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Tuesday, March 1, 2011

••◊ Vectorscope. Friend or Foe?

While I'm waiting for projects to get kicked off this month my mind is wandering into the deep forest of technical knowledge. I sought enlightenment from my desk chair man cave and began to meditate on the concept of camera color calibration. No camera, no matter film or digital, cheap or expensive, seems to be calibrated for proper color reproduction out of the box. Why can't we simplify our existence with the comfort of proper hue and saturation? Why? What if my camera could, through a insignificant amount of pain and suffering, be calibrated? Wouldn't that make the world a better place?

So I whipped out my web browser and went to the keeper of all knowledge - wikipedia. With video the common tool to examine hue, saturation, and color reproduction in general is the vectorscope. As with most seekers of enlightenment, expensive toys are out of the question. A solution? Microsoft Excel. Yes I know that Premier Pro has a vectorscope, but it only handles NTSC so it doesn't work for the modern world of HD video. Adobe After Effects has one too via Synthetic Aperture, however that scope has issues too and I still haven't found out what it really plots. A proper vectorscope works with the Y'CbCr color space, which turns out to be a simple transformation from the RGB colors in video. All I had to do was look up the correct matrix multiply and viola! ...Excel vectorscope. Now I can look at hue and saturation in the rec.709 color space (HD) without the cost of buying a brand name test and measurement device.

The picture above shows my less than glamorous vectorscope plot of a few RGB values. The empty boxes represent the standard 75% saturated color values that everyone in the industry uses for color calibration. The asterisks represent the color reproduction of a Canon 5D mark II in video mode, with the faithful color profile and saturation turned down one notch. Note that the calibration target was a Red Cambook made by DSC Labs. Ben Cain over at negativespaces.com did this measurement a while back since he owns the Red Cambook and a 5D mark II.

Now the question becomes, what do I do with this knowledge. Well... I search for the ideal of course. The solid squares in the above plot represent the idea color reproduction for HD video. Canon provides a little used piece of software with their cameras called the Picture Style Editor that actually allows you to tweak the color reproduction of the camera. So using the asterisks as the source colors and the squares as a target color I began to twist the color reproduction into shape. Blue was the color most in need of tweaking. Magenta needed a little more saturation, but other colors needed only very minor shifts.

What you see in the above pictures is first, the picture style editor with color tweaks at each of the calibration targets. Next is the final pictures. Notice that if you look closely, the color of the car has changed. The woman, the ground, her clothes...etc are all pretty much the same. There is a little more saturation with the red stripes in her shirt because they probably have a slightly magenta tint, but skin tones are pretty much unchanged.

Am I done? No. I don't have a way to verify this custom picture style without a Red Cambook. It's even difficult to rent one of these. What I can say is that after viewing many, many photos with varying content the only significant shift is in blue. I don't have a picture with much magenta in it so I can't verify that test condition. So my journey is not done. I still need to innovate a test case and prove my work. My eventual goal is to create a custom profile and release it to the world, but until then I am in search of a Red Cambook or other DSC Chroma DuMonde target.

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