#version 120

/*
 Copyright (C) 2020 MaxED
 
 This file is part of TrenchBroom.
 
 TrenchBroom is free software: you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by
 the Free Software Foundation, either version 3 of the License, or
 (at your option) any later version.
 
 TrenchBroom 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 General Public License for more details.
 
 You should have received a copy of the GNU General Public License
 along with TrenchBroom. If not, see <http://www.gnu.org/licenses/>.
 */

uniform mat4 ModelMatrix;
uniform mat4 ViewMatrix;
uniform vec3 CameraPosition;
uniform vec3 CameraDirection;
uniform vec3 CameraRight;
uniform vec3 CameraUp;

// see Orientation enum in EntityModel.h
uniform int Orientation;

varying vec4 worldCoordinates;

mat4 getScaleMatrix() {
    float sx = length(vec3(ModelMatrix[0]));
    float sy = length(vec3(ModelMatrix[1]));
    float sz = length(vec3(ModelMatrix[2]));

    return mat4(
        vec4(sx,  0.0, 0.0, 0.0),
        vec4(0.0, sy,  0.0, 0.0),
        vec4(0.0, 0.0, sz,  0.0),
        vec4(0.0, 0.0, 0.0, 1.0)
    );
}

mat4 getViewPlaneParallelUprightModelMatrix() {
    // Faces view plane, up is towards the heavens.
    vec3 right = CameraRight;
    vec3 up = vec3(0.0, 0.0, 1.0);
    vec3 normal = normalize(cross(right, up));

    return mat4(
        vec4(right, 0.0),
        vec4(up, 0.0),
        vec4(normal, 0.0),
        ModelMatrix[3]
    ) * getScaleMatrix();
}

mat4 getFacingUprightModelMatrix() {
    // Faces camera origin, up is towards the heavens.
    vec3 toCam = CameraPosition - vec3(ModelMatrix[3]);
    vec3 up = vec3(0.0, 0.0, 1.0);
    vec3 right = normalize(cross(up, toCam));
    vec3 normal = normalize(cross(right, up));

    return mat4(
        vec4(right, 0.0),
        vec4(up, 0.0),
        vec4(normal, 0.0),
        ModelMatrix[3]
    ) * getScaleMatrix();
}

mat4 getViewPlaneParallelModelMatrix() {
    // Faces view plane, up is towards the top of the screen.
    vec3 normal = -CameraDirection;
    vec3 right = CameraRight;
    vec3 up = CameraUp;

    return mat4(
        vec4(right, 0.0),
        vec4(up, 0.0),
        vec4(normal, 0.0),
        ModelMatrix[3]
    ) * getScaleMatrix();
}

float extractRollAngle(mat4 rotation) {
    if (abs(rotation[0][2]) != 1.0) {
        float theta = -asin(rotation[0][1]);
        float cosTheta = cos(theta);
        return atan(rotation[1][2] / cosTheta, rotation[2][2] / cosTheta);
    }  else if (rotation[0][2] == -1.0) {
        return atan(rotation[1][0], rotation[2][0]);
    } else {
        return atan(-rotation[1][0], -rotation[2][0]);
    }
}

mat4 getViewPlaneParallelOrientedModelMatrix() {
    // Faces view plane, but obeys roll value.

    mat4 transform = mat4(
        ModelMatrix[0],
        ModelMatrix[1],
        ModelMatrix[2],
        vec4(0.0, 0.0, 0.0, 1.0)
    );

    // the rotated unit vectors
    vec3 x = normalize((transform * vec4(1.0, 0.0, 0.0, 1.0)).xyz);
    vec3 y = normalize(cross((transform * vec4(0.0, 0.0, 1.0, 1.0)).xyz, x));
    vec3 z = normalize(cross(x, y));

    mat4 rotation = mat4(
        vec4(x, 0.0),
        vec4(y, 0.0),
        vec4(z, 0.0),
        vec4(0.0, 0.0, 0.0, 1.0)
    );

    float roll = extractRollAngle(rotation);
    float s = sin(roll);
    float c = cos(roll);

    vec3 normal = -CameraDirection;
    vec3 right = CameraRight * c + CameraUp * s;
    vec3 up = CameraRight * -s + CameraUp * c;

    return mat4(
        vec4(right, 0.0),
        vec4(up, 0.0),
        vec4(normal, 0.0),
        ModelMatrix[3]
    ) * getScaleMatrix();
}

mat4 getModelMatrix() {
    if (Orientation == 0) {
        return getViewPlaneParallelUprightModelMatrix();
    } else if (Orientation == 1) {
        return getFacingUprightModelMatrix();
    } else if (Orientation == 2) {
        return getViewPlaneParallelModelMatrix();
    } else if (Orientation == 4) {
        return getViewPlaneParallelOrientedModelMatrix();
    }

    // Pitch yaw roll are independent of camera.
    return ModelMatrix;
}

void main(void) {
    gl_Position = gl_ProjectionMatrix * ViewMatrix * getModelMatrix() * gl_Vertex;
    worldCoordinates = ModelMatrix * gl_Vertex;
    gl_TexCoord[0] = gl_MultiTexCoord0;
}