/**
 * @author Almar Klein / http://almarklein.org
 *
 * Shaders to render 3D volumes using raycasting.
 * The applied techniques are based on similar implementations in the Visvis and Vispy projects.
 * This is not the only approach, therefore it's marked 1.
 */

THREE.VolumeRenderShader1 = {
	uniforms: {
				"u_size": { value: new THREE.Vector3( 1, 1, 1 ) },
				"u_renderstyle": { value: 0 },
				"u_renderthreshold": { value: 0.5 },
				"u_clim": { value: new THREE.Vector2( 1, 1 ) },
				"u_data": { value: null },
				"u_cmdata": { value: null }
		},
		vertexShader: [
				'varying vec4 v_nearpos;',
				'varying vec4 v_farpos;',
				'varying vec3 v_position;',

				'mat4 inversemat(mat4 m) {',
						// Taken from https://github.com/stackgl/glsl-inverse/blob/master/index.glsl
						// This function is licenced by the MIT license to Mikola Lysenko
						'float',
						'a00 = m[0][0], a01 = m[0][1], a02 = m[0][2], a03 = m[0][3],',
						'a10 = m[1][0], a11 = m[1][1], a12 = m[1][2], a13 = m[1][3],',
						'a20 = m[2][0], a21 = m[2][1], a22 = m[2][2], a23 = m[2][3],',
						'a30 = m[3][0], a31 = m[3][1], a32 = m[3][2], a33 = m[3][3],',

						'b00 = a00 * a11 - a01 * a10,',
						'b01 = a00 * a12 - a02 * a10,',
						'b02 = a00 * a13 - a03 * a10,',
						'b03 = a01 * a12 - a02 * a11,',
						'b04 = a01 * a13 - a03 * a11,',
						'b05 = a02 * a13 - a03 * a12,',
						'b06 = a20 * a31 - a21 * a30,',
						'b07 = a20 * a32 - a22 * a30,',
						'b08 = a20 * a33 - a23 * a30,',
						'b09 = a21 * a32 - a22 * a31,',
						'b10 = a21 * a33 - a23 * a31,',
						'b11 = a22 * a33 - a23 * a32,',

						'det = b00 * b11 - b01 * b10 + b02 * b09 + b03 * b08 - b04 * b07 + b05 * b06;',

				'return mat4(',
						'a11 * b11 - a12 * b10 + a13 * b09,',
						'a02 * b10 - a01 * b11 - a03 * b09,',
						'a31 * b05 - a32 * b04 + a33 * b03,',
						'a22 * b04 - a21 * b05 - a23 * b03,',
						'a12 * b08 - a10 * b11 - a13 * b07,',
						'a00 * b11 - a02 * b08 + a03 * b07,',
						'a32 * b02 - a30 * b05 - a33 * b01,',
						'a20 * b05 - a22 * b02 + a23 * b01,',
						'a10 * b10 - a11 * b08 + a13 * b06,',
						'a01 * b08 - a00 * b10 - a03 * b06,',
						'a30 * b04 - a31 * b02 + a33 * b00,',
						'a21 * b02 - a20 * b04 - a23 * b00,',
						'a11 * b07 - a10 * b09 - a12 * b06,',
						'a00 * b09 - a01 * b07 + a02 * b06,',
						'a31 * b01 - a30 * b03 - a32 * b00,',
						'a20 * b03 - a21 * b01 + a22 * b00) / det;',
				'}',


				'void main() {',
						// Prepare transforms to map to "camera view". See also:
						// https://threejs.org/docs/#api/renderers/webgl/WebGLProgram
						'mat4 viewtransformf = viewMatrix;',
						'mat4 viewtransformi = inversemat(viewMatrix);',

						// Project local vertex coordinate to camera position. Then do a step
						// backward (in cam coords) to the near clipping plane, and project back. Do
						// the same for the far clipping plane. This gives us all the information we
						// need to calculate the ray and truncate it to the viewing cone.
						'vec4 position4 = vec4(position, 1.0);',
						'vec4 pos_in_cam = viewtransformf * position4;',

						// Intersection of ray and near clipping plane (z = -1 in clip coords)
						'pos_in_cam.z = -pos_in_cam.w;',
						'v_nearpos = viewtransformi * pos_in_cam;',

						// Intersection of ray and far clipping plane (z = +1 in clip coords)
						'pos_in_cam.z = pos_in_cam.w;',
						'v_farpos = viewtransformi * pos_in_cam;',

						// Set varyings and output pos
						'v_position = position;',
						'gl_Position = projectionMatrix * viewMatrix * modelMatrix * position4;',
				'}',
		].join( '\n' ),
	fragmentShader: [
				'precision highp float;',
				'precision mediump sampler3D;',

