多维数据集到球体的映射(需要反函数)

本文介绍了多维数据集到球体的映射(需要反函数)的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

我遇到了一个多维数据集到球的映射函数，该函数提供的结果比仅对坐标或其他映射方法进行标准化处理的结果更为统一.不幸的是，没有解包功能.

来源:

从左侧 cube，sphere，cube2 .颜色存储在 col 中，以更好地显示点之间的映射...

cubify 背后的想法是保留最大的坐标不变，而其他两个坐标转换为球面角，然后使用该角作为坐标.基本上与此相反.它在边缘附近有点非线性，因此将45度范围轻微移至较小的范围.此外，为避免线性化后表面上的点交叉，还需要进行另一项检查(末尾为6个ifs).

I've came across a cube to sphere mapping function that provides a more uniform result than just normalizing the coordinates or other mapping methods. Unfortunately there is no unwrapping function.

Source: mathproofs.blogspot/2005/07/mapping-cube-to-sphere.html

vec3 spherify ( vec3 v ) { float x2 = v.x * v.x; float y2 = v.y * v.y; float z2 = v.z * v.z; vec3 s; s.x = v.x * sqrt(1.0 - y2 / 2.0 - z2 / 2.0 + y2 * z2 / 3.0); s.y = v.y * sqrt(1.0 - x2 / 2.0 - z2 / 2.0 + x2 * z2 / 3.0); s.z = v.z * sqrt(1.0 - x2 / 2.0 - y2 / 2.0 + x2 * y2 / 3.0); return s; }

How could this be unwrapped back to a cube face? To wrap back to a square i use the following, though it doesn't unwrap the special squeezing of the coordinates.

vec3 cubify ( vec3 s ) { s.x = ( s.x / s.z ); s.y = ( s.y / s.z ); return s; }

Which looks as the following Mapping

解决方案

How about something like this (C++/VCL):

//--------------------------------------------------------------------------- #include <vcl.h> #pragma hdrstop #include "Unit1.h" #include "gl_simple.h" #include "glsl_math.h" //--------------------------------------------------------------------------- #pragma package(smart_init) #pragma resource "*.dfm" TForm1 *Form1; const int n=10*10*6; vec3 col[n]; vec3 cube[n]; vec3 sphere[n]; vec3 cube2[n]; //--------------------------------------------------------------------------- vec3 spherify(vec3 v) { float x2 = v.x * v.x; float y2 = v.y * v.y; float z2 = v.z * v.z; vec3 s; s.x = v.x * sqrt(1.0 - y2 / 2.0 - z2 / 2.0 + y2 * z2 / 3.0); s.y = v.y * sqrt(1.0 - x2 / 2.0 - z2 / 2.0 + x2 * z2 / 3.0); s.z = v.z * sqrt(1.0 - x2 / 2.0 - y2 / 2.0 + x2 * y2 / 3.0); return s; } //--------------------------------------------------------------------------- vec3 cubify(vec3 v) { int i; float r,a; // major axis and size a=fabs(v.x); { r=a; i=0; } a=fabs(v.y); if (r<a){ r=a; i=1; } a=fabs(v.z); if (r<a){ r=a; i=2; } v/=r; r*=1.75; a=4.0*r/M_PI; // convert of cube + linearization if (i==0){ v.y=a*atan(v.y/r); v.z=a*atan(v.z/r); } else if (i==1){ v.x=a*atan(v.x/r); v.z=a*atan(v.z/r); } else { v.x=a*atan(v.x/r); v.y=a*atan(v.y/r); } // just remedy boundaries after linearization if (v.x<-1.0) v.x=-1.0; if (v.x>+1.0) v.x=+1.0; if (v.y<-1.0) v.y=-1.0; if (v.y>+1.0) v.y=+1.0; if (v.z<-1.0) v.z=-1.0; if (v.z>+1.0) v.z=+1.0; return v; } //--------------------------------------------------------------------------- void set_cube() { float u,v,d; int m=sqrt(n/6),i,j,k; k=0; d=2.0/float(m-1); for (u=-1.0,i=0;i<m;i++,u+=d) for (v=-1.0,j=0;j<m;j++,v+=d) { col[k]=vec3(0.5,0.0,0.0); cube[k]=vec3(u,v,-1.0); k++; col[k]=vec3(1.0,0.0,0.0); cube[k]=vec3(u,v,+1.0); k++; col[k]=vec3(0.0,0.5,0.0); cube[k]=vec3(u,-1.0,v); k++; col[k]=vec3(0.0,1.0,0.0); cube[k]=vec3(u,+1.0,v); k++; col[k]=vec3(0.0,0.0,0.5); cube[k]=vec3(-1.0,u,v); k++; col[k]=vec3(0.0,0.0,1.0); cube[k]=vec3(+1.0,u,v); k++; } } //--------------------------------------------------------------------------- void gl_draw() { glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); float aspect=float(xs)/float(ys); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(60.0/aspect,aspect,0.1,100.0); glMatrixMode(GL_TEXTURE); glLoadIdentity(); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); glTranslatef(0.0,0.0,-10.5); glRotatef(-10.0,1.0,0.0,0.0); glRotatef(-20.0,0.0,1.0,0.0); glEnable(GL_DEPTH_TEST); glDisable(GL_TEXTURE_2D); int i; glPointSize(2); glMatrixMode(GL_MODELVIEW); glTranslatef(-3.0,0.0,0.0); glBegin(GL_POINTS); for (i=0;i<n;i++){ glColor3fv(col[i].dat); glVertex3fv(cube[i].dat); } glEnd(); // set_cube glTranslatef(+3.0,0.0,0.0); glBegin(GL_POINTS); for (i=0;i<n;i++){ glColor3fv(col[i].dat); glVertex3fv(sphere[i].dat); } glEnd(); // spherify glTranslatef(+3.0,0.0,0.0); glBegin(GL_POINTS); for (i=0;i<n;i++){ glColor3fv(col[i].dat); glVertex3fv(cube2[i].dat); } glEnd(); // cubify glEnd(); glPointSize(1); glFlush(); SwapBuffers(hdc); } //--------------------------------------------------------------------------- __fastcall TForm1::TForm1(TComponent* Owner):TForm(Owner) { gl_init(Handle); int i; set_cube(); for (i=0;i<n;i++) sphere[i]=spherify(cube[i]); for (i=0;i<n;i++) cube2[i]=cubify(sphere[i]); } //--------------------------------------------------------------------------- void __fastcall TForm1::FormDestroy(TObject *Sender) { gl_exit(); } //--------------------------------------------------------------------------- void __fastcall TForm1::FormPaint(TObject *Sender) { gl_draw(); } //--------------------------------------------------------------------------- void __fastcall TForm1::Timer1Timer(TObject *Sender) { gl_draw(); } //--------------------------------------------------------------------------- void __fastcall TForm1::FormResize(TObject *Sender) { gl_resize(ClientWidth,ClientHeight); gl_draw(); } //---------------------------------------------------------------------------

Just ignore the VCL stuff. Code creates uniform grid cube points using set_cube , that is converted into sphere using your spherify and that is finally converted to cube2 using mine cubify.

Here preview:

from left cube,sphere,cube2. The colors are stored in col to better show the mapping between points...

The idea behind cubify is to leave biggest coordinate as is and the other two convert into spherical angle and then use this angle as coordinate. Basically its a reverse of this. Its a bit nonlinear near edges hence the slight shifting of 45 deg range to smaller ones... Also to avoid crossings of points above surface after linearization another check is involved (the 6 ifs at the end).