Chapter 26 — Video Basics

Updated Markdown edition of the HVE User's Manual (HVE Version 5, Seventh Edition, January 2006), Chapter 26, pages 26-1 through 26-14. Verified against the current HVE application source (HVEINV-64/).

This chapter provides a general overview of the process of producing video output from HVE (many sections of this chapter apply to any video device). Beginners will learn a lot about the subject, while users with advanced knowledge will find much of this chapter unnecessary. However, all users should understand the information contained in this chapter before attempting to produce their first video.

(updated: Current HVE records video directly to compressed AVI/MPEG movie files (or single-frame image files) using standard Windows codecs — it no longer drives external video tape recorders. The sections below on tape formats, broadcast standards, color encoding and recording media are retained as legacy reference for users producing physical video media from their rendered movies; the sections on recording methods, compression and the Tips and Suggestions remain directly applicable.)

Overview

This chapter contains the following sections:

• Why Video
• Computer Graphics vs Video
• Typical System Layout (legacy)
• Basic Procedures
• Recording Methods
• Video Compression
• Video Communication Set-up (legacy)
• Preparation of Recording Media (legacy)
• Broadcast Standards (legacy)
• Color Encoding (legacy)
• Video Formats and Recording Quality (legacy)
• Tips and Suggestions

Why Video?

Video is different from photographs or diagrams in that it shows motion. The benefits from this fact are numerous:

• Realism — The life-like appearance and motion are easily understood by non-technical individuals, and also illustrate how the sequence appeared from any perspective.
• Time vs Distance Studies — The ability to visualize the motion allows the researcher to illustrate relative position vs time.
• Visibility and Avoidability Studies — The ability to visualize position vs time allows the user to understand more about a driver's opportunity to avoid a crash.

The fact that video motion is displayed at a fixed frame rate also makes it possible to produce and confirm real-time output. Thus, video is useful as an analytical tool as well.

Video is perhaps most useful because of its universal viewability: a movie file sent to your clients may be viewed easily and conveniently. (updated: the original text emphasized the widespread availability of VCRs; today the equivalent is that an AVI or MPEG file plays on any computer, and is easily converted for DVD, web or courtroom presentation.)

Computer Graphics Vs Video

Computer-generated graphics differs from broadcast video in a number of ways. Understanding these differences will help users produce better results using the HVE Video Interface. Some of the differences include:

• Interlacing — In most video standards, each row of pixels is drawn every other time the monitor is refreshed. This is called interlacing. In contrast, computer graphics monitors typically draw every row of pixels every time the screen is refreshed. This is called non-interlaced (progressive).
• Broadcast Standards — The world historically employed three different analog video standards: NTSC, PAL and SECAM. These standards are described later in this chapter (see Broadcast Standards).
• Tape Formats — Several video tape recording formats existed, such as VHS, S-VHS and Betacam. These formats are described later in this chapter (see Video Formats).
• Color Encoding — Several color-encoding methods are used, including RGB (component), YUV (component), YIQ (component), YC (several types) and composite video. These methods are described later in this chapter (see Color Encoding).

Typical System Layout (legacy)

A typical (legacy) video system layout included the computer, video hardware, video interface software, simulation software, animation controller, video recorder, VHS deck and television monitor.

Figure 26-1 — Typical System Layout. The control signal from the serial port is used for single-frame recorders and is not required for systems using a video compressor.

(updated: a current HVE video "system" is simply the computer itself — HVE renders the playback window to a movie file on disk. No external recorder or serial control connection is used.)

Basic Procedures

No matter what system configuration is used, recording a simulation sequence involves the same basic procedures:

1. Set up the video options (format, size, frame rate, compressor).

NOTE: You should only need to do this once; the settings are stored in HVE's configuration file.

1. Using HVE, create the simulation sequence to be recorded.
2. Record the video (to a movie file in current HVE; to tape on legacy systems).
3. Replay the movie to confirm it is properly recorded.
4. Distribute copies to your clients (on legacy systems this meant copying the master tape from the professional video system to VHS).

Refer to Chapter 27, Using the HVE Video Interface, for detailed, step-by-step procedures and examples.