				'uniform vec3 u_size;',
				'uniform int u_renderstyle;',
				'uniform float u_renderthreshold;',
				'uniform vec2 u_clim;',

				'uniform sampler3D u_data;',
				'uniform sampler2D u_cmdata;',

				'varying vec3 v_position;',
				'varying vec4 v_nearpos;',
				'varying vec4 v_farpos;',

				// The maximum distance through our rendering volume is sqrt(3).
				'const int MAX_STEPS = 887;	// 887 for 512^3, 1774 for 1024^3',
				'const int REFINEMENT_STEPS = 4;',
				'const float relative_step_size = 1.0;',
				'const vec4 ambient_color = vec4(0.2, 0.4, 0.2, 1.0);',
				'const vec4 diffuse_color = vec4(0.8, 0.2, 0.2, 1.0);',
				'const vec4 specular_color = vec4(1.0, 1.0, 1.0, 1.0);',
				'const float shininess = 40.0;',

				'void cast_mip(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray);',
				'void cast_iso(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray);',

				'float sample1(vec3 texcoords);',
				'vec4 apply_colormap(float val);',
				'vec4 add_lighting(float val, vec3 loc, vec3 step, vec3 view_ray);',


				'void main() {',
						// Normalize clipping plane info
						'vec3 farpos = v_farpos.xyz / v_farpos.w;',
						'vec3 nearpos = v_nearpos.xyz / v_nearpos.w;',

						// Calculate unit vector pointing in the view direction through this fragment.
						'vec3 view_ray = normalize(nearpos.xyz - farpos.xyz);',

						// Compute the (negative) distance to the front surface or near clipping plane.
						// v_position is the back face of the cuboid, so the initial distance calculated in the dot
						// product below is the distance from near clip plane to the back of the cuboid
						'float distance = dot(nearpos - v_position, view_ray);',
						'distance = max(distance, min((-0.5 - v_position.x) / view_ray.x,',
																				'(u_size.x - 0.5 - v_position.x) / view_ray.x));',
						'distance = max(distance, min((-0.5 - v_position.y) / view_ray.y,',
																				'(u_size.y - 0.5 - v_position.y) / view_ray.y));',
						'distance = max(distance, min((-0.5 - v_position.z) / view_ray.z,',
																				'(u_size.z - 0.5 - v_position.z) / view_ray.z));',

																				// Now we have the starting position on the front surface
						'vec3 front = v_position + view_ray * distance;',

						// Decide how many steps to take
						'int nsteps = int(-distance / relative_step_size + 0.5);',
						'if ( nsteps < 1 )',
								'discard;',

						// Get starting location and step vector in texture coordinates
						'vec3 step = ((v_position - front) / u_size) / float(nsteps);',
						'vec3 start_loc = front / u_size;',

						// For testing: show the number of steps. This helps to establish
						// whether the rays are correctly oriented
						//'gl_FragColor = vec4(0.0, float(nsteps) / 1.0 / u_size.x, 1.0, 1.0);',
						//'return;',

						'if (u_renderstyle == 0)',
								'cast_mip(start_loc, step, nsteps, view_ray);',
						'else if (u_renderstyle == 1)',
								'cast_iso(start_loc, step, nsteps, view_ray);',

						'if (gl_FragColor.a < 0.05)',
								'discard;',
				'}',


				'float sample1(vec3 texcoords) {',
						'/* Sample float value from a 3D texture. Assumes intensity data. */',
						'return texture(u_data, texcoords.xyz).r;',
				'}',


				'vec4 apply_colormap(float val) {',
						'val = (val - u_clim[0]) / (u_clim[1] - u_clim[0]);',
						'return texture2D(u_cmdata, vec2(val, 0.5));',
				'}',


				'void cast_mip(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray) {',

						'float max_val = -1e6;',
						'int max_i = 100;',
						'vec3 loc = start_loc;',

						// Enter the raycasting loop. In WebGL 1 the loop index cannot be compared with
						// non-constant expression. So we use a hard-coded max, and an additional condition
						// inside the loop.
						'for (int iter=0; iter<MAX_STEPS; iter++) {',
								'if (iter >= nsteps)',
										'break;',
								// Sample from the 3D texture
								'float val = sample1(loc);',
								// Apply MIP operation
								'if (val > max_val) {',
										'max_val = val;',
										'max_i = iter;',
								'}',
								// Advance location deeper into the volume
								'loc += step;',
						'}',

