Merge pull request obsproject#7 from jpark37/color-space-sycc

Add RFC: sRGB Color Space setting

accepted/0007-color-space-srgb.md

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+# Summary
+
+Add sRGB to "Color Space" output options, and default to sRGB.
+
+# Motivation
+
+There is often confusion around the "Color Space" option for output, and it has several problems.
+
+- We don't actually convert colors between RGB color spaces in OBS. We merely tag the metadata.
+- Rec. 601 output support is weird. x264 tags 601 as "undef" for everything, and jim-nvenc uses the European 625-line variant of 601 attributes.
+- Any PC applications/games that output sRGB are tagged as 601/709, and will be handled incorrectly by color-accurate applications, e.g. old versions of Chrome handled 709 properly before they decided to cheat to save power.
+
+Defaulting to sRGB is probably the least of all evils.
+
+- sRGB is identical to Rec. 709 except for the sRGB transfer function, which can be a free conversion for most GPUs. When represented as YCbCr, it makes sense to transform with Rec. 709 matrix coefficients.
+- PC applications/games that use sRGB (very common) are now tagged accurately if using sRGB. Video capture feeds that use 601/709 would not be, but we can leave behind 601/709 settings for passthrough scenarios. Actually converting the color data is beyond the scope of this RFC.
+- Chrome supports sRGB videos properly on all tested platforms.
+- Rec. 601 for output was almost always the wrong choice.
+
+Implementation:
+
+- Add VIDEO_CS_SRGB, and plumb into all switch cases.
+- Go through output encoder settings, and use sRGB metadata values.
+
+# Drawbacks
+
+People that had a working color pipeline with the old faulty settings may have to tweak their setup.
+
+There is supposedly a performance regression involving DeckLink when switching away from 601.
+
+# Additional Information
+
+None.