Frequently Asked Questions
The Digital Television (DTV) standard adopted by the US, Canada and many other countries allows broadcasters to choose among several different formats that they can transmit. Under legislation passed by US Congress - the Deficit Reduction Act of 2005 - full-power over-the-air broadcast television stations must turn off their analog channels on February 17, 2009, and broadcast only in the digital format. As well as some standard definition formats, there are two high definition formats within the DTV specification in North America.
The 720p format (SMPTE S17.392) makes a picture with 720 vertical lines, each with 1280 pixels horizontally -- so in computer display terms, it has a resolution of "1280 x 720". 720p uses progressive scanning, like computers, which sends a complete picture 60 times per second. 720p provides the smoothest possible motion rendition, but it does not have as much resolution as 1080i.
The 1080i format (SMPTE 274M) makes a picture with 1080 vertical lines, each with 1920 pixels horizontally -- so in computer display terms, it has a resolution of "1920 x 1080". 1080i uses interlaced scanning, like traditional TV, which alternates sending odd lines and even lines and thus sends a complete picture 30 times per second. 1080i provides the highest possible resolution, but has the same motion rendition as traditional TV.
CBS (204 stations), NBC (70 stations), and PBS have chosen the 1080 line, 30 fps, interlaced format (1080i) for their high definition digital broadcasts.
ABC (122 stations) and Fox have chosen the 720 line, 60 fps, progressive scan (720p) high definition format.
Non-NTSC countries will not tolerate "artifacts" in their delivery elements that are a result of the conversion from the 30-frame NTSC original. (74 countries use PAL compared to 31 countries using NTSC)
Obviously, recreating the programs in each of the standards is impractical. There is only one video format that can be used as a universal archival master, and that is 1080 line, 24 (23.976) Fps, Progressive Scan High Definition video. It is the only video format from which all current and expected future television deliveries can be made without artifacts.
From this single 1080/24P master, any of the required delivery elements can be made. With the touch of a button, the 1080/24P VTR machine will directly output 1080/60i for CBS and NBC HD delivery, down-convert to 720/60P for delivery to ABC or Fox, or down-convert to 525/60i for Standard Definition NTSC delivery. Equally simple is down-converting to PAL 625/25i for international delivery with no conversion artifacts. Also available during down-convert are the various aspect ratios: 4:3 full-frame, 4:3 center-cut, 16:9 Anamorphic and 16:9 Letterboxed, all with the touch of a button. And all at better picture quality and resolution than that of original Digital Betacam.
Finally we have a TV format which is not only pleasing to the eye and very film-like, but is truly universal.
24FPS vs. 23.976FPS
The production can choose to work in either a true 24 frame mode, or switch the camera to 23.976. Each of these methods has advantages and disadvantages as the production works its way through the post production process.
Camera is shooting at true 24 frames per second. This is mainly used for projects that will end up on film or for PAL delivery. The production may also choose to acquire at 25P keeping in mind that there is 4.1% speed change between 24 and 25 fps. Our HDCAM-SR VTR can play back a 24fps tape at 25fps with pitch correction.
For NTSC postproduction workflow, 23.976 is preferred since there is a direct relationship to 29.97.
If using double system on the set, the audio recorder typically does not have a 23.976 timecode rate. Set the audio recorder to 29.97 fps, non-drop timecode. Keep in mind that 29.97fps is the speed of the recording. Drop and non-drop timecode are both available as a counting scheme only, and do not affect the speed. Non-drop for sound is preferred because drop-frame in HD 23.976 video does not exist.
What format should I shoot on?
The ability to make a good key will depend on the quality of the supplied source material. If shooting on film, the stock should be Kodak's SFX 200T which was developed to correct blue fringe problems. Betacam SX which samples at 4:2:2 but is compressed at 10:1 using MPEG-2, should be avoided. Any of the DV native formats (DV, Mini DV, DVCAM, DVCPRO) are compressed at 5:1 with 4:1:1 sampling, so will give sub-standard results for chroma keying because of the compression and low color sampling rate. DVCPRO-HD can also be problematic in keying due to the 1280x720 image size compared to 1920x1080. In a perfect world the source would be an HDCAM-SR 4:4:4, however this is not always practical. HDCAM and Digital Betacam will give the best practical results.
What color should I use?
(The term bluescreen will be used to refer to any colored backdrop used for keying.)
Blue generally gives better results when working with skin tones, but green is better for bringing out shadows in low-light situations and is easier to work with digitally.
It is generally easier to get an even, solid coat with blue paint; the green paints tend to require repeated applications for an even coat, and touch-ups don't blend in as well.
Some software like Ultimatte works best with a very specific bluescreen color.
Make sure that your bluescreen color doesn't appear in any of the foreground subjects. Make sure that the costume, make-up, and set design people understand the color issues as early as possible.
What should I know about the shoot?
If you are shooting on video, make sure you adjust the camera's white balance before you start, and keep the same setting throughout the shoot.
Be aware that soft filters on the camera (such as Promist) will severely affect the edges of your key. Avoid them if possible.
Keep sets, talent and props as far from the bluescreen as possible.
Light the bluescreen as evenly as possible. You have to put a lot of light on the bluescreen to get an even color. Use grazing light rather that direct light to minimize blue bounce/spill. Large diffusers such as Chimera's Lightbanks cast a soft light without distinct shadows.
Try backlighting the actor, but don't go overboard so as to create a halo effect.
If the subject is in sharp focus but the bluescreen is a little blurry, this will smooth over small lighting and color inconsistencies.
Run a light meter all over your bluescreen. The luminousity of your bluescreen should be equal to or higher than the brightest object in the scene, but no more than 2 to 3 f-stops higher than the subject's hot spots.