Recording Methods

The three basic methods of recording computer graphics on video are:

• Recording Live From Screen
• Single-frame Recording
• Real-time Recording

Each of these methods has advantages.

Recording Live From Screen

Recording live from the computer screen is the simplest form of recording (e.g., a screen-capture utility, or on legacy systems a VCR wired to the display output). It requires no special recording interface: anything displayed on the computer screen is transferred to the recording. However, there is no control over frame rate; thus, there is no constant relationship between the time domain in the recorded sequence and real time. Because of this major limitation, recording live from screen is not generally useful for recording real-time simulations.

Single-frame Recording

Single-frame recording involves the controlled transfer of individual computer-generated images to the recording medium one frame at a time. The advantage of single-frame recording is that the frame rate is carefully controlled (normally 25 or 30 frames per second, depending on the video standard used for recording). Thus, when results recorded using single-frame recording are played back, the sequence is shown in real time (i.e., 1.0 second of recorded simulation is exactly one second on a stopwatch).

(legacy) On tape-based systems this process required special video hardware and software and a computer-controlled, frame-accurate video tape recorder (VTR), and owing to the required pre-roll and post-roll it took about 10 seconds to record each frame (about 25 minutes to record a 5-second simulation sequence).

(updated: current HVE offers a Single Frames video format option in the Video Options dialog which writes each rendered frame to an individual image file — useful for producing frame-accurate stills or for assembling video in an external editing package.)

Real-time Recording

Real-time recording also involves the controlled transfer of individual computer-generated images, but the individually rendered frames are first transferred to the computer's hard disk, where they are stored in a movie file. After all the frames are rendered and stored, the file plays back in real time. This is the method used by current HVE: each frame of the playback sequence is rendered and appended to a compressed AVI (or MPEG) file at the selected frame rate; the resulting movie is then played back in real time in the Playback Window (or any media player). The sequence may be previewed on the computer's monitor before it is distributed.

To achieve real-time frame rates for complex scenes, the frames are compressed. A video compressor reduces the amount of storage required for each frame by eliminating or reducing the number of pixels having the same color and intensity. The trade-off is a slight loss of quality, although modern video compression algorithms produce excellent results. Both hardware and software compression are available; current HVE uses the software codecs installed in Windows.

Video Compression

Rendering a single frame of video takes a while. Even a powerful computer graphics system cannot render a complex scene fast enough to display in real time (i.e., 25–30 frames per second). One solution to this limitation is to compress the data in each frame of video and store the result on the computer's hard disk. Once this is done, the frames may be played back in real time from the disk to the screen.

Several types of video compression exist. Two of the most popular families are:

• MPEG (Moving Picture Experts Group)
• JPEG (Joint Photographic Experts Group)

NOTE: An AVI (Video for Windows) type video compression system is standard equipment on Windows computers. (updated: HVE enumerates the codecs installed in Windows through the standard compressor-selection dialog; "Full Frames (Uncompressed)" is used if no compressor is chosen. The legacy manual recommended the Cinepak codec, which is obsolete — use any modern codec installed on your system.)

The selected compressor is chosen using HVE's Video Options dialog (see Chapter 27).

Video Communication Set-up (legacy)

Before any video input or output could be performed on tape-based systems, the computer's video hardware had to be configured to communicate with the video device (e.g., video tape recorder). This process was called video communication set-up, and the procedures were specific to the video output device installed in the computer. (updated: no such set-up is required in current HVE — output goes directly to a disk file.)

Preparation of Recording Media (legacy)

The following procedures were recommended when recording to video tape:

• Use only high-quality tape. Be sure the correct type of medium is used. Never use regular VHS tape for S-VHS recording.
• Avoid the use of thin-based tape, such as tape longer than T-120.

NOTE: A typical simulation analysis requires less than 5 minutes of tape; thus, it is convenient to use the shortest tape available.

• Prior to using the tape, pack the tape by running the VTR in fast forward to the end of the tape, then rewinding it.
• Start recording several minutes into the tape. Avoid recording in the leader area.
• Avoid reusing tapes that have already been used for recording. Use the tape only for limited playback, using it as a master copy for duplication onto regular VHS tapes for distribution to your clients.

