Stereoscopic Encoding Guide for Windows Media

Content

• Introduction
• Let's Start
• Encoding
• Multiplexing
• Advanced Features

Introduction

This guide explains how to encode stereoscopic video files for playback using Stereoscopic Player. It is assumed you have already edited the video in some video editing application and want to a efficiently compressed file for distribution now. We will use the Windows Media-based Dual Stream format (spezification) for the following reasons (a legacy guide for DivX is also available):

• Compatible with 2D video playback software (shows left view only)
• Makes use of dual core CPUs
• Provides best encoding efficiency vs. decoding complexity ratio
• Supports streaming
• Free encoder available

Let's Start

First, export left and right view from your video editing application to separate files. To obtain best quality, use an uncompressed format or a lossless codec (e.g. Huffyuv).

If you are using side-by-side or over/under format for editing, convert it to separate left and right files. Many video editing applications allow you to specify cropping when exporting the file. If your video processing application does, export left and right view successively by cropping the right or bottom half in the first pass and the left or top half in the second pass. If you cannot specify cropping, export to side-by-side or over/under format and create left and right views in VirtualDub: Click File->Open video file and Video->Filters. Press the Add button to insert the null transform filter. Click on the Cropping button and crop half of the image (adjust X2 offset for side-by-side format or Y2 offset for over/under format). File->Save as AVI will save the left movie to the hard disk. Adjust the cropping settings for the right view (X1 or Y1 offset) and save the right movie to hard disk.

VirtualDub can also convert interlaced (= field-sequential) sources  to separate left and right files. Instead of the null transform filter, use the deinterlace filters and select Discard field 1 in the first and Discard field 2 in the second pass.

Encoding

Now that we have an left and right file available, we encode both in Windows Media Encoder. The following steps must be executed for both files. If you do not have Windows Media Encoder installed yet, download it from the Windows Media website.

Run Windows Media Encoder an select Custom session in the New Session dialog. Click Tools->Options and move the bottom slider to the most-right position. If you accept slightly lower quality for faster encoding speed, you can leave the slider at its default position. Press Apply to all new session to make the new settings permanent.

If the session properties window is not visible, press the Properties button in the toolbar. On the Source tab, select File source and browse for your left (or right) video. On the Output tab, select Encode to file and enter the output filename.

The Compression tab allows to select and configure the audio and video codec. Proper compression settings are crucial for a good playback experience, so choose them with care! Click Edit to create a custom encoding profile. We recommend to click Import and load one of the recommended profiles. This package includes profiles for streaming quality, 480p, 576p, 720p and 1080p resolutions at frame rates of 24 (film), 25 (PAL) and 29.97 / 30 (NTSC) frames per second.

We recommend to author your content in one of the following standard video formats. Italic formats are highly recommended because they correspond with broadcast standards. In case your content differs from these formats, derive your profile from one of the recommended profile which matches in frame rate and has a similar resolution.

30 fps formats are valid at 29.97 fps as well. Captured content is more likely to show 29.97 fps whereas computer generated content is often rendered at 30 fps. Nevertheless, make sure your enter the exact frame rate in the Custom Encoding Settings dialog. 24 fps should be used for high definition formats (1280 x 720, 1440 x 1080 and 1920 x 1080) only. High definition formats formats support 24 fps, 25 fps, 29.97 fps as well as 30 fps.

All table values are just recommended values, you can choose lower or higher bitrates if necessary. Increasing buffer size and key frame interval may improve the codec's efficiency, but cause longer delays when starting playback and longer seeking durations, respectively.

Use the following codecs (if they are not available, upgrade to the latest Windows Media Player and Windows Media Encoder):

• Windows Media Video 9
• Windows Media Audio 9.1 for mono or stereo soundtracks
• Windows Media Audio 9.1 Professional for multi channel or high definition sound tracks

The Windows Media Video 9 Advanced Profile codec currently does not provide any quality advantage over the Windows Media Video 9 codec, but is pre-installed on less systems. Therefore, we advise against using it. The Windows Media Video 9 codec supports both constant bit rate (CBR) and variable bit rate (VBR) encoding. Bit rate VBR (Peak) is recommended because it adopts best to a sequence of static and high motions scenes while limiting the bit rate peaks to a reasonable value. Bit rate peaks may cause dropped frames during playback due to CPU overload or bandwidth limitations. The peak bitrate should be about 150% of the average bitrate.

