

<pre class="c">/* Initialize codec */
if&#40;vpx_codec_dec_init&#40;&amp;codec, interface, NULL, flags&#41;&#41;
 die_codec&#40;&amp;codec, &quot;Failed to initialize decoder&quot;&#41;;</pre>

<h2>Decoding A Frame</h2>

Once the frame has been read into memory, it is decoded using the
<code>vpx_codec_decode</code> function. The call takes a pointer to the data
(<code>frame</code>) and the length of the data (<code>frame_sz</code>). No application data
is associated with the frame in this example, so the <code>user_priv</code>
parameter is NULL. The <code>deadline</code> parameter is left at zero for this
example. This parameter is generally only used when doing adaptive
postprocessing.

<pre class="c">/* Decode the frame */
if&#40;vpx_codec_decode&#40;&amp;codec, frame, frame_sz, NULL, 0&#41;&#41;
 die_codec&#40;&amp;codec, &quot;Failed to decode frame&quot;&#41;;</pre>

Codecs may produce a variable number of output frames for every call to
<code>vpx_codec_decode</code>. These frames are retrieved by the
<code>vpx_codec_get_frame</code> iterator function. The iterator variable <code>iter</code> is
initialized to NULL each time <code>vpx_codec_decode</code> is called.
<code>vpx_codec_get_frame</code> is called in a loop, returning a pointer to a
decoded image or NULL to indicate the end of list.

<pre class="c">while&#40;&#40;img = vpx_codec_get_frame&#40;&amp;codec, &amp;iter&#41;&#41;&#41; &#123;</pre>

<h2>Processing The Decoded Data</h2>

In this example, we simply write the encoded data to disk. It is
important to honor the image&#8217;s <code>stride</code> values.

<pre class="c">for&#40;plane=0; plane &lt; 3; plane++&#41; &#123;
 unsigned char *buf =img-&gt;planes&#91;plane&#93;;
&nbsp;
 for&#40;y=0; y &lt; &#40;plane ? &#40;img-&gt;d_h + 1&#41; &gt;&gt; 1 : img-&gt;d_h&#41;; y++&#41; &#123;
 &#40;void&#41; fwrite&#40;buf, 1, &#40;plane ? &#40;img-&gt;d_w + 1&#41; &gt;&gt; 1 : img-&gt;d_w&#41;,
 outfile&#41;;
 buf += img-&gt;stride&#91;plane&#93;;
 &#125;
&#125;</pre>

<h2>Cleanup</h2>

The <code>vpx_codec_destroy</code> call frees any memory allocated by the codec.

<pre class="c">if&#40;vpx_codec_destroy&#40;&amp;codec&#41;&#41;
 die_codec&#40;&amp;codec, &quot;Failed to destroy codec&quot;&#41;;</pre>

<h2>Error Handling</h2>

This example does not special case any error return codes. If there was
an error, a descriptive message is printed and the program exits. With
few exeptions, vpx_codec functions return an enumerated error status,
with the value <code>0</code> indicating success.

<pre class="c">static void die_codec&#40;vpx_codec_ctx_t *ctx, const char *s&#41; &#123;
 const char *detail = vpx_codec_error_detail&#40;ctx&#41;;
&nbsp;
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html">printf</a>&#40;&quot;%s: %s\n&quot;, s, vpx_codec_error&#40;ctx&#41;&#41;;
 if&#40;detail&#41;
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html">printf</a>&#40;&quot; %s\n&quot;,detail&#41;;
 exit&#40;EXIT_FAILURE&#41;;
&#125;</pre>

<h1>Putting It All Together</h1>

<pre class="c">/*
 Copyright (c) 2010 The WebM project authors. All Rights Reserved.
 
 Use of this source code is governed by a BSD-style license
 that can be found in the LICENSE file in the root of the source
 tree. An additional intellectual property rights grant can be found
 in the file PATENTS. All contributing project authors may
 be found in the AUTHORS file in the root of the source tree.
 */
&nbsp;
&nbsp;
/*
 This is an example of a simple decoder loop. It takes an input file
 containing the compressed data (in IVF format), passes it through the
 decoder, and writes the decompressed frames to disk. Other decoder
 examples build upon this one.
 