						// Refine location, gives crispier images
						'vec3 iloc = start_loc + step * (float(max_i) - 0.5);',
						'vec3 istep = step / float(REFINEMENT_STEPS);',
						'for (int i=0; i<REFINEMENT_STEPS; i++) {',
								'max_val = max(max_val, sample1(iloc));',
								'iloc += istep;',
						'}',

						// Resolve final color
						'gl_FragColor = apply_colormap(max_val);',
				'}',


				'void cast_iso(vec3 start_loc, vec3 step, int nsteps, vec3 view_ray) {',

						'gl_FragColor = vec4(0.0);	// init transparent',
						'vec4 color3 = vec4(0.0);	// final color',
						'vec3 dstep = 1.5 / u_size;	// step to sample derivative',
						'vec3 loc = start_loc;',

						'float low_threshold = u_renderthreshold - 0.02 * (u_clim[1] - u_clim[0]);',

						// Enter the raycasting loop. In WebGL 1 the loop index cannot be compared with
						// non-constant expression. So we use a hard-coded max, and an additional condition
						// inside the loop.
						'for (int iter=0; iter<MAX_STEPS; iter++) {',
								'if (iter >= nsteps)',
										'break;',

										// Sample from the 3D texture
								'float val = sample1(loc);',

								'if (val > low_threshold) {',
								// Take the last interval in smaller steps
										'vec3 iloc = loc - 0.5 * step;',
										'vec3 istep = step / float(REFINEMENT_STEPS);',
										'for (int i=0; i<REFINEMENT_STEPS; i++) {',
												'val = sample1(iloc);',
												'if (val > u_renderthreshold) {',
														'gl_FragColor = add_lighting(val, iloc, dstep, view_ray);',
														'return;',
												'}',
												'iloc += istep;',
										'}',
								'}',

								// Advance location deeper into the volume
								'loc += step;',
						'}',
				'}',


				'vec4 add_lighting(float val, vec3 loc, vec3 step, vec3 view_ray)',
				'{',
						// Calculate color by incorporating lighting

						// View direction
						'vec3 V = normalize(view_ray);',

						// calculate normal vector from gradient
						'vec3 N;',
						'float val1, val2;',
						'val1 = sample1(loc + vec3(-step[0], 0.0, 0.0));',
						'val2 = sample1(loc + vec3(+step[0], 0.0, 0.0));',
						'N[0] = val1 - val2;',
						'val = max(max(val1, val2), val);',
						'val1 = sample1(loc + vec3(0.0, -step[1], 0.0));',
						'val2 = sample1(loc + vec3(0.0, +step[1], 0.0));',
						'N[1] = val1 - val2;',
						'val = max(max(val1, val2), val);',
						'val1 = sample1(loc + vec3(0.0, 0.0, -step[2]));',
						'val2 = sample1(loc + vec3(0.0, 0.0, +step[2]));',
						'N[2] = val1 - val2;',
						'val = max(max(val1, val2), val);',

						'float gm = length(N); // gradient magnitude',
						'N = normalize(N);',

						// Flip normal so it points towards viewer
						'float Nselect = float(dot(N, V) > 0.0);',
						'N = (2.0 * Nselect - 1.0) * N;	// ==	Nselect * N - (1.0-Nselect)*N;',

						// Init colors
						'vec4 ambient_color = vec4(0.0, 0.0, 0.0, 0.0);',
						'vec4 diffuse_color = vec4(0.0, 0.0, 0.0, 0.0);',
						'vec4 specular_color = vec4(0.0, 0.0, 0.0, 0.0);',

						// note: could allow multiple lights
						'for (int i=0; i<1; i++)',
						'{',
								 // Get light direction (make sure to prevent zero devision)
								'vec3 L = normalize(view_ray);	//lightDirs[i];',
								'float lightEnabled = float( length(L) > 0.0 );',
								'L = normalize(L + (1.0 - lightEnabled));',

								// Calculate lighting properties
								'float lambertTerm = clamp(dot(N, L), 0.0, 1.0);',
								'vec3 H = normalize(L+V); // Halfway vector',
								'float specularTerm = pow(max(dot(H, N), 0.0), shininess);',

								// Calculate mask
								'float mask1 = lightEnabled;',

								// Calculate colors
								'ambient_color +=	mask1 * ambient_color;	// * gl_LightSource[i].ambient;',
								'diffuse_color +=	mask1 * lambertTerm;',
								'specular_color += mask1 * specularTerm * specular_color;',
						'}',

						// Calculate final color by componing different components
						'vec4 final_color;',
						'vec4 color = apply_colormap(val);',
						'final_color = color * (ambient_color + diffuse_color) + specular_color;',
						'final_color.a = color.a;',
						'return final_color;',
				'}',
	].join( '\n' )
};
