Update deinterlacing methods to current best practices

For many years, hardware devices and online streaming platforms
have been capable of 50p or 60p output, so the older advice of
deinterlacing 25i/30i to 25p/30p is no longer necessary.  For matching
the fidelity of interlaced sources as they were originally viewed on
period hardware, each field should be output to its own frame, producing
50p content from 25i sources, and 60p content from 30i sources.  These
updates to ffmprovisr reflect that.

Additionally, the deinterlacing filter advice of yadif or w3fdif has
been changed to bwdif, which combines the best attributes of both older
filters.

index.html

@@ -1354,15 +1354,17 @@ <h2 id="interlacing">Work with interlaced video</h2>
     <input type="checkbox" id="ntsc_to_h264">
     <div class="hiding">
       <h5>Upscaled, Pillar-boxed HD H.264 Access Files from SD NTSC source</h5>
-      <p><code>ffmpeg -i <em>input_file</em> -c:v libx264 -filter:v "yadif, scale=1440:1080:flags=lanczos, pad=1920:1080:(ow-iw)/2:(oh-ih)/2, format=yuv420p" <em>output_file</em></code></p>
+      <p><code>ffmpeg -i <em>input_file</em> -c:v libx264 -filter:v "idet, bwdif, scale=1440:1080:flags=lanczos, pad=1920:1080:(ow-iw)/2:(oh-ih)/2, format=yuv420p" <em>output_file</em></code></p>
       <dl>
         <dt>ffmpeg</dt><dd>starts the command</dd>
         <dt>-i <em>input_file</em></dt><dd>path, name and extension of the input file</dd>
         <dt>-c:v libx264</dt><dd>encodes video stream with libx264 (h264)</dd>
         <dt>-filter:v</dt><dd>a video filter will be used</dd>
         <dt>"</dt><dd>quotation mark to start filtergraph</dd>
-        <dt>yadif</dt><dd>deinterlacing filter (‘yet another deinterlacing filter’)<br>
-         By default, <a href="https://ffmpeg.org/ffmpeg-filters.html#yadif-1" target="_blank">yadif</a> will output one frame for each frame. Outputting one frame for each <em>field</em> (thereby doubling the frame rate) with <code>yadif=1</code> may produce visually better results.</dd>
+        <dt>idet</dt><dd>detect interlaced video field order<br>
+         <a href="https://ffmpeg.org/ffmpeg-filters.html#idet" target="_blank">idet</a> will try to detect if the video is interlaced, and if so, what the order of the fields are (top-field-first, or bottom-field-first).  This is done to ensure the output of the deinterlacing filter is correct.</dd>
+        <dt>bwdif</dt><dd>deinterlacing filter (‘Bob Weaver Deinterlacing Filter’)<br>
+         By default, <a href="https://ffmpeg.org/ffmpeg-filters.html#bwdif-1" target="_blank">bwdif</a> will output one frame for each field, matching the visual cadence of interlaced video. Outputting one frame for each <em>frame</em> (thereby halving the number of output frames per second) with <code>bwdif=mode=send_frame</code> can be used when the presentation device is not capable of reproducing 50 (PAL) or 60 (NTSC) frames per second.</dd>
         <dt>scale=1440:1080:flags=lanczos</dt><dd>resizes the image to 1440x1080, using the Lanczos scaling algorithm, which is slower but better than the default bilinear algorithm.</dd>
         <dt>pad=1920:1080:(ow-iw)/2:(oh-ih)/2</dt><dd>pads the area around the 4:3 input video to create a 16:9 output video</dd>
         <dt>format=yuv420p</dt><dd>specifies a pixel format of Y′C<sub>B</sub>C<sub>R</sub> 4:2:0</dd>
@@ -1379,24 +1381,26 @@ <h5>Upscaled, Pillar-boxed HD H.264 Access Files from SD NTSC source</h5>
     <input type="checkbox" id="deinterlace">
     <div class="hiding">
       <h5>Deinterlace a video</h5>
-      <p><code>ffmpeg -i <em>input_file</em> -c:v libx264 -vf "yadif,format=yuv420p" <em>output_file</em></code></p>
+      <p><code>ffmpeg -i <em>input_file</em> -c:v libx264 -vf "idet,bwdif,format=yuv420p" <em>output_file</em></code></p>
       <p>This command takes an interlaced input file and outputs a deinterlaced H.264 MP4.</p>
       <dl>
         <dt>ffmpeg</dt><dd>starts the command</dd>