 The details of the IVF format have been elided from this example for
 simplicity of presentation, as IVF files will not generally be used by
 your application. In general, an IVF file consists of a file header,
 followed by a variable number of frames. Each frame consists of a frame
 header followed by a variable length payload. The length of the payload
 is specified in the first four bytes of the frame header. The payload is
 the raw compressed data.
 */
#include &lt;stdarg.h&gt;
#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
#define VPX_CODEC_DISABLE_COMPAT 1
#include &quot;./vpx_config.h&quot;
#include &quot;vpx/vp8dx.h&quot;
#include &quot;vpx/vpx_decoder.h&quot;
#define interface (vpx_codec_vp8_dx())
&nbsp;
&nbsp;
#define IVF_FILE_HDR_SZ (32)
#define IVF_FRAME_HDR_SZ (12)
&nbsp;
static unsigned int mem_get_le32&#40;const unsigned char *mem&#41; &#123;
 return &#40;mem&#91;3&#93; &lt;&lt; 24&#41;|&#40;mem&#91;2&#93; &lt;&lt; 16&#41;|&#40;mem&#91;1&#93; &lt;&lt; 8&#41;|&#40;mem&#91;0&#93;&#41;;
&#125;
&nbsp;
static void die&#40;const char *fmt, ...&#41; &#123;
 va_list ap;
&nbsp;
 va_start&#40;ap, fmt&#41;;
 vprintf&#40;fmt, ap&#41;;
 if&#40;fmt&#91;strlen&#40;fmt&#41;-1&#93; != '\n'&#41;
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html">printf</a>&#40;&quot;\n&quot;&#41;;
 exit&#40;EXIT_FAILURE&#41;;
&#125;
&nbsp;
static void die_codec&#40;vpx_codec_ctx_t *ctx, const char *s&#41; &#123; //
 const char *detail = vpx_codec_error_detail&#40;ctx&#41;; //
 //
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html">printf</a>&#40;&quot;%s: %s\n&quot;, s, vpx_codec_error&#40;ctx&#41;&#41;; //
 if&#40;detail&#41; //
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html">printf</a>&#40;&quot; %s\n&quot;,detail&#41;; //
 exit&#40;EXIT_FAILURE&#41;; //
&#125; //
&nbsp;
&nbsp;
int main&#40;int argc, char **argv&#41; &#123;
 FILE *infile, *outfile;
 vpx_codec_ctx_t codec;
 int flags = 0, frame_cnt = 0;
 unsigned char file_hdr&#91;IVF_FILE_HDR_SZ&#93;;
 unsigned char frame_hdr&#91;IVF_FRAME_HDR_SZ&#93;;
 unsigned char frame&#91;256*1024&#93;;
 vpx_codec_err_t res;
&nbsp;
 &#40;void&#41;res;
 /* Open files */
 if&#40;argc!=3&#41;
 die&#40;&quot;Usage: %s &lt;infile&gt; &lt;outfile&gt;\n&quot;, argv&#91;0&#93;&#41;;
 if&#40;!&#40;infile = fopen&#40;argv&#91;1&#93;, &quot;rb&quot;&#41;&#41;&#41;
 die&#40;&quot;Failed to open %s for reading&quot;, argv&#91;1&#93;&#41;;
 if&#40;!&#40;outfile = fopen&#40;argv&#91;2&#93;, &quot;wb&quot;&#41;&#41;&#41;
 die&#40;&quot;Failed to open %s for writing&quot;, argv&#91;2&#93;&#41;;
&nbsp;
 /* Read file header */
 if&#40;!&#40;fread&#40;file_hdr, 1, IVF_FILE_HDR_SZ, infile&#41; == IVF_FILE_HDR_SZ
 &amp;&amp; file_hdr&#91;0&#93;=='D' &amp;&amp; file_hdr&#91;1&#93;=='K' &amp;&amp; file_hdr&#91;2&#93;=='I'
 &amp;&amp; file_hdr&#91;3&#93;=='F'&#41;&#41;
 die&#40;&quot;%s is not an IVF file.&quot;, argv&#91;1&#93;&#41;;
&nbsp;
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html">printf</a>&#40;&quot;Using %s\n&quot;,vpx_codec_iface_name&#40;interface&#41;&#41;;
 /* Initialize codec */ //
 if&#40;vpx_codec_dec_init&#40;&amp;codec, interface, NULL, flags&#41;&#41; //
 die_codec&#40;&amp;codec, &quot;Failed to initialize decoder&quot;&#41;; //
&nbsp;
 /* Read each frame */
 while&#40;fread&#40;frame_hdr, 1, IVF_FRAME_HDR_SZ, infile&#41; == IVF_FRAME_HDR_SZ&#41; &#123;
 int frame_sz = mem_get_le32&#40;frame_hdr&#41;;
 vpx_codec_iter_t iter = NULL;
 vpx_image_t *img;
&nbsp;
&nbsp;
 frame_cnt++;
 if&#40;frame_sz &gt; sizeof&#40;frame&#41;&#41;
 die&#40;&quot;Frame %d data too big for example code buffer&quot;, frame_sz&#41;;
 if&#40;fread&#40;frame, 1, frame_sz, infile&#41; != frame_sz&#41;
 die&#40;&quot;Frame %d failed to read complete frame&quot;, frame_cnt&#41;;
&nbsp;
 /* Decode the frame */ //
 if&#40;vpx_codec_decode&#40;&amp;codec, frame, frame_sz, NULL, 0&#41;&#41; //
 die_codec&#40;&amp;codec, &quot;Failed to decode frame&quot;&#41;; //
&nbsp;
 /* Write decoded data to disk */
 while&#40;&#40;img = vpx_codec_get_frame&#40;&amp;codec, &amp;iter&#41;&#41;&#41; &#123; //
 unsigned int plane, y;
&nbsp;
 for&#40;plane=0; plane &lt; 3; plane++&#41; &#123; //
 unsigned char *buf =img-&gt;planes&#91;plane&#93;; //
 //
 for&#40;y=0; y &lt; &#40;plane ? &#40;img-&gt;d_h + 1&#41; &gt;&gt; 1 : img-&gt;d_h&#41;; y++&#41; &#123; //
 &#40;void&#41; fwrite&#40;buf, 1, &#40;plane ? &#40;img-&gt;d_w + 1&#41; &gt;&gt; 1 : img-&gt;d_w&#41;,//
 outfile&#41;; //
 buf += img-&gt;stride&#91;plane&#93;; //
 &#125; //
 &#125; //
 &#125;
 &#125;
 <a href="http://www.opengroup.org/onlinepubs/009695399/functions/printf.html">printf</a>&#40;&quot;Processed %d frames.\n&quot;,frame_cnt&#41;;
 if&#40;vpx_codec_destroy&#40;&amp;codec&#41;&#41; //
 die_codec&#40;&amp;codec, &quot;Failed to destroy codec&quot;&#41;; //
&nbsp;
 fclose&#40;outfile&#41;;
 fclose&#40;infile&#41;;
 return EXIT_SUCCESS;
&#125;</pre>
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