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, 11 insertions, 0 deletions

diff --git a/video/out/gl_video_shaders.glsl b/video/out/gl_video_shaders.glsl
index 2f5940c26f..4037e42449 100644
--- a/video/out/gl_video_shaders.glsl
+++ b/video/out/gl_video_shaders.glsl
@@ -179,6 +179,10 @@ uniform mat2 dither_trafo;
 uniform vec3 inv_gamma;
 uniform float input_gamma;
 uniform float conv_gamma;
+uniform float sig_center;
+uniform float sig_slope;
+uniform float sig_scale;
+uniform float sig_offset;
 uniform float dither_quantization;
 uniform float dither_center;
 uniform float filter_param1_l;
@@ -425,7 +429,14 @@ void main() {
     // The BT.2020 specification says Yc = 0.2627*R + 0.6780*G + 0.0593*B
     color.g = (color.g - 0.2627*color.r - 0.0593*color.b)/0.6780;
 #endif
+#ifdef USE_SIGMOID
+    color = sig_center - log(1.0/(color * sig_scale + sig_offset) - 1.0)/sig_slope;
+#endif
     // Image upscaling happens roughly here
+#ifdef USE_SIGMOID_INV
+    // Inverse of USE_SIGMOID
+    color = (1.0/(1.0 + exp(sig_slope * (sig_center - color))) - sig_offset) / sig_scale;
+#endif
 #ifdef USE_GAMMA_POW
     // User-defined gamma correction factor (via the gamma sub-option)
     color = pow(color, inv_gamma);