Ultimatte is More than a Chroma Key Device - Here we Explain Why
An Ultimatte differs from a chroma keyer in three ways:
- It uses a fully additive mix to combine the foreground and background so that it can reproduce extremely subtle detail in the foreground subject such as reflections in transparent objects.
- It uses patented algorithms to generate the key signal which enables it to better distinguish the background from the foreground subject.
- It processes the foreground with patented algorithms which suppress the backing and remove blue spill while at the same time permitting many shades of blue to be reproduced in the foreground.
Mixing vs. Switching
Ultimatte combines images by summing them rather than switching back and forth between them, or to use the current jargon, it does a fully additive mix rather than a non-additive mix. The foreground and the background are first processed separately and then added together to produce the composite image.
Originally keyers used a non-additive mix. That is to say they were either/or devices which switched back and forth between two source signals during the scanning to create the composite image. The transition from background to foreground source could be soft or hard depending on the design and adjustment of the keyer, but there was nonetheless a choice to be made at every point in the image between background and foreground source. The limitations of this approach to composite are fairly obvious. There is no way a transparent or translucent foreground object can be added to the background scene in such a way that the background is still visible through it. You either see the foreground object by itself or the background -- depending on which way the switch goes. Also since there is a switch involved, there is a limit to how fast it can switch back and forth between foreground and background. This results in a limit on the ability of the system to isolate fine detail in the foreground.
The camera can resolve detail that is much finer than the scanning time required for the switch to turn on and off. The loss of detail will probably be most apparent around the edges of a person's hair. Softening or blending the edges by slowing down the switch and overlapping foreground and background during the transition can produce a more attractive compromise in the image, but it actually sacrifices even more fine detail. The only real solution is to eliminate the switching function altogether, which is what Ultimatte and linear keyers do.
Linear keyers combine the background and foreground in proportions determined by the level of the key signal rather than simply switching between the two signals. A linear key can make a foreground object appear transparent, but the effect is not the same as an Ultimatte composite. A linear keyer does not do a fully additive mix. The foreground must be attenuated if the background is to be turned on. An Ultimatte does not attenuate the foreground subject when the foreground is processed. It subtracts only the blue (or green) component which represents the backing.
In an Ultimatte composite the processed foreground and background are added together without limited the total signal. This is what enables Ultimatte to reproduce transparent or translucent objects in the foreground and to reproduce extremely fine detail in the foreground scene such as wisps of hair or smoke.
In some situations the difference between a linear chroma key and an Ultimatte composite may not be all that noticeable, but with others it can be dramatic. It is most apparent when compositing a bright background scene with a foreground containing subtle detail such as smoke or reflections. A reflection in a window in a bluescreen foreground can be reproduced in an Ultimatte composite, but will be lost completely in a chroma key. The window may appear to be there, but the reflection will be lost. This is because the signal levels involved in the reflection are so slight that attenuating them in order to turn on the background will cause them to disappear. Smoke in the foreground of a composite made by a linear keyer will either be lost or appear darkened. Where the smoke is thick enough to obscure the backing it will appear normal, but as it thins out it will get darker because both it and the background are being attenuated.
Edges of out of focus foreground objects which the Ultimatte can composite with complete fidelity will also not composite well with a linear keyer. Shadows composited by a linear shadow chroma key are often noisy in the blacks. The shadow reproducing ability of a linear keyer should be tested with shadows which range from light to extremely dense and compared to the same shadows composited with an Ultimatte.
The difference between the fully additive mix in an Ultimatte composite and the mix accomplished by a linear chroma keyer in a switcher can be seen by taking the matte signal generated by an Ultimatte and using it as an external key in the linear keyer to composite the Ultimatte's processed foreground with a background. Using a linear keyer will not yield the same results as using an Ultimatte to put together the final composite.
If the foreground and background are to be added together, it is necessary to process the foreground to remove the backing before the mix. Ultimatte does this by means of the patented algorithms which selectively subtract the levels representing the backing. A linear chroma keyer processes the foreground by nulling the chroma at the hue specified for the backing. The remaining luminance level is eliminated when the foreground is attenuated in the mix, but in some instances shades of blue in the foreground subject may also go to gray.
Matte Signal Generation
The second feature which distinguishes an Ultimatte from any chroma key device is the patented algorithm by which the matte signal is generated from the foreground scene. The process by which Ultimatte derives the matte signal from the foreground RGB is a more complex and more flexible than the process by which a chroma key signal is derived. The net result is that the Ultimatte can better distinguish between the color of the backing and other colors in the foreground subject. The Ultimatte operator has more control over how pure the color is which is used to generate the matte signal. Shades of blue and cyan in a foreground subject in front of a bluescreen can be held by an Ultimatte more easily than by a chroma key.
Blue Spill Suppression
The Ultimatte also has circuitry which removes the discoloration of the foreground subject caused by spill light from the backing. It does this as part of the foreground processing which takes place separately from the generation of the matte signal.
The processed foreground generated by the Ultimatte is one in which the blue (or green) of the backing has been suppressed to black, and blue (or green) in other areas of the foreground has been reduced to an appropriate level. It does this by selectively reducing the blue (or green) component of the foreground rather than by nulling the chroma for a specific hue as is the case with some chroma keyers which claim to remove blue spill. The difference is that blues in the foreground subject of a chroma key composite will turn gray (if the chroma key can hold them at all) whereas with Ultimatte the blue can be reproduced as blue. There are some foreground colors which will be changed by the blue spill removal circuitry in an Ultimatte, but the operator has complete control over the circuits. Any tradeoff between spill removal and color fidelity can be optimized for each particular setup. With a blue backing the only colors which are altered are ones in which the blue content exceeds the red (magenta, pink, purple). (For further information see the chart Technical Bulletin #2 explaining the function of Gates 1, 2 and 3 with both blue and green backings.)
Some sophisticated linear chroma keyers claim to be able to reproduce transparent objects and shadows cast on the backing while at the same time removing blue spill from the foreground subject. In practice, however, they cannot create a composite that is as realistic as an Ultimatte composite in many situations. Smoke, reflections, out of focus objects and extremely fine hair detail will not look as good in a composite made by a linear chroma keyer. Shades of blue will be difficult to hold in the foreground and will turn gray in a chroma key composite while they can be held and accurately reproduced by an Ultimatte or Newsmatte.The Screen Correction feature in CineFusion and the Ultimatte 8 is another way in which the Ultimatte process differs from chroma keying. (See "Screen Correction".)