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Video::FrameGrab CPAN Module
Perl
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FrameGrab.pm
MANIFEST
MANIFEST.SKIP
Makefile.PL
README

README

######################################################################
    Video::FrameGrab 0.08
######################################################################

NAME
    Video::FrameGrab - Grab a frame or metadata from a video

SYNOPSIS
        use Video::FrameGrab;

        my $grabber = Video::FrameGrab->new( video => "movie.avi" );

        my $jpg_data = $grabber->snap( "00:00:10" );
        $grabber->jpeg_save("snapshot.jpg");

        print "This movie is ", 
              $grabber->meta_data()->{length}, 
              " seconds long\n";

          # Snap 10 frames at constant intervals throughout the movie
        for my $p ( $grabber->equidistant_snap_times(10) ) {
            $grabber->snap( $p );
            $grabber->jpeg_save("frame-at-$p.jpg");
        }

DESCRIPTION
    Video::FrameGrab grabs a frame at the specified point in time from the
    specified video file and returns its JPEG data.

    It uses mplayer for the heavy lifting behind the scenes and therefore
    requires it to be installed somewhere in the PATH. If mplayer is
    somewhere else, its location can be provided to the constructor:

        my $grabber = Video::FrameGrab->new( mplayer => "/path/to/mplayer",
                                             video   => "movie.avi"
                                           );

  METHODS
    snap( $time )
        Grabs a frame from the movie at time $time. Time is given as
        HH::MM::SS, just as mplayer likes it. Returns the raw jpeg data of
        the captured frame on success and undef if an error occurs.

    jpeg_save( $jpg_file_name )
        Save a grabbed frame as a jpeg image in $file on disk.

    meta_data()
        Runs mplayer's identify() function and returns a reference to a hash
        containing something like

            demuxer          => MOV
            video_format     => AVC1
            video_bitrate    => 0
            video_width      => 320
            video_height     => 240
            video_fps        => 29.970
            video_aspect     => 0.0000
            audio_format     => MP4A
            audio_bitrate    => 0
            audio_rate       => 48000
            audio_nch        => 2
            length           => 9515.94

    equidistant_snap_times( $howmany, [$opts] )
        If you want to snap N frames at constant intervals throughout the
        movie, use equidistant_snap_times( $n ) to get a list of timestamps
        you can use later pass to snap(). For example, on a two hour movie,
        equidistant_snap_times( 5 ) will return

            00:20:00
            00:40:00
            01:00:00
            01:20:00
            01:40:00

        as a list of strings. The movie length is determined by a call to
        meta data, but some formats don't allow retrieving the movie length
        that way, therefore the optional options hash can set the
        movie_length entry to the movie length (or the length of the overall
        interval to perform the snapshots in) in seconds.

            my @times =
              $fg->equidistant_snap_times( $howmany, { movie_length => 3600 } );

    cropdetect( $time, [$opts] )
        If this is a 16:9 movie converted to 4:3 format, the black bars at
        the bottom and the top of the screen should be cropped out. To help
        with this task, "cropdetect" will return a list of ($width, $height,
        $x, $y) to be passed to mplayer/mencoder in the form "-vf
        crop=w:h:x:y" to accomplish the suggested cropping.

        The default algorithm is a homegrown detection mechanism "{algorithm
        => "schilli"}", which first blurs the image with the Gaussian Blur
        algorithm with a radius of "$opts->{gaussian_blur_radius}" (which
        defaults to 3), and then measures if any of the left, right, upper
        or lower border pixel lines of the snapped frame average an
        intensity of less than "$opts->{min_intensity_average}", which
        defaults to 20.

        Note that this is just a guess and might be incorrect at times. In a
        dark scene, black pixels might protrude far into the video, making
        it impossible to detect the border reliably. However, if you overlay
        a number of frames, obtained at several times during the movie (e.g.
        by using the equidistant_snap_times method described above), the
        result is fairly predicatblye and accurate. "cropdetect_average",
        described below, does exactly that.

        The alternative algorithm, "mplayer", asks mplayer to come up with a
        recommendation on how to crop the video. This technique delivers
        incorrect results if there are sporadic white spots within the dark
        bars.

    cropdetect_average( $number_of_probes, [$opts] )
        Takes $number_of_probes from the movie at equidistant intervals,
        overlays the frames and performs a border detection on the resulting
        images, which is almost white in the viewing area.

        See "equidistant_snap_times" for setting the movie length in the
        optional $opts parameter.

    aspect_ratio_guess( ["16:9", "4:3"] )
        This function will take the width and height of the video and map it
        to the best matching aspect ratio given in a reference to an array.

    dimensions()
        Snaps a frame in the middle of the movie, determines its width and
        height and returns them in a list:

            my($width, $height) = $grabber->dimensions();

        Dimensions are usually also available via the meta_data() call.
        dimensions() works even in absence of meta data.

CAVEATS
        Note that the mplayer-based frame grabbing mechanism used in this
        module allows you to snap a picture about every 10 seconds into the
        movie, on shorter intervals, you'll get the same frame back.

LEGALESE
    Copyright 2009 by Mike Schilli, all rights reserved. This program is
    free software, you can redistribute it and/or modify it under the same
    terms as Perl itself.

AUTHOR
    2009, Mike Schilli <cpan@perlmeister.com>

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