The selected audio sampling frequency (44 kHz, 48 kHz, 96 kHz) and quantization (16 bits, 24 bits) should match the source audio format. Recommended audio bitrates for stereo content:

Language should match the language of your audio track. If there is no sound track or it is language neutral, select English (USA) [en-us]. Check Allow nonsquare pixel output if the video pixels are not square (anamorphic pixels). The pixel aspect ratio can be chosen on the Video size tab. Select [Custom] and enter the pixel aspect ratio. Please note that pixel aspect ratio and display aspect ratio are not the same! To calculate the required pixel aspect ratio, divide the desired display ratio by the source resolution. Example: A 720 x 576 pixel source should be displayed at 16:9 display aspect ratio. This requires a pixel aspect ratio of (16:9) / (720:576) = 64:45. The following table lists common pixel aspect ratios:

* Using these values, the video will be played back in the desired display aspect ratio, but they do not take analog blank into account. As a result, the image might be slightly distorted (details). Anyway, you will not notice the difference ;-)

Stereoscopic Player requires progressive content. If you did not deinterlace your content during editing, enable Deinterlacing on the Processing tab.

Before you start encoding, you should add some metadata on the Attributes pages. Besides the default attributes Title, Author, Copyright and Description, Stereoscopic Player also evaluates the Source attribute. It should contain information where to obtain the file, usually a reference to a website.

Finally, press the Start Encoding button and have a coffee break ;-) Afterwards, encode the right view.

Multiplexing

In the previous chapter, we created a left and right Windows Media files. Using Stereoscopic Player's File->Open Left and Right File feature, it would be possible to play them in 3D. However, we would like to merge them to a single file, because only the single file/dual stream format offers best usability.

We use the Windows Media Stereo Muxer command line tool to multiplex left and right file. Open a command line window (Start->Run, enter Cmd) and type WmxMux.exe -i1 "Oldtimers 1080p Left.wmv" -i2 "Oldtimers 1080p Right.wmv" -o "Output.wmv" (adjust the filenames). If WmvMux.exe and your video files are not located in the same directory, it is necessary to specify paths, e.g. "C:\Left.wmv". Input one (-i1) must be the left file, input two (-i2) the right file. Run WmvMux.exe without parameters to get a list of all available options.

Stereoscopic Player displays a preview image for each video in the built-in video library. The preview image can be embedded in the Windows Media file, so that no additional metadata file is required. The following restrictions apply for preview images:

• JPEG format
• 160 x 120 pixels resolution
• Maximum size of 10 kB (10240 Bytes)

To embed the preview image, use the -p <file> parameter, for example -p "Oldtimers.jpg".

By default, the output file will contain the audio track of the first (left) input file only. To keep all audio tracks, specify the -a parameter. Use this option only if your left input file doesn't contain an audio track but only the right one. Avoid to multiplex two audio tracks into the output file (except multi language audio tracks, see below)!

If you have forgotten to choose the proper language in Windows Media Encoder, the Windows Media Stereo Muxer allows to fix it. Specify -l1 "en-us" and -l2 "en-us" to set the language of all streams to English. A list of valid language codes is available here. The complete command will look like this:

Windows Media Stereo Muxer will display an error message if the input files differ in length. If you edited and encoded left and right files properly, this should never happen. Nevertheless, parameter -x allows to ignore different lengths and -d <time> limits the duration of the output file. For example -x -t 6000 will copy the first minute of input files differing in length to the output file. Use these options only if you know exactly what you are doing!

To check and understand the final output file, open it in the Windows Media ASF View (available from Microsoft). Make sure there are three streams (three Stream Properties Objects), two video streams and one audio stream. There should also be an Advanced Mutual Exclusion Object including the video streams. It prevents 2D players from showing both video streams.

Advanced Features

• Multi stream files: Specify additional input files -i3 <file>, -i4 <file> and so on. Input files must be ordered from left to right, -i1 is the most-left view.
   
• Multi language audio tracks: Inputs -i1 and -i2 should contain video streams only. To remove audio streams from existing Windows Media files, use the Windows Media Stream Editor, which is shipped together with Windows Media Encoder. Encode each audio track separately. The same profile must be used to encode the audio tracks! The Windows Media Stereo Muxer's command line will be like this: WmvMux.exe -i1 "Left.wmv" -l1 "en-us" -i2 "Right.wmv" -l2 "en-us" -i3 "English.wma" -l3 "en-us" -i4 "German.wma" -l4 "de-de" -p "Preview.jpg" -a
   
• 2D Intro: It is possible to define an intro at the beginning of the file which will be played back by 2D video players only, but skipped by Stereoscopic Player. The intro can be used to display a message like 'This is a stereoscopic video file. Use Stereoscopic Player to play it in 3D.' The intro must be present in the original left and right files. The -t <time> parameter is used to define the intro duration. For example, -t 500 sets the intro duration to 5 second. Click here to download uncompressed 25 fps intro files in 288p, 576p, 720p and 1080p resolutions.
   
• Digital Rights Management: Encrypting a file requires the Windows Media Rights Manager SDK. You can obtain it from Microsoft. Dual stream files are encrypted the same way as ordinary video files. Please consult the Windows Media Rights Manager SDK for more details. Do not use player individualization because this mechanism is not supported by Stereoscopic Player yet. Don't use advanced security levels either.