67 lines
1.9 KiB
GLSL
67 lines
1.9 KiB
GLSL
#version 330 core
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layout(location = 0) out vec4 fragColor;
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in vec2 texcoord;
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in vec4 cloud_color;
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in vec4 ap_color;
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uniform sampler2D base_tex;
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uniform mat4 osg_ProjectionMatrix;
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uniform vec4 fg_Viewport;
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uniform vec3 fg_SunDirection;
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uniform float density = 30.0;
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uniform float max_sample_dist = 0.05;
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const int STEPS = 8;
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// aerial_perspective.glsl
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vec3 mix_aerial_perspective(vec3 color, vec4 ap);
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void main()
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{
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vec4 base = texture(base_tex, texcoord);
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// Directly discard fragments below a threshold
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if (base.a < 0.02)
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discard;
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// Pixel position in screen space [-1, 1]
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vec2 screen_uv = ((gl_FragCoord.xy - fg_Viewport.xy) / fg_Viewport.zw) * 2.0 - 1.0;
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// XXX: Sun's screen-space position. This should be passed as an uniform
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vec4 sun_dir_screen = osg_ProjectionMatrix * vec4(fg_SunDirection, 0.0);
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sun_dir_screen.xyz /= sun_dir_screen.w;
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sun_dir_screen.xyz = normalize(sun_dir_screen.xyz);
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// Direction from pixel to Sun in screen space
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vec2 sun_dir = screen_uv - sun_dir_screen.xy;
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// Flip the x axis
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sun_dir.x = -sun_dir.x;
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float dt = max_sample_dist / STEPS;
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// 2D ray march along the Sun's direction to estimate the transmittance
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float T = 1.0;
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for (int i = 0; i < STEPS; ++i) {
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float t = (float(i) + 0.5) * dt;
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vec2 uv_t = texcoord - sun_dir * t;
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vec4 texel = texture(base_tex, uv_t);
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// Beer-Lambert's law
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T *= exp(-texel.a * dt * density);
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}
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// When the camera is facing perpendicularly to the Sun, the Sun's
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// screen-space location can tend toward infinity. Fade the effect toward
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// the perpendicular.
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float fade = smoothstep(0.1, 0.5, dot(vec3(0.0, 0.0, -1.0), fg_SunDirection));
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vec4 color = base * cloud_color;
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color.rgb *= base.a * mix(1.0, T, fade);
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color.rgb = mix_aerial_perspective(color.rgb, ap_color);
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fragColor = color;
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}
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