vo_gpu: improve tone mapping desaturation

Instead of desaturating towards luma, we desaturate towards the
per-channel tone mapped version. This essentially proves a smooth
roll-off towards the "hollywood"-style (non-chromatic) tone mapping
algorithm, which works better for bright content, while continuing to
use the "linear" style (chromatic) tone mapping algorithm for primarily
in-gamut content.

We also split up the desaturation algorithm into strength and exponent,
which allows users to use less aggressive desaturation settings without
affecting the overall curve.

1 files changed, 20 insertions, 11 deletions

diff --git a/DOCS/man/options.rst b/DOCS/man/options.rst
index c6b34f3171..1c08917d7a 100644
--- a/DOCS/man/options.rst
+++ b/DOCS/man/options.rst
@@ -5245,17 +5245,26 @@ The following video options are currently all specific to ``--vo=gpu`` and
     The special value ``auto`` (default) will enable HDR peak computation
     automatically if compute shaders and SSBOs are supported.
 
-``--tone-mapping-desaturate=<value>``
-    Apply desaturation for highlights. The parameter essentially controls the
-    steepness of the desaturation curve. The higher the parameter, the more
-    aggressively colors will be desaturated. This setting helps prevent
-    unnaturally blown-out colors for super-highlights, by (smoothly) turning
-    into white instead. This makes images feel more natural, at the cost of
-    reducing information about out-of-range colors.
-
-    The default of 0.5 provides a good balance. This value is weaker than the
-    ACES ODT curves' recommendation, but works better for most content in
-    practice. A setting of 0.0 disables this option.
+``--tone-mapping-desaturate=<0.0..1.0>``
+    Apply desaturation for highlights (default: 0.75). The parameter controls
+    the strength of the desaturation curve. A value of 0.0 completely disables
+    it, while a value of 1.0 means that overly bright colors will tend towards
+    white. (This is not always the case, especially not for highlights that are
+    near primary colors)
+
+    Values in between apply progressively more/less aggressive desaturation.
+    This setting helps prevent unnaturally oversaturated colors for
+    super-highlights, by (smoothly) turning them into less saturated (per
+    channel tone mapped) colors instead. This makes images feel more natural,
+    at the cost of chromatic distortions for out-of-range colors. The default
+    value of 0.75 provides a good balance. Setting this to 0.0 preserves the
+    chromatic accuracy of the tone mapping process.
+
+``--tone-mapping-desaturate-exponent=<0.0..20.0>``
+    This setting controls the exponent of the desaturation curve, which
+    controls how bright a color needs to be in order to start being
+    desaturated. The default of 1.5 provides a reasonable balance.  Decreasing
+    this exponent makes the curve more aggressive.
 
 ``--gamut-warning``
     If enabled, mpv will mark all clipped/out-of-gamut pixels that exceed a