|Issue: April 1, 2008
POST POSITIONS: THE IMPORTANCE OF TEST & MEASUREMENT IN POST
By: Roger Fawcett
|Today’s increasing pressure on post production for a speedy turn around is supported by a plethora of color grading and other color correction equipment. Visually pleasing results can readily be obtained. However, the types of transformation applied by this equipment also affect the technical quality of the resulting images. Color gamut is particularly likely to be adversely affected. While video legalizers can ensure that the resulting video output remains within range, the best results are achieved with the support of dedicated test and measurement equipment.
FIXING IT IN POST
The economics of film production has seen a significant trend toward shooting film to a schedule dictated by the availability of the participants and leaving post production to compensate for takes made in less-than- ideal conditions. A wide range of color correction equipment is available to assist in this process, offering all sorts of color grading and image effects with which to modify the images coming from the film scanner.
However, color balance and lighting are not the only things that need to be considered. Just as important is the suitability of the images for all the different media in which they will be transmitted because the requirements on HD video differ from those on either digital cinema or SD analogue transmission. You also need to be aware that the different processes of color correction and format conversion can themselves have unwanted side effects.
COLOR GAMUT ISSUES
The extent of the problem can be seen by considering the effect of the different stages of production on the color gamut of digitized video.
There are two main points in the process at which color gamut issues can appear. Firstly, the color correction procedure itself can cause parts of the image to go out of gamut. The combined effect of the various transformations that are applied can very easily result in color space values that lie outside the legal range.
The color gamut can also be affected at any point in the process where there is a change of video format. Even what appears to be the same color space can have different legal ranges according to the image format that is selected, a case in point being the YUV color space, which has different vectors in HD formats to those used in SD.
Gamut changes are particularly evident when material generated for digital cinema is converted for broadcast. The defining characteristic of the XYZ color space used for digital cinema production is its ability to represent every color to which the human eye can respond. Switching to RGB or the YUV color space used in broadcast is likely to result in a significant proportion of an image becoming out of gamut due to the more limited range of colors represented in RGB.
To ensure that the output from the color correction process is kept within legal values, post production suites often include a video legalizer. However, video legalizers provide a less-than-perfect answer through simply clipping illegal values. This approach can introduce a range of image artifacts, notably fringing effects. There can also be an overall dulling of the image as a result of reducing the color saturation.
ADDRESSING THE PROBLEM
Small adjustments, for example to the parameters applied by the film scanner, can significantly reduce the level of image degradation. It may also be possible to achieve a visually better result if you are prepared to accept that a proportion of the image is out of gamut. Provided the proportion of the image that is out of range, and the degree to which it goes out of range, is too small to be detected, the end result could appear both clearer and brighter.
Making such adjustments requires detailed knowledge of the image properties at each stage of the post production process. This in turn requires the use of a high-resolution waveform display and image analyzer, supporting all the different video and digital film formats that may be employed.
Analyzers, such as the OmniTek XR, are now providing colorists with high levels of detail and accuracy. As well as waveform and vector scope displays, it provides histogram and gamut charts, indicating the distribution of pixel values, in all the different color spaces. It also offers high-resolution video proxy displays to allow comparison of pre- and post-transform images and the ability to isolate particular regions of a source image for detailed color analysis. Displays such as these give the colorist immediate feedback on the effect of any adjustment made, not just on that particular stage of the process but also on all the different formats in which the images are to be distributed.
The OmniTek system supports dual-link video formats and XYZ color space specified for digital cinema, as well as for the HSDL format used to transfer data from film scanners.
Incorporating such an analyzer in the color grading suite offers further advantages through the other aspects of the video that can be monitored. For instance, OmniTek XR also provides analysis of audio channels delivered alongside video images. Support is also offered where these channels are configured for Dolby E Surround Sound, with full interpretation of the Dolby E metadata delivered alongside the audio. Another benefit is comprehensive logging of all monitored parameters, so after the final tape has been produced, detailed performance information can be generated and logged in a file for the customer.
OTHER MEASUREMENT OPTIONS
Another company offering a range of test and measurement gear is Tektronix. They have solutions that provide measurement and analysis for multiformat environments, from analog to advanced digital video and audio. And DK-Technologies recently introduced a waveform monitor called the DK PT0760, which can serve as four different SD/HD waveform monitors in one box.
Roger Fawcett is the Managing Director of OmniTek (www.omnitek.tv) in Bashingstoke, England.