video: Add sigmoidal upscaling to avoid ringing artifacts

This avoids issues when upscaling directly in linear light, and is the
recommended way to upscale images according to imagemagick.

The default slope of 6.5 offers a reasonable compromise between
ringing artifacts eliminated and ringing artifacts introduced by
sigmoid-upscaling. Same goes for the default center of 0.75.

1 files changed, 13 insertions, 1 deletions

diff --git a/DOCS/man/vo.rst b/DOCS/man/vo.rst
index c296e705be..0566fd6817 100644
--- a/DOCS/man/vo.rst
+++ b/DOCS/man/vo.rst
@@ -457,6 +457,18 @@ Available video output drivers are:
         This is automatically disabled for anamorphic video, because this
         feature doesn't work correctly with this.
 
+    ``sigmoid-upscaling``
+        When upscaling in linear light, use a sigmoidal color transform
+        to avoid emphasizing ringing artifacts.
+
+    ``sigmoid-center``
+        The center of the sigmoid curve used for ``sigmoid-upscaling``, must
+        be a float between 0.0 and 1.0. Defaults to 0.75 if not specified.
+
+    ``sigmoid-slope``
+        The slope of the sigmoid curve used for ``sigmoid-upscaling``, must
+        be a float between 1.0 and 20.0. Defaults to 6.5 if not specified.
+
     ``no-npot``
         Force use of power-of-2 texture sizes. For debugging only.
         Borders will be distorted due to filtering.
@@ -598,7 +610,7 @@ Available video output drivers are:
 
     This is equivalent to::
 
-        --vo=opengl:lscale=spline36:dither-depth=auto:fbo-format=rgba16:fancy-downscaling
+        --vo=opengl:lscale=spline36:dither-depth=auto:fbo-format=rgba16:fancy-downscaling:sigmoid-upscaling
 
     Note that some cheaper LCDs do dithering that gravely interferes with
     ``opengl``'s dithering. Disabling dithering with ``dither-depth=no`` helps.