[gnome-shell] blur-effect: Use sigma value instead of blur radius

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit f351cfa2f7ffde1459680fce31ec273a07ff773c
Author: Jonas Dreßler <verdre v0yd nl>
Date:   Mon Feb 10 15:34:57 2020 +0100

    blur-effect: Use sigma value instead of blur radius
    
    Almost all implementations, including CSS [1] of gaussian blurs use the
    sigma/standard deviation value as the input parameter, even if they call
    that value "radius". Since using sigma is more correct mathematically
    and avoids confusion for people used to other blur implementations, use
    that parameter here, too.
    
    [1] https://www.w3.org/TR/filter-effects-1/#funcdef-filter-blur
    
    https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/991

 js/ui/unlockDialog.js   |  4 +--
 src/shell-blur-effect.c | 77 ++++++++++++++++++++++++-------------------------
 src/shell-blur-effect.h |  6 ++--
 3 files changed, 43 insertions(+), 44 deletions(-)
---
diff --git a/js/ui/unlockDialog.js b/js/ui/unlockDialog.js
index 55dfa2cc4f..9dc809b4d4 100644
--- a/js/ui/unlockDialog.js
+++ b/js/ui/unlockDialog.js
@@ -19,7 +19,7 @@ const SCREENSAVER_SCHEMA = 'org.gnome.desktop.screensaver';
 const CROSSFADE_TIME = 300;
 
 const BLUR_BRIGHTNESS = 0.55;
-const BLUR_RADIUS = 200;
+const BLUR_SIGMA = 60;
 
 const SUMMARY_ICON_SIZE = 32;
 
@@ -516,7 +516,7 @@ var UnlockDialog = GObject.registerClass({
 
         widget.add_effect(new Shell.BlurEffect({
             brightness: BLUR_BRIGHTNESS,
-            blur_radius: BLUR_RADIUS,
+            sigma: BLUR_SIGMA,
         }));
     }
 
diff --git a/src/shell-blur-effect.c b/src/shell-blur-effect.c
index 500bad12c5..01dd797bd9 100644
--- a/src/shell-blur-effect.c
+++ b/src/shell-blur-effect.c
@@ -80,16 +80,13 @@
  */
 
 static const gchar *gaussian_blur_glsl_declarations =
-"uniform float blur_radius;                                                \n"
+"uniform float sigma;                                                      \n"
 "uniform float pixel_step;                                                 \n"
 "uniform int vertical;                                                     \n";
 
 static const gchar *gaussian_blur_glsl =
 "  int horizontal = 1 - vertical;                                          \n"
 "                                                                          \n"
-"  int n_steps = int (ceil (blur_radius));                                 \n"
-"  float sigma = float (n_steps) / 3.0;                                    \n"
-"                                                                          \n"
 "  vec2 uv = vec2 (cogl_tex_coord.st);                                     \n"
 "                                                                          \n"
 "  vec3 gauss_coefficient;                                                 \n"
@@ -102,6 +99,8 @@ static const gchar *gaussian_blur_glsl =
 "  vec4 ret = texture2D (cogl_sampler, uv) * gauss_coefficient.x;          \n"
 "  gauss_coefficient.xy *= gauss_coefficient.yz;                           \n"
 "                                                                          \n"
+"  int n_steps = int (ceil (3 * sigma));                                   \n"
+"                                                                          \n"
 "  for (int i = 1; i < n_steps; i += 2) {                                  \n"
 "    float coefficient_subtotal = gauss_coefficient.x;                     \n"
 "    gauss_coefficient.xy *= gauss_coefficient.yz;                         \n"
@@ -129,7 +128,7 @@ static const gchar *brightness_glsl =
 "  cogl_color_out.rgb *= brightness;                                       \n";
 
 #define MIN_DOWNSCALE_SIZE 256.f
-#define MAX_BLUR_RADIUS 10.f
+#define MAX_SIGMA 6.f
 
 typedef enum
 {
@@ -154,7 +153,7 @@ typedef struct
 {
   FramebufferData data;
   BlurType type;
-  int blur_radius_uniform;
+  int sigma_uniform;
   int pixel_step_uniform;
   int vertical_uniform;
 } BlurData;
@@ -182,14 +181,14 @@ struct _ShellBlurEffect
   ShellBlurMode mode;
   float downscale_factor;
   float brightness;
-  int blur_radius;
+  int sigma;
 };
 
