/** 
 * @file WLCloudsV.glsl
 *
 * $LicenseInfo:firstyear=2005&license=viewerlgpl$
 * Second Life Viewer Source Code
 * Copyright (C) 2005, 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 mat4 modelview_projection_matrix;

ATTRIBUTE vec3 position;
ATTRIBUTE vec2 texcoord0;

//////////////////////////////////////////////////////////////////////////
// The vertex shader for creating the atmospheric sky
///////////////////////////////////////////////////////////////////////////////

// Output parameters
VARYING vec4 vary_CloudColorSun;
VARYING vec4 vary_CloudColorAmbient;
VARYING float vary_CloudDensity;
VARYING vec2 vary_texcoord0;
VARYING vec2 vary_texcoord1;
VARYING vec2 vary_texcoord2;
VARYING vec2 vary_texcoord3;

// Inputs
uniform vec3 camPosLocal;

uniform vec4 lightnorm;
uniform vec4 sunlight_color;
uniform vec4 ambient;
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 max_y;

uniform vec4 glow;
uniform float sun_up_factor;

uniform vec4 cloud_color;

uniform float cloud_scale;

void main()
{

    // World / view / projection
    gl_Position = modelview_projection_matrix * vec4(position.xyz, 1.0);

    vary_texcoord0 = texcoord0;

    // Get relative position
    vec3 P = position.xyz - camPosLocal.xyz + vec3(0,50,0);

    // Set altitude
    if (P.y > 0.)
    {
        P *= (max_y / P.y);
    }
    else
    {
        P *= (-32000. / P.y);
    }

    // Can normalize then
    vec3 Pn = normalize(P);
    float  Plen = length(P);

    // Initialize temp variables
    vec4 temp1 = vec4(0.);
    vec4 temp2 = vec4(0.);
    vec4 blue_weight;
    vec4 haze_weight;
    vec4 sunlight = sunlight_color;
    vec4 light_atten;


    // 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)) * (density_multiplier * max_y);

    // Calculate relative weights
    temp1 = blue_density + haze_density;
    blue_weight = blue_density / temp1;
    haze_weight = haze_density / temp1;

    // Compute sunlight from P & lightnorm (for long rays like sky)
    temp2.y = max(0., max(0., Pn.y) * 1.0 + lightnorm.y );
    temp2.y = 1. / temp2.y;
    sunlight *= exp( - light_atten * temp2.y);

    // Distance
    temp2.z = Plen * density_multiplier;

    // Transparency (-> temp1)
    // ATI Bugfix -- can't store temp1*temp2.z in a variable because the ati
    // compiler gets confused.
    temp1 = exp(-temp1 * temp2.z);


    // Compute haze glow
    temp2.x = dot(Pn, lightnorm.xyz);
    temp2.x = 1. - temp2.x;
        // temp2.x is 0 at the sun and increases away from sun
    temp2.x = max(temp2.x, .001);   
        // 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

    //temp2.x *= sun_up_factor;

    // Add "minimum anti-solar illumination"
    temp2.x += .25;

    // Increase ambient when there are more clouds
    vec4 tmpAmbient = ambient;
    tmpAmbient += (1. - tmpAmbient) * cloud_shadow * 0.5; 

    // Dim sunlight by cloud shadow percentage
    sunlight *= (1. - cloud_shadow);

    // Haze color below cloud
    vec4 additiveColorBelowCloud = (      blue_horizon * blue_weight * (sunlight + tmpAmbient)
                + (haze_horizon * haze_weight) * (sunlight * temp2.x + tmpAmbient)
             ); 

    // CLOUDS

    sunlight = sunlight_color;
    temp2.y = max(0., lightnorm.y * 2.);
    temp2.y = 1. / temp2.y;
    sunlight *= exp( - light_atten * temp2.y);

    // Cloud color out
    vary_CloudColorSun = (sunlight * temp2.x) * cloud_color;
    vary_CloudColorAmbient = tmpAmbient * cloud_color;
    
    // Attenuate cloud color by atmosphere
    temp1 = sqrt(temp1);    //less atmos opacity (more transparency) below clouds
    vary_CloudColorSun *= temp1;
    vary_CloudColorAmbient *= temp1;
    vec4 oHazeColorBelowCloud = additiveColorBelowCloud * (1. - temp1);

    // Make a nice cloud density based on the cloud_shadow value that was passed in.
    vary_CloudDensity = 2. * (cloud_shadow - 0.25);


    // Texture coords
    vary_texcoord0 = texcoord0;
    vary_texcoord0.xy -= 0.5;
    vary_texcoord0.xy /= max(0.001, cloud_scale);
    vary_texcoord0.xy += 0.5;

    vary_texcoord1 = vary_texcoord0;
    vary_texcoord1.x += lightnorm.x * 0.0125;
    vary_texcoord1.y += lightnorm.z * 0.0125;

    vary_texcoord2 = vary_texcoord0 * 16.;
    vary_texcoord3 = vary_texcoord1 * 16.;

    // Combine these to minimize register use
    vary_CloudColorAmbient += oHazeColorBelowCloud;

    // needs this to compile on mac
    //vary_AtmosAttenuation = vec3(0.0,0.0,0.0);

    // END CLOUDS
}