         <dt>-i <em>input file</em></dt><dd>path, name and extension of the input file</dd>
         <dt>-c:v libx264</dt><dd>tells FFmpeg to encode the video stream as H.264</dd>
         <dt>-vf</dt><dd>video filtering will be used (<code>-vf</code> is an alias of <code>-filter:v</code>)</dd>
         <dt>"</dt><dd>start of filtergraph (see below)</dd>
-        <dt>yadif</dt><dd>deinterlacing filter (‘yet another deinterlacing filter’)<br>
-        By default, <a href="https://ffmpeg.org/ffmpeg-filters.html#yadif-1" target="_blank">yadif</a> will output one frame for each frame. Outputting one frame for each <em>field</em> (thereby doubling the frame rate) with <code>yadif=1</code> may produce visually better results.</dd>
+        <dt>idet</dt><dd>detect interlaced video field order<br>
+         <a href="https://ffmpeg.org/ffmpeg-filters.html#idet" target="_blank">idet</a> will try to detect if the video is interlaced, and if so, what the order of the fields are (top-field-first, or bottom-field-first).  This is done to ensure the output of the deinterlacing filter is correct.</dd>
+        <dt>bwdif</dt><dd>deinterlacing filter (‘Bob Weaver Deinterlacing Filter’)<br>
+         By default, <a href="https://ffmpeg.org/ffmpeg-filters.html#bwdif-1" target="_blank">bwdif</a> will output one frame for each field, matching the visual cadence of interlaced video. Outputting one frame for each <em>frame</em> (thereby halving the number of output frames per second) with <code>bwdif=mode=send_frame</code> can be used when the presentation device is not capable of reproducing 50 (PAL) or 60 (NTSC) frames per second.</dd>
         <dt>,</dt><dd>separates filters</dd>
         <dt>format=yuv420p</dt><dd>chroma subsampling set to 4:2:0<br>
         By default, <code>libx264</code> will use a chroma subsampling scheme that is the closest match to that of the input. This can result in Y′C<sub>B</sub>C<sub>R</sub> 4:2:0, 4:2:2, or 4:4:4 chroma subsampling. QuickTime and most other non-FFmpeg based players can’t decode H.264 files that are not 4:2:0, therefore it’s advisable to specify 4:2:0 chroma subsampling.</dd>
         <dt>"</dt><dd>end of filtergraph</dd>
         <dt><em>output file</em></dt><dd>path, name and extension of the output file</dd>
       </dl>
-      <p><code>"yadif,format=yuv420p"</code> is an FFmpeg <a href="https://trac.ffmpeg.org/wiki/FilteringGuide#FiltergraphChainFilterrelationship" target="_blank">filtergraph</a>. Here the filtergraph is made up of one filter chain, which is itself made up of the two filters (separated by the comma).<br>
-      The enclosing quote marks are necessary when you use spaces within the filtergraph, e.g. <code>-vf "yadif, format=yuv420p"</code>, and are included above as an example of good practice.</p>
+      <p><code>"idet,bwdif,format=yuv420p"</code> is an FFmpeg <a href="https://trac.ffmpeg.org/wiki/FilteringGuide#FiltergraphChainFilterrelationship" target="_blank">filtergraph</a>. Here the filtergraph is made up of one filter chain, which is itself made up of the three filters (separated by the comma).<br>
+      The enclosing quote marks are necessary when you use spaces within the filtergraph, e.g. <code>-vf "idet, bwdif, format=yuv420p"</code>, and are included above as an example of good practice.</p>
       <p><strong>Note:</strong> FFmpeg includes several deinterlacers apart from <a href="https://ffmpeg.org/ffmpeg-filters.html#yadif-1" target="_blank">yadif</a>: <a href="https://ffmpeg.org/ffmpeg-filters.html#bwdif" target="_blank">bwdif</a>, <a href="https://ffmpeg.org/ffmpeg-filters.html#w3fdif" target="_blank">w3fdif</a>, <a href="https://ffmpeg.org/ffmpeg-filters.html#kerndeint" target="_blank">kerndeint</a>, and <a href="https://ffmpeg.org/ffmpeg-filters.html#nnedi" target="_blank">nnedi</a>.</p>
       <p>For more H.264 encoding options, see the latter section of the <a href="#transcode_h264">encode H.264 command</a>.</p>
       <div class="sample-image">