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Saturday, October 31, 2015

••◊ Color Correcting Web Videos

I've recently been doing more color correction with Sony's S-log3 and learning as I go.  A problem that keeps creeping up is that Dominique's skin tones always look too dark.  He has very dark skin and tends to wear typical set work black clothes during our blog videos.  I was wondering why in the world does it look so dark when I get the footage into post on my computer(?)  I use a rec.709 (800%) LUT on the monitor and it looks perfect there.  Is there something wrong with my computer monitor?

As with most technical endeavors, I first set out to make a model of the problem in Excel; but first a brief explanation:  Computer monitors use a standard called sRGB.  This color space is pretty much what the web uses today.  Rec.709 is a television and video monitor standard.  Although both standard have the same color primaries, they differ in the encoded gamma curve.  ...and that's where my problem lies.  When I was viewing the footage for rec.709 it wasn't correct for display on an sRGB display, nor web videos.

...so how far off was my video?  That's where Excel comes in. I started by plotting percentage reflectance of an object being recorded versus the output IRE value.  So if you had a test target that gradually increased reflectance from 0% (absolute black) to 100% (absolute white) then this is what the luma waveform monitor would show for each standard.  The biggest take away here is that sRGB produces higher IRE values for any given reflectance (blue curve).  So if you have an 18% gray card in the scene then the standard says it would produce a 44 IRE level with rec.709 and a 50 IRE level with sRGB (assuming data levels 0-255, not broadcast or "legal" levels).

To further describe the relationship of sRGB to rec.709 I plotted the two against one another.  The take away here is that sRGB produces higher values, especially in the dark tones - i.e. where Dominique's skin and clothing are.  The process to fix this in Adobe Premiere is pretty easy.  All we have to do is apply a "luma curve" effect that emulates the blue line in the graph below and we'll have a correction from rec.709 to sRGB - and a much better looking web video.

The question now is how does this look in the real world?  I added the luma curve effect to the project below and set it to use approximately the same curve as the blue line in the previous graph.  Notice how Dominique's skin now has much more detail and his clothes do too?  Much better, I think.

So my learning here is that if you want to output the video for television, use a rec.709 monitor.  If you want to output to the web and you shot in rec.709, you might want to use the luma curve above to correct your footage.  Next time I know.  Don't assume one equals the other.

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