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Wednesday, July 21, 2010

••◊ Feeling techie, part 2: Isn't brightness supposed to be easy?

I was slightly wrong with the previous post, but for good reason. The DSC documentation is misleading for a mentally impaired monkey boy, such as myself. Going back over the documentation, the IRE levels they recommend didn't make sense. I mean what color space are we talking about here? I hate it when numbers don't make sense when I place so much trust in them everyday. When DSC says RGB does that mean sRGB, rec.709 (HD), or 601 (SD) color? Being an engineer this kept me awake at least one night until I got a grasp on what they were talking about. Charles Poynton's excellent FAQ helped to a certain degree.

Long story short, the RGB values DSC provides for the neutral tiles are straight up γ (i.e gamma) equals 0.45, so it's mostly correlated to the 601 color space. So for instance if the reflectance equals 0.18, then the IRE level = 0.18^0.45 = 46.13 IRE, 8-bit RGB value = 118. Follow with me folks because we're about to go down the rabbit hole.

The web and pretty much all computers (except the screwy Apple brand) follow an sRGB gamma curve. This calculated as follows... If the Reflectance (R) is greater than 0.0031308 then the gamma curve is calculated as E'=1.055*(R^(1/2.4))-0.055. So if the reflectance is 0.18 again then the IRE level = 46.22 and the 8-bit RGB value is 118. OK, that's pretty close to a straight up gamma at this reflectance.

Now we move on to rec.709 gamma, which is used for HDTV. If reflectance is greater than 0.018 then it's calculated as 1.099*(R^0.45)-0.099. So we end up with an IRE level of 40.9 and an 8-bit RGB value of 104.

They are all slightly different.

The 5Dm2 uses the sRGB gamma curve, which fits closely with straight γ=0.45. However it seems for using the camera in broadcast HD situations you really want to adjust the tone curve to compensate. How much...? Pretty much all footage is tweaked in post and this aspect might not be any different. Editing in sRGB and displaying in rec.709 will likely make your footage have less contrast than intended. The graph below shows how the 11 neutral tiles on the DSC Chroma du Monde Red test chart compare under each of the color spaces. See how rec.709 is quite a ways different than sRGB?

So for those of you brave enough to be reading until this point you may ask, "how much different is that little gap at the bottom of the curve?" Well, here's another graph that shows the exact amount. Anything above 5% just mentally seems like a lot. In this case there's up to 38% difference in the darker neutral tiles.

I thought, "cool, I have a test chart that will help settle my color issues out." "Oh...wait a minute, this doesn't make sense." "Oh crud, there goes my night of sleep." All this and I have yet to figure out vector scopes and color coordinates completely. I might require a melatonin pill this evening.

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