ESTABLISHING A CONSISTENT BASELINE REFERENCE
FOR HD CAMERAS
D. P. Adams and T. L. Scott
THE HARLEQUIN CHART
- With the rise of HD cameras, consistent picture quality is critical if the end-user is to enjoy to the full what the system has to offer
- The service provider must give camera line-up the attention it needs
- Camera line-up is more involved than a simple black and white balance
- Various factors need to be considered to maintain consistency across cameras and shooting days
- Recent advances in line-up charts have made this easier, however there are still limitations
- The Harlequin chart helps overcome many of these limitations
FAVOURING SKIN TONES
- 60% favours skin tones by moving them up the gamma curve
- Noise is perceivably less
- Lower dynamic range makes for more comfortable viewing experience
COLORIMETRIC ANALYSIS
- Line-up shouldn’t just involve checking the greyscale; we transmit in colour too!
- With a distinctive cartwheel and spokes pattern, differences between cameras (I.e. saturation or matrix) can be clearly identified
- Dots should appear at certain pre-determined points on a waveform monitor and vectorscope under ITU-709 recommendations
- Colour chips arranged horizontally are easily viewed at line-rate in RGB on a waveform monitor
- The more ‘linear’ the amplitude of the video signal is for the colour chips, then the better the colorimetric performance of the camera
Some of these images have been digitally sharpened to increase their clarity, however the principles discussed remain the same.
RGB & PERFECT WAVEFORMS
OTHER FEATURES & WHEN IS IT NOT ENOUGH
Other Features
- Skin tone swatches
- Front-lit so that lighting level and colour temperature correspond to actual studio conditions
- Front-lit is more compact than rear-lit making it easier to store
- Absence of crossed greyscales declutters the chart
Not Enough
- Shading – neutral greyscale
- Back focus – back focus chart
- Aperture correction – zone plate chart
- Gamma – waveform monitor
WHITES, BLACKS & UNIFORMITY
WHAT IS IN STORE FOR THE FUTURE?
CONCLUSIONS
- The Harlequin Chart favours skin tones in scene composition by embracing the ‘less is more’ adage
- Sophisticated analysis of the camera’s colorimetric performance can be easily achieved
- Allows identification of areas of deficiency and gives the user the tools necessary to make improvements
- Camera architecture is rapidly moving towards C-MOS and a colour difference equation specifically formulated for it is recommended
REFERENCES
- 1. Foad, G. 2007. Are Six Colors Enough? DSC Tech Tips – dsclabs.com/tech_tips.htm. September, 2007.
- 2. Corley, D. C.; Scott, T. L.; and Adams, D. P. 2009. Selecting Production Parameters to Ensure that Picture Quality Accommodates the Intended and Possible Future Imaging Systems. Proceedings of the SMPTE Annual Tech Conference. October, 2009. pp. 8 to 11, slides 27-36.
ACKNOWLEDGEMENTS
The authors would like to thank David Corley of DSC Labs for his contributions to the development of this project and his continued support.
They would also like to thank Rien van Trotsenburg of Grass Valley Nederland B.V. and Nigel Arnott of Grass Valley (UK) Ltd for their contributions and involvement.