#version 300 es
// Copyright 2020 The Tilt Brush Authors
// Updated to OpenGL ES 3.0 by the Icosa Gallery Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Brush-specific shader for GlTF web preview, based on General generator
// with parameters lit=1, a=1.

precision mediump float;

out vec4 fragColor;

in vec4 v_color;
in vec3 v_position;
in vec2 v_texcoord0;
uniform sampler2D u_MainTex;

// Copyright 2020 The Tilt Brush Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Fogging support
uniform vec3 u_fogColor;
uniform float u_fogDensity;
in float f_fog_coord;

// This fog function emulates the exponential fog used in Tilt Brush
//
// Details:
//   * For exponential fog, Unity defines u_density = density / ln(2) on the CPU, sp that they can
//        convert the base from e to 2 and use exp2 rather than exp. We might as well do the same.
//   * The fog on Plya does not precisely match that in Unity, though it's very close.  Two known
//        reasons for this are
//          1) Clipping plans on Poly are different than in Tilt Brush.  Poly is .1:2000, Tilt
//             Brush is .5:10000.
//          2) Poly applies post processing (vignettes, etc...) that can subtly change the look
//             of the fog.
//   * Finally, Tilt Brush uses "decimeters" for legacy reasons.
//        In order to convert Density values from TB to Poly, we multiply by 10.0 in order to
//        convert decimeters to meters.
//

vec3 ApplyFog(vec3 color) {
  // Per the top comment, we must modify the density value by Unity's ln(2) modification as well as
  // a decimeter conversion
  float density = (u_fogDensity / .693147) * 10.;

  // This exponential fog function is copied directly from unity (see UnityCG.inc).
  float fogFactor = f_fog_coord * density;
  fogFactor = exp2(-fogFactor);
  fogFactor = clamp( fogFactor, 0.0, 1.0 );
  return mix(u_fogColor, color.xyz, fogFactor);
}

void main() {
  vec4 brush_tex = texture(u_MainTex, v_texcoord0);
  fragColor.rgb = ApplyFog(brush_tex.a * v_color.rgb);
  fragColor.a = 1.0;
}