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許多影像處理軟體都會提供濾鏡效果，其中之一的效果是魚眼鏡頭(Fisheye Lens)，會寫程式且對影像處理感興趣的人，多少會想知道如何實做魚眼效果，所以我就動手來做做看囉！

標準的魚眼鏡頭可以由數學式子推導，詳細內容可參照Fisheye Projection。

網路上已經有人實做出來，所以我直接拿來玩:P前提示要先安裝OpenCV喔！

/**
	Theme: Simulate Fisheye Lens
	Compiler: Dev C++ 4.9.9.2
	Date: 101/01/03
	Author: HappyMan
	Blog: https://cg2010studio.wordpress.com/
*/
#include <cv.h>
#include <highgui.h>
#include <math.h>
#include <unistd.h>
#include <getopt.h>
#include <iostream>
void sampleImage(const IplImage* arr, float idx0, float idx1, CvScalar& res){
  if(idx0<0 || idx1<0 || idx0>(cvGetSize(arr).height-1) || idx1>(cvGetSize(arr).width-1)){
    res.val[0]=0;
    res.val[1]=0;
    res.val[2]=0;
    res.val[3]=0;
    return;
  }
  float idx0_fl=floor(idx0);
  float idx0_cl=ceil(idx0);
  float idx1_fl=floor(idx1);
  float idx1_cl=ceil(idx1);

  CvScalar s1=cvGet2D(arr,(int)idx0_fl,(int)idx1_fl);
  CvScalar s2=cvGet2D(arr,(int)idx0_fl,(int)idx1_cl);
  CvScalar s3=cvGet2D(arr,(int)idx0_cl,(int)idx1_cl);
  CvScalar s4=cvGet2D(arr,(int)idx0_cl,(int)idx1_fl);
  float x = idx0 - idx0_fl;
  float y = idx1 - idx1_fl;
  res.val[0]= s1.val[0]*(1-x)*(1-y) + s2.val[0]*(1-x)*y + s3.val[0]*x*y + s4.val[0]*x*(1-y);
  res.val[1]= s1.val[1]*(1-x)*(1-y) + s2.val[1]*(1-x)*y + s3.val[1]*x*y + s4.val[1]*x*(1-y);
  res.val[2]= s1.val[2]*(1-x)*(1-y) + s2.val[2]*(1-x)*y + s3.val[2]*x*y + s4.val[2]*x*(1-y);
  res.val[3]= s1.val[3]*(1-x)*(1-y) + s2.val[3]*(1-x)*y + s3.val[3]*x*y + s4.val[3]*x*(1-y);
}
float xscale;
float yscale;
float xshift;
float yshift;
float getRadialX(float x,float y,float cx,float cy,float k){
  x = (x*xscale+xshift);
  y = (y*yscale+yshift);
  float res = x+((x-cx)*k*((x-cx)*(x-cx)+(y-cy)*(y-cy)));
  return res;
}
float getRadialY(float x,float y,float cx,float cy,float k){
  x = (x*xscale+xshift);
  y = (y*yscale+yshift);
  float res = y+((y-cy)*k*((x-cx)*(x-cx)+(y-cy)*(y-cy)));
  return res;
}
float thresh = 1;
float calc_shift(float x1,float x2,float cx,float k){
  float x3 = x1+(x2-x1)*0.5;
  float res1 = x1+((x1-cx)*k*((x1-cx)*(x1-cx)));
  float res3 = x3+((x3-cx)*k*((x3-cx)*(x3-cx)));
  //  std::cerr<<"x1: "<<x1<<" - "<<res1<<" x3: "<<x3<<" - "<<res3<<std::endl;
  if(res1>-thresh and res1 < thresh)
    return x1;
  if(res3<0){
    return calc_shift(x3,x2,cx,k);
  }
  else{
    return calc_shift(x1,x3,cx,k);
  }
}
int main(int argc, char** argv){
  IplImage* src = cvLoadImage( argv[1], 1 );
  IplImage* dst = cvCreateImage(cvGetSize(src),src->depth,src->nChannels);
  IplImage* dst2 = cvCreateImage(cvGetSize(src),src->depth,src->nChannels);
  float K=atof(argv[3]);
  float centerX=atoi(argv[4]);
  float centerY=atoi(argv[5]);
  int width = cvGetSize(src).width;
  int height = cvGetSize(src).height;

  xshift = calc_shift(0,centerX-1,centerX,K);
  float newcenterX = width-centerX;
  float xshift_2 = calc_shift(0,newcenterX-1,newcenterX,K);

  yshift = calc_shift(0,centerY-1,centerY,K);
  float newcenterY = height-centerY;
  float yshift_2 = calc_shift(0,newcenterY-1,newcenterY,K);
  //  scale = (centerX-xshift)/centerX;
  xscale = (width-xshift-xshift_2)/width;
  yscale = (height-yshift-yshift_2)/height;

  std::cerr<<xshift<<" "<<yshift<<" "<<xscale<<" "<<yscale<<std::endl;
  std::cerr<<cvGetSize(src).height<<std::endl;
  std::cerr<<cvGetSize(src).width<<std::endl;

  for(int j=0;j<cvGetSize(dst).height;j++){
    for(int i=0;i<cvGetSize(dst).width;i++){
      CvScalar s;
      float x = getRadialX((float)i,(float)j,centerX,centerY,K);
      float y = getRadialY((float)i,(float)j,centerX,centerY,K);
      sampleImage(src,y,x,s);
      cvSet2D(dst,j,i,s);
    }
  }
#if 0
  cvNamedWindow( "Source1", 1 );
  cvShowImage( "Source1", dst);
  cvWaitKey(0);
#endif
  cvSaveImage(argv[2],dst,0);
#if 0
  for(int j=0;j<cvGetSize(src).height;j++){
    for(int i=0;i<cvGetSize(src).width;i++){
      CvScalar s;
      sampleImage(src,j+0.25,i+0.25,s);
      cvSet2D(dst,j,i,s);
    }
  }
  cvNamedWindow( "Source1", 1 );
  cvShowImage( "Source1", src);
  cvWaitKey(0);
#endif
}

編譯後產生執行檔，可以直接在命令提示字元中下指令，格式有五個參數：|input image| |output image| |K which controlls amount of distortion (typically try values around 0.001)| |x coordinate of center of distortion| |y coordinate of center of distortion|

我使用黑白相間的棋盤，來看看魚眼效果如何。

C:\test>xx chessboard.jpg “1 382 382.jpg" 1 382 382
374.785 374.785 0.0188681 0.0188867
764
765

C:\test>xx chessboard.jpg “0.1 382 382.jpg" 0.1 382 382
366.571 366.571 0.0403572 0.040391
764
765

C:\test>xx chessboard.jpg “0.01 382 382.jpg" 0.01 382 382
349.281 349.281 0.0855572 0.0856518
764
765

C:\test>xx chessboard.jpg “0.001 382 382.jpg" 0.001 382 382
314.027 314.027 0.177814 0.177939
764
765

C:\test>xx chessboard.jpg “0.0001 382 382.jpg" 0.0001 382 382
246.776 246.776 0.353744 0.35399
764
765

例子：xx是執行檔的名字，chessboard.jpg是輸入影像名稱，"0.001 382 382.jpg"是輸出影像名字，0.001是變形參數，382 382分別是x和y的變形中心點。

變形參數大於0.001效果就變得很奇怪，0表示無任何變形。原本想要使用負數來表現反魚眼效果，可惜該數學模式沒辦法做到。

參考：Fisheye Projection、How to simulate fisheye lens effect by openCV?。