Merged in euclid-12978 (pull request #286)

SL-12978 Refactor sun intensity calculation for smooth transitions.

Approved-by: Michael Pohoreski

indra/newview/app_settings/shaders/class1/windlight/atmosphericsFuncs.glsl

-/** 
+/**
  * @file class1\windlight\atmosphericsFuncs.glsl
  *
  * $LicenseInfo:firstyear=2005&license=viewerlgpl$
  * Second Life Viewer Source Code
  * Copyright (C) 2019, Linden Research, Inc.
- * 
+ *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public
  * License as published by the Free Software Foundation;
  * version 2.1 of the License only.
- * 
+ *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Lesser General Public License for more details.
- * 
+ *
  * You should have received a copy of the GNU Lesser General Public
  * License along with this library; if not, write to the Free Software
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
- * 
+ *
  * Linden Research, Inc., 945 Battery Street, San Francisco, CA  94111  USA
  * $/LicenseInfo$
  */
-uniform vec4 lightnorm;
-uniform vec4 sunlight_color;
-uniform vec4 moonlight_color;
-uniform int sun_up_factor;
-uniform vec4 ambient_color;
-uniform vec4 blue_horizon;
-uniform vec4 blue_density;
+uniform vec4  lightnorm;
+uniform vec4  sunlight_color;
+uniform vec4  moonlight_color;
+uniform int   sun_up_factor;
+uniform vec4  ambient_color;
+uniform vec4  blue_horizon;
+uniform vec4  blue_density;
 uniform float haze_horizon;
 uniform float haze_density;
 uniform float cloud_shadow;
 uniform float density_multiplier;
 uniform float distance_multiplier;
 uniform float max_y;
-uniform vec4 glow;
+uniform vec4  glow;
 uniform float scene_light_strength;
-uniform mat3 ssao_effect_mat;
-uniform int no_atmo;
+uniform mat3  ssao_effect_mat;
+uniform int   no_atmo;
 uniform float sun_moon_glow_factor;
 
-float getAmbientClamp()
-{
-    return 1.0f;
-}
+float getAmbientClamp() { return 1.0f; }
 
-
-void calcAtmosphericVars(vec3 inPositionEye, vec3 light_dir, float ambFactor, out vec3 sunlit, out vec3 amblit, out vec3 additive, out vec3 atten, bool use_ao)
+void calcAtmosphericVars(vec3 inPositionEye, vec3 light_dir, float ambFactor, out vec3 sunlit, out vec3 amblit, out vec3 additive,
+                         out vec3 atten, bool use_ao)
 {
-    vec3 P = inPositionEye;
-   
-    //(TERRAIN) limit altitude
-    if (P.y >  max_y) P *= ( max_y / P.y);
-    if (P.y < -max_y) P *= (-max_y / P.y);
-
-    vec3 tmpLightnorm = lightnorm.xyz;
+    vec3 rel_pos = inPositionEye;
 
-    vec3 Pn = normalize(P);
-    float Plen = length(P);
-
-    vec4 temp1 = vec4(0);
-    vec3 temp2 = vec3(0);
-    vec4 blue_weight;
-    vec4 haze_weight;
-    vec4 sunlight = (sun_up_factor == 1) ? sunlight_color : moonlight_color;
-    vec4 light_atten;
+    //(TERRAIN) limit altitude
+    rel_pos.y = clamp(rel_pos.y, -max_y, max_y);
 
-    float dens_mul = density_multiplier;
-    float dist_mul = distance_multiplier;
+    vec3  rel_pos_norm = normalize(rel_pos);
+    float rel_pos_len  = length(rel_pos);
+    vec4  sunlight     = (sun_up_factor == 1) ? sunlight_color : moonlight_color;
 
-    //sunlight attenuation effect (hue and brightness) due to atmosphere
-    //this is used later for sunlight modulation at various altitudes
-    light_atten = (blue_density + vec4(haze_density * 0.25)) * (dens_mul * max_y);
-        //I had thought blue_density and haze_density should have equal weighting,
-        //but attenuation due to haze_density tends to seem too strong
+    // sunlight attenuation effect (hue and brightness) due to atmosphere
+    // this is used later for sunlight modulation at various altitudes
+    vec4 light_atten = (blue_density + vec4(haze_density * 0.25)) * (density_multiplier * max_y);
+    // I had thought blue_density and haze_density should have equal weighting,
+    // but attenuation due to haze_density tends to seem too strong
 
-    temp1 = blue_density + vec4(haze_density);
-    blue_weight = blue_density / temp1;
-    haze_weight = vec4(haze_density) / temp1;
+    vec4 combined_haze = blue_density + vec4(haze_density);
+    vec4 blue_weight   = blue_density / combined_haze;
+    vec4 haze_weight   = vec4(haze_density) / combined_haze;
 
-    //(TERRAIN) compute sunlight from lightnorm only (for short rays like terrain)
-    temp2.y = max(0.0, tmpLightnorm.y);
-    if (abs(temp2.y) > 0.000001f)
-    {
-        temp2.y = 1. / abs(temp2.y);
-    }
-    temp2.y = max(0.0000001f, temp2.y);
-    sunlight *= exp(-light_atten * temp2.y);
+    //(TERRAIN) compute sunlight from lightnorm y component. Factor is roughly cosecant(sun elevation) (for short rays like terrain)
+    float above_horizon_factor = 1.0 / max(1e-6, lightnorm.y);
+    sunlight *= exp(-light_atten * above_horizon_factor);  // for sun [horizon..overhead] this maps to an exp curve [0..1]
 