 G_DEFINE_TYPE (ShellBlurEffect, shell_blur_effect, CLUTTER_TYPE_EFFECT)
 
 enum {
   PROP_0,
-  PROP_BLUR_RADIUS,
+  PROP_SIGMA,
   PROP_BRIGHTNESS,
   PROP_MODE,
   N_PROPS
@@ -272,8 +271,8 @@ setup_blur (BlurData *blur,
   blur->type = type;
   blur->data.pipeline = create_blur_pipeline ();
 
-  blur->blur_radius_uniform =
-    cogl_pipeline_get_uniform_location (blur->data.pipeline, "blur_radius");
+  blur->sigma_uniform =
+    cogl_pipeline_get_uniform_location (blur->data.pipeline, "sigma");
   blur->pixel_step_uniform =
     cogl_pipeline_get_uniform_location (blur->data.pipeline, "pixel_step");
   blur->vertical_uniform =
@@ -300,11 +299,11 @@ update_blur_uniforms (ShellBlurEffect *self,
                                     pixel_step);
     }
 
-  if (blur->blur_radius_uniform > -1)
+  if (blur->sigma_uniform > -1)
     {
       cogl_pipeline_set_uniform_1f (blur->data.pipeline,
-                                    blur->blur_radius_uniform,
-                                    self->blur_radius / self->downscale_factor);
+                                    blur->sigma_uniform,
+                                    self->sigma / self->downscale_factor);
     }
 
   if (blur->vertical_uniform > -1)
@@ -462,18 +461,18 @@ clear_framebuffer_data (FramebufferData *fb_data)
 static float
 calculate_downscale_factor (float width,
                             float height,
-                            float blur_radius)
+                            float sigma)
 {
   float downscale_factor = 1.0;
   float scaled_width = width;
   float scaled_height = height;
-  float scaled_radius = blur_radius;
+  float scaled_sigma = sigma;
 
   /* This is the algorithm used by Firefox; keep downscaling until either the
    * blur radius is lower than the threshold, or the downscaled texture is too
    * small.
    */
-  while (scaled_radius > MAX_BLUR_RADIUS &&
+  while (scaled_sigma > MAX_SIGMA &&
          scaled_width > MIN_DOWNSCALE_SIZE &&
          scaled_height > MIN_DOWNSCALE_SIZE)
     {
@@ -481,7 +480,7 @@ calculate_downscale_factor (float width,
 
       scaled_width = width / downscale_factor;
       scaled_height = height / downscale_factor;
-      scaled_radius = blur_radius / downscale_factor;
+      scaled_sigma = sigma / downscale_factor;
     }
 
   return downscale_factor;
@@ -689,7 +688,7 @@ update_framebuffers (ShellBlurEffect *self)
     return FALSE;
 
   get_target_size (self, &width, &height);
-  downscale_factor = calculate_downscale_factor (width, height, self->blur_radius);
+  downscale_factor = calculate_downscale_factor (width, height, self->sigma);
 
   updated =
     update_actor_fbo (self, width, height, downscale_factor) &&
@@ -777,7 +776,7 @@ shell_blur_effect_paint (ClutterEffect           *effect,
   ShellBlurEffect *self = SHELL_BLUR_EFFECT (effect);
   uint8_t paint_opacity;
 
-  if (self->blur_radius > 0)
+  if (self->sigma > 0)
     {
       if (needs_repaint (self, flags))
         {
@@ -841,11 +840,11 @@ shell_blur_effect_modify_paint_volume (ClutterEffect      *effect,
   width = clutter_paint_volume_get_width (volume);
   height = clutter_paint_volume_get_height (volume);
 
-  origin.y -= self->blur_radius;
-  origin.x -= self->blur_radius;
+  origin.y -= self->sigma;
+  origin.x -= self->sigma;
 
