好的。我有一个功能齐全的解决方案。这适用于任何发现这个问题想要做同样事情的人。我的代码可能不是很优雅，但它可以完成工作。
获取 FFMPEG 的输出
首先我必须从 ffmpeg -i 获取输出，这本身就是一个挑战。感谢 hegemon 在my other question 上的回答，我终于能够在我的命令结束时让它与2>&1 一起工作。并且感谢 Evert 对这个问题的回答，我能够使用 preg_match 解析输出以找到原始文件的高度和宽度。
function get_vid_dim($file)
{
$command = '/usr/bin/ffmpeg -i ' . escapeshellarg($file) . ' 2>&1';
$dimensions = array();
exec($command,$output,$status);
if (!preg_match('/Stream #(?:[0-9\.]+)(?:.*)\: Video: (?P<videocodec>.*) (?P<width>[0-9]*)x(?P<height>[0-9]*)/',implode('\n',$output),$matches))
{
preg_match('/Could not find codec parameters \(Video: (?P<videocodec>.*) (?P<width>[0-9]*)x(?P<height>[0-9]*)\)/',implode('\n',$output),$matches);
}
if(!empty($matches['width']) && !empty($matches['height']))
{
$dimensions['width'] = $matches['width'];
$dimensions['height'] = $matches['height'];
}
return $dimensions;
}
确定最佳尺寸
我编写了这个函数来确定用于转换的最佳尺寸。它采用原件的尺寸、目标尺寸以及是否强制转换为目标纵横比（由其宽度/高度确定）。目标维度将始终是此函数可以返回的最大结果。
function get_dimensions($original_width,$original_height,$target_width,$target_height,$force_aspect = true)
{
// Array to be returned by this function
$target = array();
// Target aspect ratio (width / height)
$aspect = $target_width / $target_height;
// Target reciprocal aspect ratio (height / width)
$raspect = $target_height / $target_width;
if($original_width/$original_height !== $aspect)
{
// Aspect ratio is different
if($original_width/$original_height > $aspect)
{
// Width is the greater of the two dimensions relative to the target dimensions
if($original_width < $target_width)
{
// Original video is smaller. Scale down dimensions for conversion
$target_width = $original_width;
$target_height = round($raspect * $target_width);
}
// Calculate height from width
$original_height = round($original_height / $original_width * $target_width);
$original_width = $target_width;
if($force_aspect)
{
// Pad top and bottom
$dif = round(($target_height - $original_height) / 2);
$target['padtop'] = $dif;
$target['padbottom'] = $dif;
}
}
else
{
// Height is the greater of the two dimensions relative to the target dimensions
if($original_height < $target_height)
{
// Original video is smaller. Scale down dimensions for conversion
$target_height = $original_height;
$target_width = round($aspect * $target_height);
}
//Calculate width from height
$original_width = round($original_width / $original_height * $target_height);
$original_height = $target_height;
if($force_aspect)
{
// Pad left and right
$dif = round(($target_width - $original_width) / 2);
$target['padleft'] = $dif;
$target['padright'] = $dif;
}
}
}
else
{
// The aspect ratio is the same
if($original_width !== $target_width)
{
if($original_width < $target_width)
{
// The original video is smaller. Use its resolution for conversion
$target_width = $original_width;
$target_height = $original_height;
}
else
{
// The original video is larger, Use the target dimensions for conversion
$original_width = $target_width;
$original_height = $target_height;
}
}
}
if($force_aspect)
{
// Use the target_ vars because they contain dimensions relative to the target aspect ratio
$target['width'] = $target_width;
$target['height'] = $target_height;
}
else
{
// Use the original_ vars because they contain dimensions relative to the original's aspect ratio
$target['width'] = $original_width;
$target['height'] = $original_height;
}
return $target;
}
用法
以下是您将从get_dimensions() 获得的一些示例，以使事情更清楚：
get_dimensions(480,360,640,480,true);
-returns: Array([width] => 480, [height] => 360)
get_dimensions(480,182,640,480,true);
-returns: Array([padtop] => 89, [padbottom] => 89, [width] => 480, [height] => 360)
get_dimensions(480,182,640,480,false);
-returns: Array([width] => 480, [height] => 182)
get_dimensions(640,480,480,182,true);
-returns: Array([padleft] => 119, [padright] => 119, [width] => 480, [height] => 182)
get_dimensions(720,480,640,480,true);
-returns: Array([padtop] => 27, [padbottom] => 27, [width] => 640, [height] => 480)
get_dimensions(720,480,640,480,false);
-returns: Array([width] => 640, [height] => 427)
成品
现在，把它们放在一起：
$src = '/var/videos/originals/original.mpg';
$original = get_vid_dim($src);
if(!empty($original['width']) && !empty($original['height']))
{
$target = get_dimensions($original['width'],$original['height'],640,480,true);
$command = '/usr/bin/ffmpeg -i ' . $src . ' -ab 96k -b 700k -ar 44100 -s ' . $target['width'] . 'x' . $target['height'];
$command .= (!empty($target['padtop']) ? ' -padtop ' . $target['padtop'] : '');
$command .= (!empty($target['padbottom']) ? ' -padbottom ' . $target['padbottom'] : '');
$command .= (!empty($target['padleft']) ? ' -padleft ' . $target['padleft'] : '');
$command .= (!empty($target['padright']) ? ' -padright ' . $target['padright'] : '');
$command .= ' -acodec mp3 /var/videos/converted/target.flv 2>&1';
exec($command,$output,$status);
if($status == 0)
{
// Success
echo 'Woohoo!';
}
else
{
// Error. $output has the details
echo '<pre>',join('\n',$output),'</pre>';
}
}