     // main atmospheric scattering line integral
-    temp2.z = Plen * dens_mul;
+    float density_dist = rel_pos_len * density_multiplier;
 
-    // Transparency (-> temp1)
-    // ATI Bugfix -- can't store temp1*temp2.z*dist_mul in a variable because the ati
+    // Transparency (-> combined_haze)
+    // ATI Bugfix -- can't store combined_haze*density_dist*distance_multiplier in a variable because the ati
     // compiler gets confused.
-    temp1 = exp(-temp1 * temp2.z * dist_mul);
+    combined_haze = exp(-combined_haze * density_dist * distance_multiplier);
+
+    // final atmosphere attenuation factor
+    atten = combined_haze.rgb;
 
-    //final atmosphere attenuation factor
-    atten = temp1.rgb;
-    
-    //compute haze glow
-    //(can use temp2.x as temp because we haven't used it yet)
-    temp2.x = dot(Pn, tmpLightnorm.xyz);
+    // compute haze glow
+    float haze_glow = dot(rel_pos_norm, lightnorm.xyz);
 
     // dampen sun additive contrib when not facing it...
-	// SL-13539: This "if" clause causes an "additive" white artifact at roughly 77 degreees.
+    // SL-13539: This "if" clause causes an "additive" white artifact at roughly 77 degreees.
     //    if (length(light_dir) > 0.01)
-    {
-        temp2.x *= max(0.0f, dot(light_dir, Pn));
-    }
-    temp2.x = 1. - temp2.x;
-        //temp2.x is 0 at the sun and increases away from sun
-    temp2.x = max(temp2.x, .001);    //was glow.y
-        //set a minimum "angle" (smaller glow.y allows tighter, brighter hotspot)
-    temp2.x *= glow.x;
-        //higher glow.x gives dimmer glow (because next step is 1 / "angle")
-    temp2.x = pow(temp2.x, glow.z);
-        //glow.z should be negative, so we're doing a sort of (1 / "angle") function
-
-    //add "minimum anti-solar illumination"
-    temp2.x += .25;
-
-    temp2.x *= sun_moon_glow_factor;
- 
-    vec4 amb_color = ambient_color; 
-    
-    //increase ambient when there are more clouds
+    haze_glow *= max(0.0f, dot(light_dir, rel_pos_norm));
+
+    haze_glow = 1. - haze_glow;
+    // haze_glow is 0 at the sun and increases away from sun
+    haze_glow = max(haze_glow, .001);  // set a minimum "angle" (smaller glow.y allows tighter, brighter hotspot)
+    haze_glow *= glow.x;
+    // higher glow.x gives dimmer glow (because next step is 1 / "angle")
+    haze_glow = pow(haze_glow, glow.z);
+    // glow.z should be negative, so we're doing a sort of (1 / "angle") function
+
+    // add "minimum anti-solar illumination"
+    haze_glow += .25;
+
+    haze_glow *= sun_moon_glow_factor;
+
+    vec4 amb_color = ambient_color;
+
+    // increase ambient when there are more clouds
     vec4 tmpAmbient = amb_color + (vec4(1.) - amb_color) * cloud_shadow * 0.5;
-    
+
     /*  decrease value and saturation (that in HSV, not HSL) for occluded areas
      * // for HSV color/geometry used here, see http://gimp-savvy.com/BOOK/index.html?node52.html
      * // The following line of code performs the equivalent of:
      * float ambAlpha = tmpAmbient.a;
      * float ambValue = dot(vec3(tmpAmbient), vec3(0.577)); // projection onto <1/rt(3), 1/rt(3), 1/rt(3)>, the neutral white-black axis
      * vec3 ambHueSat = vec3(tmpAmbient) - vec3(ambValue);
-     * tmpAmbient = vec4(RenderSSAOEffect.valueFactor * vec3(ambValue) + RenderSSAOEffect.saturationFactor *(1.0 - ambFactor) * ambHueSat, ambAlpha);
+     * tmpAmbient = vec4(RenderSSAOEffect.valueFactor * vec3(ambValue) + RenderSSAOEffect.saturationFactor *(1.0 - ambFactor) * ambHueSat,
+     * ambAlpha);
      */
     if (use_ao)
     {
@@ -145,13 +123,12 @@ void calcAtmosphericVars(vec3 inPositionEye, vec3 light_dir, float ambFactor, ou
     // Similar/Shared Algorithms:
     //     indra\llinventory\llsettingssky.cpp                                        -- LLSettingsSky::calculateLightSettings()
     //     indra\newview\app_settings\shaders\class1\windlight\atmosphericsFuncs.glsl -- calcAtmosphericVars()
-    //haze color
-        vec3 cs  = sunlight.rgb * (1.-cloud_shadow);
-        additive = (blue_horizon.rgb * blue_weight.rgb) * (cs           + tmpAmbient.rgb)
-                 + (haze_horizon     * haze_weight.rgb) * (cs * temp2.x + tmpAmbient.rgb);
+    // haze color
+    vec3 cs = sunlight.rgb * (1. - cloud_shadow);
+    additive = (blue_horizon.rgb * blue_weight.rgb) * (cs + tmpAmbient.rgb) + (haze_horizon * haze_weight.rgb) * (cs * haze_glow + tmpAmbient.rgb);
 
-    //brightness of surface both sunlight and ambient
+    // brightness of surface both sunlight and ambient
     sunlit = sunlight.rgb * 0.5;
     amblit = tmpAmbient.rgb * .25;
-    additive *= vec3(1.0 - temp1);
+    additive *= vec3(1.0 - combined_haze);
 }