NOTE: Legacy HVE video systems used special video formats requiring professional broadcast video hardware. Because clients generally did not possess such hardware, a consumer-quality VHS duplication of the master tape was normally produced for distribution.

• Keep the VTR's mechanism clean and well maintained, according to the manufacturer's recommendations. If you suspect a tape is worn, do not insert it into the VTR; it may damage the mechanism.

Broadcast Standards (legacy)

Broadcast standards, or video timing formats, are ways of encoding video information for broadcast to television receivers. The three recognized analog broadcast standards were:

• NTSC (National Television Systems Committee) — Used in all of North and South America, except Brazil, and much of East Asia.
• PAL (Phase Alternated by Line) — Used in the United Kingdom and Western Europe, except France, and in parts of East Asia, including Australia.
• SECAM (Séquentiel Couleur avec Mémoire) — Used in France, Eastern Europe, the Near East and Mideast, and in parts of Africa and the Caribbean.

NTSC uses a total of 525 horizontal lines per frame, with two fields of 262.5 lines each. Each field refreshes at 60 Hz (actually 59.94). NTSC encodes brightness, color and synchronizing information in the same signal.

PAL uses a total of 625 horizontal lines per frame, with two fields of 312.5 lines each. Each field refreshes at 50 Hz. PAL encodes brightness, color and synchronizing information in the same signal, but in a different way from NTSC.

SECAM transmits the same number of lines at the same rate as PAL, but transmits each color difference signal on alternating lines, using frequency modulation for the subcarrier.

NOTE: The actual lines of active video are fewer than the total lines noted above. Typically, NTSC displays 485 lines and PAL displays 575 lines. The remainder are used for synchronizing and other information.

(updated: analog broadcasting has been replaced by digital television; the 30 fps (NTSC) and 25 fps (PAL) frame-rate conventions survive in current video work. HVE's Video Options dialog offers movie sizes from the default window size through DVD, 480p, 720i, 1080i, 1080p and 4K, with a selectable frame rate.)

Color Encoding (legacy)

Color encoding refers to how the video signal is generated and conducted between devices (e.g., computer and recorder). The common methods are:

• RGB (3-component signal)
• YUV (3-component signal)
• YIQ (3-component signal)
• YC (2-component signal)
• Composite Video (1-component, single signal)

RGB

RGB is the color encoding method used by most computers, as well as some professional-quality video cameras. The three colors red, green and blue are generated separately and carried on three separate wires.

YUV

YUV is a 3-signal (and 3-wire) method used by PAL and some video cameras and VCRs. Like RGB, YUV is also a component color-encoding method, but instead of conducting individual colors, YUV carries the brightness, or luminance, on the Y signal, and color, or chrominance, on the U and V signals. YUV color encoding can be recorded digitally, according to the CCIR 601 standard. This recording technique is referred to as D1.

YIQ

YIQ is a 3-signal (and 3-wire) method typically used by the NTSC format. Like YUV, it encodes luminance onto one signal, Y. It encodes color onto two signals, called I and Q (intermodulation and quadrature, respectively), that differ from the method used by YUV.

YC (YC-358, YC-443, or S-Video)

YC is a two-wire signal that uses luminance (Y) and chrominance (C). YC results when I and Q are combined into a single color signal, C. YC-358 is the most common NTSC version of this format; YC-443 is the most common PAL version. These formats are also known as S-Video, and are used in S-VHS recorders.

Composite Video

The composite color-encoding methods encode the brightness and color into a single signal. There are NTSC and PAL versions of this format.

Video Formats and Recording Quality (legacy)

Video recorders were available for analog and digital recording in a number of formats. They were also informally classified according to Consumer, Industrial/Professional and Broadcast market types, a measure of recording quality (Consumer being the lowest quality and Broadcast being the highest quality).

