video: refactor HDR implementation

List of changes:

1. Kill nom_peak, since it's a pointless non-field that stores nothing
   of value and is _always_ derived from ref_white anyway.

2. Kill ref_white/--target-brightness, because the only case it really
   existed for (PQ) actually doesn't need to be this general: According
   to ITU-R BT.2100, PQ *always* assumes a reference monitor with a
   white point of 100 cd/m².

3. Improve documentation and comments surrounding this stuff.
4. Clean up some of the code in general. Move stuff where it belongs.

1 files changed, 8 insertions, 3 deletions

diff --git a/video/csputils.h b/video/csputils.h
index 9eaafbe75d..ec7369fa67 100644
--- a/video/csputils.h
+++ b/video/csputils.h
@@ -121,10 +121,15 @@ struct mp_colorspace {
     enum mp_csp_levels levels;
     enum mp_csp_prim primaries;
     enum mp_csp_trc gamma;
-    float nom_peak; // nominal (absolute) peak. 0 = auto/unknown
-    float sig_peak; // signal peak, highest value that occurs in the source
+    float sig_peak; // highest relative value in signal. 0 = unknown/auto
 };
 
+// For many colorspace conversions, in particular those involving HDR, an
+// implicit reference white level is needed. Since this magic constant shows up
+// a lot, give it an explicit name. The value of 100 cd/m² comes from ITU-R
+// documents such as ITU-R BT.2100
+#define MP_REF_WHITE 100.0
+
 // Replaces unknown values in the first struct by those of the second struct
 void mp_colorspace_merge(struct mp_colorspace *orig, struct mp_colorspace *new);
 
@@ -230,7 +235,7 @@ int mp_chroma_location_to_av(enum mp_chroma_location mploc);
 void mp_get_chroma_location(enum mp_chroma_location loc, int *x, int *y);
 
 struct mp_csp_primaries mp_get_csp_primaries(enum mp_csp_prim csp);
-float mp_csp_trc_nom_peak(enum mp_csp_trc trc, float ref_peak);
+float mp_trc_nom_peak(enum mp_csp_trc trc);
 bool mp_trc_is_hdr(enum mp_csp_trc trc);
 
 /* Color conversion matrix: RGB = m * YUV + c