#version 450 core

layout(quads, fractional_even_spacing, cw) in;

layout(binding = 0) uniform sampler2D heightTexture;

layout(std140, binding = 4) uniform GeoUniforms
{
    mat4 PV;
    mat4 M;
    
    float minTessellation;
    float maxTessellation;
    float displacementScale;
    int gridDensity;
};

in TC_OUT {
    vec3 position;
    vec2 texCoord;
    float tessLevel;
} te_in[];

out TE_OUT {
    vec3 position;
    vec2 texCoord;
    vec3 normal;
} te_out;

float getHeight(vec2 uv) {
    return texture(heightTexture, uv).r * displacementScale;
}

float getNeighbor(float tessFactor) {
    return (float(gridDensity) * tessFactor + 1.0f);
}

vec3 calculateSmoothNormal(vec3 pos, vec2 coord) {
    // Currently hardcoded to match patch density
    //float step = 1.0f / (float(gridDensity) * tessLevel + 1.0f);
    float stepU = 1.0f / (float(gridDensity) * gl_TessLevelInner[0]);
    float stepV = 1.0f / (float(gridDensity) * gl_TessLevelInner[1]);

    // Find surrounding heights
    float hLeft = getHeight(coord + vec2(-stepU, 0.0f));
    float hRight = getHeight(coord + vec2(stepU, 0.0f));
    float hUp = getHeight(coord + vec2(0.0f, stepV));
    float hDown = getHeight(coord + vec2(0.0f, -stepV));
    
    // Calculate tangents
    vec3 tangentX = normalize(vec3(stepU, hRight - hLeft, 0.0));
    vec3 tangentY = normalize(vec3(0.0, hUp - hDown, stepV));

    return normalize(cross(tangentY, tangentX));
}

void main() {
    // Interpolate texture coordinates
    vec2 texCoord = mix(
        mix(te_in[0].texCoord, te_in[1].texCoord, gl_TessCoord[0]),
        mix(te_in[3].texCoord, te_in[2].texCoord, gl_TessCoord[0]),
        gl_TessCoord[1]
    );

    // Interpolate the original position
    vec3 position = mix(
        mix(te_in[0].position, te_in[1].position, gl_TessCoord[0]),
        mix(te_in[3].position, te_in[2].position, gl_TessCoord[0]),
        gl_TessCoord[1]
    );

    position.z += texture(heightTexture, texCoord).r * displacementScale;

    te_out.position = (M*vec4(position, 1.0f)).xyz; // Position in world-space
    te_out.texCoord = texCoord;
    te_out.normal   = calculateSmoothNormal(position, texCoord);  
    gl_Position = PV*M*vec4(position, 1.0f);        // Position in screen-space
}