Analog Formats

The following were the available analog videotape formats:

• VHS — Consumer quality format using a composite signal
• S-VHS — Consumer quality format using a YC or composite signal
• S-Video — Consumer and Industrial/Professional quality format using a YC-358 or YC-443 signal
• Beta — Consumer quality format using a composite signal
• 8mm — Consumer quality format using a composite signal
• Hi-8mm — Consumer and Industrial/Professional quality format using a composite or YC signal
• U-Matic — Industrial/Professional quality format (SP) cassette, 3/4 inch, using a composite signal
• Type C — Broadcast quality format, reel-to-reel, 1 inch tape, using a composite signal
• Type B — Broadcast quality format (Europe) using a composite signal
• Betacam — Broadcast quality format using a component (RGB) signal
• Betacam SP — Broadcast quality format using YUV, YIQ or composite signal
• MII — Broadcast quality format using YUV, YIQ or composite signal

Figure 26-2 — Video Panel (Analog Video Input Source), used on legacy systems when the video communication was set up.

Digital Formats

The available digital tape formats were D1 525 (YUV), D1 625 (YUV), D2 525 (NTSC) and D2 625 (PAL). All digital formats were used in post-production and were typically not used for recording original information.

Tips and Suggestions

Video differs from computer graphics in many ways. By being aware of these differences, the video quality may be greatly improved. Some important issues are addressed below.

Color Saturation

Video recording devices do NOT record saturated colors well.

NOTE: A saturated color is 100 percent red, blue or green (as defined by its RGB values).

Objects with saturated colors seem to have a halo and tend to glow. When assigning any color value using an HVE color wheel editor or slider, choose values less than 256 (move the hot spot slightly away from the edge of a color wheel and the right end of the brightness slider).

Anti-aliasing

Anti-aliasing significantly increases rendering time. There is really no benefit to anti-aliasing until you want to produce a video. At this point, use the Rendering Options dialog to increase the Anti-aliasing value to about 3 (on a scale of 1 to 10; the improvement becomes less and less visible as anti-aliasing is increased).

Keep Objects Close

This suggestion simply comes from experience. Closer objects look better and tend to command your attention. Objects farther away lose their definition and become lost in the environment.

"Camera Car"

Pretend you are directing an action movie. As the vehicle moves through the scene, you want to follow it! A single view from a distance would not focus on the items of importance and would not connect with the viewer. The solution is to move the camera along with the vehicle.

Using HVE, you can create a Camera Car. A camera car is a vehicle to which you have attached a camera.

NOTE: See View Menu, Camera Set-up (Object-based Cameras).

Set up an event that moves this camera through the scene and points towards the desired action. This takes a little planning and practice, but has a huge payback. You'll also know the exact position of the view at each timestep (this information is available from the camera car's Variable Output Window).

Sizing Issue (TV will crop)

Nearly all video display devices crop some information (in addition to issues related to saturated colors, described earlier). View your results on the actual presentation display (television, projector, courtroom monitor) before you send anything to a client. By viewing the results on the target display, you will confirm the quality as well as the actual information displayed on the screen.

Flicker

Horizontal white lines cause flicker! This is true of even the very best broadcast video systems. (Have you ever seen the TV weatherman wear a tie with horizontal white stripes? Probably not very often.)

Think about your environment when composing your view. If the viewer is filled with pavement stripes or other information running horizontally across the screen, adjust the viewer to a slightly oblique view. The result will be significantly better.

Computer to S-VHS to VHS, Not Computer to VHS (legacy)

On tape-based systems, although creating composite video directly on the VTR might have seemed more efficient, the quality was not as good as creating an S-VHS master first and using it for composite copies. (updated: the modern equivalent of this advice is to keep a high-quality, lightly compressed master movie file, and produce more heavily compressed copies for distribution from that master rather than re-recording.)

Tape Problems (legacy)

Use the tips listed under Preparation of Recording Media, earlier in this chapter, to avoid problems that may occur during recording of real-time simulations to video tape.

Screen Saver, Other Windows on top of Playback Window

HVE's video sub-system reads the rendered contents of the playback window region of the computer's frame buffer. As it turns out, a lot of objects can draw into this area! To help prevent unexpected results, do the following:

• Turn off the screen saver. Otherwise, your video may turn black when the screen saver comes on!

NOTE: Remember, it can take up to 25 minutes or more to record a 20-second simulation movie if you have used a highly detailed environment model, a moving camera position and increased settings for anti-aliasing and render quality. If you walk away and return when it's done, you might be surprised about half-way through the sequence!

• Be sure there are no windows or dialogs lying within the limits of the Playback Window. These will appear in your video!
• Make sure the mouse cursor isn't sitting within the window!

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