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Monday, January 17, 2000

Luminance Ranges in Video

Different video systems have differing internal definitions of “black” and “white.”

Case Study: 3D Renders

Many 3D animators do not have a PAL or NTSC video card in their workstations. However, it is desirable to be able to make demo reels to attract clients, as well as videotape proofs of your work before delivering the final version.

The ease with which DV equipment can be interfaced to computers has enticed many to go this route to create video proofs. However, with many DV codecs default to working with the 601 luminance range, rather than the computer range most 3D software uses. Therefore, if you render direct from your 3D program to DV, the final result might have a different contrast range than you were expecting.

To get around this, you need to render to an intermediate codec, such as Animation, and then process the render through a program such as After Effects to change its luminance range. Again, you can do this by adding a Levels effect to the footage, and setting Output Black to 16 and Output White to 235. Hopefully, more 3D software programmers will catch onto this issue, and include this as an option in their file-saving routines to save this extra step.

If you are using captured video as a texture map on 3D objects, the issues are similar to the compositing example above: Does this footage use my 3D software’s computer luminance range, or does it use the 601 range? If the latter, it means you will need to translate it before applying it as a texture map.

Since most 3D programs don’t know how to handle video’s frame rate and fields to begin with, you’re probably pre-processing video maps through another program anyway. If that’s the case, and your capture came from a 601-based system, add a Levels effect with Input Black and White set to 16 and 235, respectively. For 3D programmers considering adding necessary features such as field separation to their texture map modules, you should also add an option to do this luminance stretch automatically.

Case Study: Graphics for Editors

Now let’s turn the tables, and say you are working in a video editing system that lives in the 601 luminance range. You want to process a few still images from scans and your CD library to include in your work. Remember The Three Questions from the first page: Your new sources are in computer space, but your software is in 601 space.

If this is the case, you may need to do some translations. Some systems, such as the Avid, have an import option that can perform this scaling for you. Note this is only available for sources not already rendered to their codec. Otherwise, process your stills in a paint program such as Adobe Photoshop, and apply Levels, with Output Black set to 16 and Output White to 235. This will squeeze the luminance range of your stills into the 601 range.

How can you verify if your editing system is working in the 601 space? Through a combination of test images and a video waveform monitor. Create a simple image that includes four swatches of different levels of gray: 0/0/0, 16/16/16, 235/235/235, and 255/255/255 respectively. Import this into your editing program, and look at its video output. If you see only two swatches now, it definitely is working in the 601 range (16-235), and has squeezed out the colors that were outside this range.

If you still see four swatches, place a waveform monitor on the output. If you see areas above 100 IRE, and below your set-up pedestal (0 or 7.5 IRE, depending on how you have your hardware set), then your system again is working in the 601 range, and the two swatches at 0/0/0 and 255/255/255 are illegal colors - because they are darker than black and brighter than white, according to the 601 specification.

If you see four swatches, and your waveform monitor says nothing is illegal, then the values below 16 and above 235 got through cleanly. This means your editing system is working inside the computer’s luminance range, or translating the files for your automatically. If you don’t have any test equipment, create a test tape and have someone with a waveform monitor look at it for you. And don’t adjust the 0/7.5 IRE setup switch as a way of “fixing” this - it’s the wrong tool for the job, and won’t help the whites anyway. (IRE issues are discussed on page 5 of this article.)

Editing • Motion Graphics • Post Production • Visual Effects •

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Favorite Technical References

Chris Meyer | 02/15- 07:45 PM

The main reason for the 16-235 range is not for cameras, but for digital decks dubbing old analogue tapes where the defined voltage range for the output signal can be a little loose. The headroom and footroom is for that, to ensure no signal is lost off the top or bottom on transfer.

Posted by Graeme Nattress  on  02/26  at  04:40 AM

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