-  height += 2 * self->blur_radius;
-  width += 2 * self->blur_radius;
+  height += 2 * self->sigma;
+  width += 2 * self->sigma;
 
   clutter_paint_volume_set_origin (volume, &origin);
   clutter_paint_volume_set_width (volume, width);
@@ -884,8 +883,8 @@ shell_blur_effect_get_property (GObject    *object,
 
   switch (prop_id)
     {
-    case PROP_BLUR_RADIUS:
-      g_value_set_int (value, self->blur_radius);
+    case PROP_SIGMA:
+      g_value_set_int (value, self->sigma);
       break;
 
     case PROP_BRIGHTNESS:
@@ -911,8 +910,8 @@ shell_blur_effect_set_property (GObject      *object,
 
   switch (prop_id)
     {
-    case PROP_BLUR_RADIUS:
-      shell_blur_effect_set_blur_radius (self, g_value_get_int (value));
+    case PROP_SIGMA:
+      shell_blur_effect_set_sigma (self, g_value_get_int (value));
       break;
 
     case PROP_BRIGHTNESS:
@@ -944,10 +943,10 @@ shell_blur_effect_class_init (ShellBlurEffectClass *klass)
   effect_class->paint = shell_blur_effect_paint;
   effect_class->modify_paint_volume = shell_blur_effect_modify_paint_volume;
 
-  properties[PROP_BLUR_RADIUS] =
-    g_param_spec_int ("blur-radius",
-                      "Blur radius",
-                      "Blur radius",
+  properties[PROP_SIGMA] =
+    g_param_spec_int ("sigma",
+                      "Sigma",
+                      "Sigma",
                       0, G_MAXINT, 0,
                       G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);
 
@@ -973,7 +972,7 @@ static void
 shell_blur_effect_init (ShellBlurEffect *self)
 {
   self->mode = SHELL_BLUR_MODE_ACTOR;
-  self->blur_radius = 0;
+  self->sigma = 0;
   self->brightness = 1.f;
 
   self->actor_fb.pipeline = create_base_pipeline ();
@@ -993,29 +992,29 @@ shell_blur_effect_new (void)
 }
 
 int
-shell_blur_effect_get_blur_radius (ShellBlurEffect *self)
+shell_blur_effect_get_sigma (ShellBlurEffect *self)
 {
   g_return_val_if_fail (SHELL_IS_BLUR_EFFECT (self), -1);
 
-  return self->blur_radius;
+  return self->sigma;
 }
 
 void
-shell_blur_effect_set_blur_radius (ShellBlurEffect *self,
-                                   int              radius)
+shell_blur_effect_set_sigma (ShellBlurEffect *self,
+                             int              sigma)
 {
   g_return_if_fail (SHELL_IS_BLUR_EFFECT (self));
 
-  if (self->blur_radius == radius)
+  if (self->sigma == sigma)
     return;
 
-  self->blur_radius = radius;
+  self->sigma = sigma;
   self->cache_flags &= ~BLUR_APPLIED;
 
   if (self->actor)
     clutter_effect_queue_repaint (CLUTTER_EFFECT (self));
 
-  g_object_notify_by_pspec (G_OBJECT (self), properties[PROP_BLUR_RADIUS]);
+  g_object_notify_by_pspec (G_OBJECT (self), properties[PROP_SIGMA]);
 }
 
 float
diff --git a/src/shell-blur-effect.h b/src/shell-blur-effect.h
index 94d24f6b51..a7486ccde0 100644
--- a/src/shell-blur-effect.h
+++ b/src/shell-blur-effect.h
@@ -42,9 +42,9 @@ G_DECLARE_FINAL_TYPE (ShellBlurEffect, shell_blur_effect, SHELL, BLUR_EFFECT, Cl
 
 ShellBlurEffect *shell_blur_effect_new (void);
 
-int shell_blur_effect_get_blur_radius (ShellBlurEffect *self);
-void shell_blur_effect_set_blur_radius (ShellBlurEffect *self,
-                                        int              radius);
+int shell_blur_effect_get_sigma (ShellBlurEffect *self);
+void shell_blur_effect_set_sigma (ShellBlurEffect *self,
+                                  int              sigma);
 
 float shell_blur_effect_get_brightness (ShellBlurEffect *self);
 void shell_blur_effect_set_brightness (ShellBlurEffect *self,