mlt/demo

MLT Demo Notes

Before running the demo script, make sure you '. setenv' from the parent
directory. Also, please create clips clip1.dv, clip2.dv, clip3.dv, clip1.mpeg,
clip2.mpeg, clip3.mpeg, and music1.ogg. Please make sure clips are at least 500 
frames duration.

These notes explain the the concepts presented in each demonstration and
what details to look for.

First, a note on consumers. When you start the script, the main menu asks
you to choose a consumer. A consumer is like a viewer, but it could also
write to a stream/file. The "SDL" consumer is the popular Simple DirectMedia
Layer audio and video output. The "Westley" consumer generates an XML
representation of the service network. That can be played directly due to the
westley producer plugin. See docs/westley.txt for more information. The
"MainConcept DV" consumer refers to the proprietary MLT plugin required to
use MLT with MainConcept DV, DVCPro, and MPEG codecs. "/dev/dv1394/0" refers
to a device file for transmitting DV over FireWire using the Linux dv1394 kernel
module. The "BlueFish444" consumer is another proprietary plugin to use
the BlueFish444 manufactured SDI video/audio output cards with MLT.

And now the demos...

All clips

	Simply builds a playlist containing each video clip, and you can transport
	between them using j and k keys.

Filter in/out

	A video filter can be applied to a portion of a producer (clip, playlist,
	or multitrack). This examples shows the greyscale filter.

Watermark

	A graphic can overlay video in realtime with support for alpha channel.
	This example uses a PNG file with an alpha channel. Distortion is explicitly
	enabled here so the otherwise circular graphic is scaled to fill the 
	compositing region. By default, compositing honours the aspect ratio of the
	overlay.

My name is...

	Titles are very easy to composite in realtime. The titler uses Pango
	with the FreeType2 rendering backend. This means it supports high
	quality scalable font rendering with anti-aliasing, unicode (UTF-8),
	and Pango markup capabilities. The compsiting here respects the aspect
	ratio of the rendered title in the first two title pieces but distorts
	the final one. This demo also shows the motion and scaling capabilities
	of the compositor in conjunction with honouring aspect. The compositor
	is doing field-based rendering. So, when displayed non-progressively
	with SDL, you can see motion artifacts during animation. 
	
A composite transition

	The compositor also handles video over video as demonstrated in this
	usage of the compositor to create a special transition. This demonstration
	also crossfades the audio during the transition! Progressive rendering
	is explicitly enabled on the compositor due to the poor results that
	would otherwise occur due to scaling an interleaved video frame and moving
	the video in a reverse direction horizontally.
	
Fade in and out

	A simple series of transitions betwen 3 clips using dissolves and audio 
	crossfades. This is easy :-).

Clock in and out
	
	Wipe transitions are very easy and highly extensible as they are generated 
	using a very convenient lookup table based upon the luma of an image.
	This image can be a 16 bit PGM (grayscale bitmap) or the luma channel of
	any video producer. A number of high quality wipes can be downloaded from
	http://mlt.sf.net/. It also performs field rendering.
	The second wipe demonstrates the ability to control the direction of the
	wipe as well.
	
Obscure

	A popular requirement in news production is to obscure a face, obscenity,
	or trademarked logo. This demonstrates using a simple rectangular 
	obscure filter applied to a region of the image. The second example is more
	advanced and shows using the "region" filter to select the image area and a 
	property of the region filter to "shape" the region using the alpha channel
	of another image (circle.png) and another property to "filter" the region
	using the obscure filter.

Audio Stuff

	A music bed sound track can be mixed with a video. The sound track of the
	video clip has a "floating" amplitude normalisation filter applied.
	Typically, audio normalisation applies a constant gain factor across the
	entire duration of an audio segment from a single source where the 
	gain factor is automatically determined by anaylsing the maximum "power"
	or peak levels. However, in news production, a popular requirement is to
	to dynamically boost the amplitude in soft areas and reduce the amplitude
	in louder areas. Thus, the gain analysis is performed using a "sliding
	window" approach. This example also applies a constant gain factor of 
	0.5 (50%) to the normalised audio of the video clip (to get a nicer
	mix level).
	
