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Saturday, December 1, 2012

••◊ Matching the GoPro Hero 3 Black to a 5D mark II, part3

I wasn't going to do this post, but the thought of not doing it seemed silly this morning.  Instead of matching the Hero 3 color to just an old Canon 5D mark II camera, what about to color calibrated cameras?  How "good" can we make the camera color rendition.  Note that I put good in quotations because color for films is all over the place.  It never looks like real life.  What I mean by "good" is sufficiently accurate to rec.709.  On a commercial shoot, as well as on TV, quite a few productions try to line up their color rendition to rec.709 so billion dollar Oprah doesn't look like she belongs in Willy Wonka's chocolate factory (fired!).  Directors I work with are all too familiar with me having to tweak the camera settings to take care of oopah loopah skin tone issues.

I'm going to spare many details of my last post because you could see my methodology there.  However, the idea is basically to use the DSC color chart to align the colors in the chart to the target boxes on a vectorscope.  Again, let's start with a picture showing the GoPro Hero 3 versus the Canon rec.709 custom picture style I made.

The color rendition is obviously a bit off.  There's a vast difference in the greens and a slight shift in other colors.  The vectorscope plots below show why.  The yellows-reds-magentas-blues all look quite reasonable and would normally just require a little saturation to be calibrated.  However the greens and cyans are quite a bit desaturated for whatever reason.  Also, the green hue angle seems to be way off.  Maybe this is by design?  Dunno. 

The first step is that I went back to my handy-dandy Excel spreadsheet calculator, as featured in the last post, and calculated up another 3x3 matrix that adjusts the Hero 3 color to better match the DSC alignment chart.  Here's the matrix...

[ 0.93   -0.15    0.02]                [R-gpro]                [R-rec709]
[-0.08    1.00   -0.08]       x       [G-gpro]      =       [G-rec709]
[-0.08    0.23    0.70]                [B-gpro]                [B-rec709]
Note that this just adjusts the color to the semi-correct hues.  It doesn't compensate for the saturation difference.  Also, this adjustment just gets us much **closer** to rec.709 color alignment.  It doesn't nail it completely, as you'll see later on in this article.  We apply the matrix using the same Channel Mixer effect in Premier Pro that we did in the last blog article.  The picture below shows the effect settings and the resulting adjusted colors.

You can now see that the point on the vectorscope graph that is supposed to be pointing at the yellow target box has now moved down more towards green.  The good news is that this has only had a minor impact on skin tones.  So user beware.  Fields of wheat blowing in the wind probably won't match well - don't do any shoots in front of a McDonald's!  Also cyan is slightly desaturated from where it should be, but the hue angle is probably OK.  Perhaps this is why GoPro desaturated the greens and cyans. They probably wanted to get the yellows spot on.  The last step in this process is to add a bit of saturation to the picture.  I found that with the above adjustment matrix we need about 138% saturation to be *technically* correct.  I personally found that I don't like this much, but like I wrote...color is an opinion...and as filmmakers we definitely have opinions!

The last step is to look at the two adjusted charts.  You'll see a bit of difference in the yellow swatch, but it's not so far off it's easily objectionable in my opinion.  However the real truth isn't in these scientific measurements, it's in the footage.  That always requires more testing than can be done in my spare bedroom.

When adjusting some test footage I did notice a slight exposure shift with the Channel Mixer effect settings.  You may experience the same thing, which will likely require an adjustment with the Luma Curve effect to compensate.

I hope this helps someone out there.  Let me know your results in the comments section below. 

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