The advent of digital cinema has significantly impacted those involved in post production of images for theatre presentation. The specifications put together by SMPTE and the Digital Cinema Initiatives group introduced several novel features such as JPEG2000 image compression and the use of new high-resolution 2K and 4K video formats.
They have also specified use of the CIE XYZ color space for encoding of master source material. This color space was created in 1931 following studies of the perception of color by the human eye. Its defining characteristic is its ability to represent every color to which the human eye can respond as a unique combination of three positive parameters that doesn't depend on the devices used to produce the light.
The issue at hand
The problem for post production is that the RGB color space typically used in broadcast is limited in the range of colors it can represent as a result of the way the responses of the red, green and blue cones of the human eye overlap. Blue-green colors are a particular problem. A monochromatic light source of approximately 500nm should only produce a response in blue and green, but any color produced by mixing monochromatic green and blue light sources will also stimulate red cones. It is therefore impossible to represent this color in the RGB color space.
The scope of the RGB color space is readily illustrated on the CIE chromaticity diagram, as shown in Figure 1. The outer curved boundary of the colored region represents the eye's response to monochromatic light in the visible part of the spectrum, while the inner region represents its response to mixtures of those colors. The vertices of the triangle represent the colors produced by red, blue and green photo devices, while the triangle encloses all the different colors that can be created using these devices.
Conversion of image material encoded using XYZ color space into RGB color space uses a matrix that maps XYZ values into RGB values. Only positive RGB values have any meaning for an RGB display, so any negative values that result from the conversion are replaced by zero. Colors that fall within the gamut of the RGB triangle have positive values in RGB color space and so translate directly. Conversion of colors that fall outside the RGB color space, however, result in negative values for either R, G or B. The result of this is to substitute the original color by one on the edge of the RGB triangle.
The effect of this color conversion on the resulting images is crucial to colorists working in the broadcast industry not just for aesthetic reasons, but also for the range of fringing and other artifacts that can result. Reducing the color saturation of the image helps by pulling each color in toward the white point at the center of the chromaticity diagram. However, doing this in a controlled manner requires detailed information about the location and extent of the problem areas in the video images. This in turn requires test and measurement equipment that can both handle the video formats used by digital cinema and provide information about color gamut in both XYZ color space and RGB color space on a pixel-by-pixel basis.
The OmniTek XR waveform monitor and image analyzer provides such support. It can take either the source or the output from the colorist's color grading equipment and, in real time, display waveforms and histograms, not just of the XYZ source but also of the equivalent RGB or YUV video.
Of particular value to the XYZ conversion problem is the color gamut display. This may be set up to display equivalent RGB values as bar graphs, with values that exceed acceptable thresholds shown in a contrasting color. It also keeps a count of the number of pixels for which the conversion produces values outside the required range and measures of these effects as a percentage of the overall area.
The XR system also has the ability to highlight the pixels affected as part of a live display either on a monitor or the built-in video proxy, thereby giving the colorist an easy way of seeing the scope of the problem that needs to be dealt with.
Mike Hodson is president of OmniTek.