Audio and Video Levels

	Audio can be normalised by setting a target amplitude level in decibels.
	A gamma curve can be applied to the luma channel of video.

Shadowed Title and Watermark

	Two instances of the titler are used to create a shadow effect.
	The aspect ratio of the watermark in this example is not distorted. Since
	the original image is a circle with square pixels--a computer-generated
	image--and ITU BT.601 video is not composed of square samples. Therefore,
	the compositor normalises the pixel aspect ratio of the overlay to the 
	destination image, and the circular image remains circular on the analog
	video output. Finally, a greyscale filter is applied to the watermark
	while its opacity is set at 30%.

Station Promo into Story?

	Here is fun demo that might show using a still graphic with some music
	to introduce a show. A luma wipe with an audio crossfade transitions from
	the show title or station promotional material.

Voiceover 2 clips with title

	A common news production requirement to have a "voiceover" audio track
	to a clip or even multiple clips as demonstrated here. Likewise, it is 
	common to place a title caption on the video at the same time! This
	demo has a little fun with the titler at the sake of practicality :-)
	The foreground of the title is transparent while the opacity of the 
	background is reduced to blend with the video. Meanwhile, the compositor
	stretches the image to fill the bottom slice of the video--not suitable
	for overscan displays ;-)
	
	Also, pay close attention to the mixing levels of the audio tracks.
	The audio of the video fades out as the voiceover track (just music
	in this demo) fades in. Then, the voiceover remains mixed with the 
	ambient audio at a 60% level. Finally, the voiceover fades out smoothly 
	from the 60% level to nothing.

GJ-TTAvantika title

	This demo requires a special TrueType font called Avantika. If you have the
	font, register it with fontconfig using the fc-cache utility. This 
	demonstrates i18n capabilities of the titler and the alignment capabilities 
	of both the titler and the compositor. The titler centre aligns
	the two lines of text, and the compositor centre aligns the title 
	horizontally on the frame. 

Title over graphic

	You can superimpose a title over a graphic over video! Also,
	you can apply a luma wipe to the compositor!

Slideshow

	This demo requires any number of JPEG images with the extension ".jpg"
	in a subdirectory named "Scotland."

Bouncy, Bouncy

	The "watermark" filter encapsulates the compositor, and you have full
	control over the compositor properties. Who says a watermark can not 
	also be a video?!

Bouncy, Bouncy Ball

	A variation on the above Bouncy, Bouncy demo that applies a shape, or
	alpha producer, to the the compositing region.
	
Breaking News

	This demonstrates layout capabilities of the compositor.

Squeeze Transitions

	This demonstrates a distorting barndoor-like wipe.


J Cut

	A J cut is an edit where the audio cuts before the video.
	It gets its name from the way it looks on a NLE timeline user interface.
	When the audio cuts over, it does an audio crossfade over the duration of 
	one frame. This makes the audio cut slightly less abrupt and avoids any 
	"click" due to mismatched sample levels at the edit point. The video edit
	is a hard cut.

L Cut

	An L cut is an edit where the video cuts before the audio.
	It gets its name from the way it looks on a NLE timeline user interface.
	This demo shows a very quick dissolve over 5 frames for a soft video cut.
	Like the J Cut demo, an audio crossfade for the duration of one frame makes
	an audio edit nearly instantaneous while being slightly softened and 
	avoiding aberrations.

Fade from/to black/silence

	Of course, it is possible using MLT to fade from black on video and silence
	on audio as well fade to black and silence.
	
Push wipe

	A push wipe is a somewhat fancier transition than most standard wipes
	because it involves motion. The new video clip "pushes" the old video
	clip off one edge. If you can preview on an analog monitor you will notice
	how smooth the motion is due to field-based rendering.
	
Ticker tape

	A very minimal reverse crawling title neard the bottom of the screen.
	The goal of the demo is show fluid motion of the field-based rendering of
	the compositor when viewed on an analog monitor using a DV or BlueFish444
	consumer. The demo also shows the potientional for using and extending the
	existing set of services for a full blown news ticker implementation.