#ifdef __cplusplus extern c #endif 的作用

其实很简单，作用就是
如果此头文件是在c++中使用的（就是一个.cpp文件include此头文件，而不是一个.c文件），
那么函数名称粉碎方式仍然使用C中的方式。(就是此函数编译后在object文件中的内部名称和C中是兼容的)

比如
void foo1();
如果c文件包含了它，生成的object文件中此函数被命名为 c_foo1,
而在cpp文件中包含了它，生成的object中此函数命名为cpp_foo1,使用extern "C"后，可以强制使其仍然为"c_foo1"。

这么做的目的是为了在c和cpp中互相调用函数命名一致，否则会出现链接错误。

/******************************************************************************   ImageUtils*   Handy utility functions for dealing with images in OpenCV (desktop or Android).*   The most recent version of this will always be available from "http://shervinemami.info/openCV.html"*******************************************************************************   by Shervin Emami, 5th Dec 2012 ([e-mail])*   http://www.shervinemami.info/*******************************************************************************   Ch1 of the book "Mastering OpenCV with Practical Computer Vision Projects"*   Copyright Packt Publishing 2012.*   http://www.packtpub.com/cool-projects-with-opencv/book*****************************************************************************/#ifndef IMAGEUTILS_0_7_H_#define IMAGEUTILS_0_7_H_// OpenCV#include <cv.h>//#include <cvaux.h>#include <cxcore.h>#ifdef USE_HIGHGUI    #include <highgui.h>#endif#include <stdio.h>#if defined WIN32 || defined _WIN32    #include <conio.h>        // For _kbhit() on Windows    #include <direct.h>        // For mkdir(path) on Windows    #define snprintf sprintf_s    // Visual Studio on Windows comes with sprintf_s() instead of snprintf()#else    #include <stdio.h>        // For getchar() on Linux    #include <termios.h>    // For kbhit() on Linux    #include <unistd.h>    #include <sys/types.h>    #include <sys/stat.h>    // For mkdir(path, options) on Linux#endif#include <vector>#include <string>#include <iostream>            // for printing streams in C++//#include <stdarg.h>        // for variable-length args in C//#include <sstream>        // for printing floats in C++//#include <fstream>        // for opening files in C++// These functions will print using the LOG() function, using the same format as printf(). If you want it to be printed using a different// function (such as for Android logcat output), then define LOG as your output function, otherwise it will use printf() by default.#ifndef LOG    #ifdef __ANDROID__        // For Android debug logging to logcat:        #include <android/log.h>        #define LOG(fmt, args...) (__android_log_print(ANDROID_LOG_INFO, "........", fmt, ## args))    #else        // For stdout debug logging, with a new-line character on the end:        #ifndef _MSC_VER            // Compiles on GCC but not MSVC:            #define LOG(fmt, args...) do {printf(fmt, ## args); printf("\n"); fflush(stdout);} while (0)    //        #define LOG printf        #else            #define LOG printf        #endif    #endif#endif#ifdef __cplusplusextern "C"{#endif// Allow 'UCHAR' variables in both C and C++ code.#ifndef UCHAR    typedef unsigned char UCHAR;#endif// Allow 'bool' variables in both C and C++ code.#ifndef __cplusplus    typedef int bool;    #define true (1)    #define false (0)#endif// Allow default args in C++ code, but explicit args in C code.#ifdef __cplusplus    #define DEFAULT(val) = val#else    #define DEFAULT(val)#endif//------------------------------------------------------------------------------// Timer functions//------------------------------------------------------------------------------// Record the execution time of some code, in milliseconds. By Shervin Emami, 27th May 2012.// eg://  DECLARE_TIMING(myTimer);//    ...//  START_TIMING(myTimer);//    printf("A slow calc = %f\n", pow(1.2, 3.4) );//  STOP_TIMING(myTimer);//  AVERAGE_TIMING(myTimer);#define DECLARE_TIMING(s)           int64 timeStart_##s; int64 timeDiff_##s; int64 timeTally_##s = 0; int64 countTally_##s = 0; double timeMin_##s = DBL_MAX; double timeMax_##s = 0; int64 timeEnd_##s;#define START_TIMING(s)             timeStart_##s = cvGetTickCount()#define STOP_TIMING(s)              do {    timeEnd_##s = cvGetTickCount(); timeDiff_##s = (timeEnd_##s - timeStart_##s); timeTally_##s += timeDiff_##s; countTally_##s++; timeMin_##s = MIN(timeMin_##s, timeDiff_##s); timeMax_##s = MAX(timeMax_##s, timeDiff_##s);    } while (0)#define GET_TIMING(s)               (double)(0.001 * ( (double)timeDiff_##s / (double)cvGetTickFrequency() ))#define GET_MIN_TIMING(s)           (double)(0.001 * ( (double)timeMin_##s / (double)cvGetTickFrequency() ))#define GET_MAX_TIMING(s)           (double)(0.001 * ( (double)timeMax_##s / (double)cvGetTickFrequency() ))#define GET_AVERAGE_TIMING(s)       (double)(countTally_##s ? 0.001 * ( (double)timeTally_##s / ((double)countTally_##s * cvGetTickFrequency()) ) : 0)#define GET_TOTAL_TIMING(s)         (double)(0.001 * ( (double)timeTally_##s / ((double)cvGetTickFrequency()) ))#define GET_TIMING_COUNT(s)         (int)(countTally_##s)#define CLEAR_AVERAGE_TIMING(s)     do {    timeTally_##s = 0; countTally_##s = 0;     } while (0)#define SHOW_TIMING(s, msg)         LOG("%s time:\t %dms\t (ave=%dms min=%dms max=%dms, across %d runs).", msg, cvRound(GET_TIMING(s)), cvRound(GET_AVERAGE_TIMING(s)), cvRound(GET_MIN_TIMING(s)), cvRound(GET_MAX_TIMING(s)), GET_TIMING_COUNT(s) )#define SHOW_TOTAL_TIMING(s, msg)   LOG("%s total:\t %dms\t (ave=%dms min=%dms max=%dms, across %d runs).", msg, cvRound(GET_TOTAL_TIMING(s)), cvRound(GET_AVERAGE_TIMING(s)), cvRound(GET_MIN_TIMING(s)), cvRound(GET_MAX_TIMING(s)), GET_TIMING_COUNT(s) )#define AVERAGE_TIMING(s)           SHOW_TIMING(s, #s)#define TOTAL_TIMING(s)             do {    SHOW_TOTAL_TIMING(s, #s); CLEAR_AVERAGE_TIMING(s);     } while (0)// Convert a float number to an int by rounding to nearest int using a certain method.// Replace the code with the fastest method detected by 'testTiming_FloatConversion()'.inline int roundFloat(float f);//------------------------------------------------------------------------------// Graphing functions//------------------------------------------------------------------------------// Draw the graph of an array of floats into imageDst or a new image, between minV & maxV if given.// Remember to free the newly created image if imageDst is not given.IplImage* drawFloatGraph(const float *arraySrc, int nArrayLength, IplImage *imageDst DEFAULT(0), float minV DEFAULT(0.0), float maxV DEFAULT(0.0), int width DEFAULT(0), int height DEFAULT(0), char *graphLabel DEFAULT(0), bool showScale DEFAULT(true));// Draw the graph of an array of ints into imageDst or a new image, between minV & maxV if given.// Remember to free the newly created image if imageDst is not given.IplImage* drawIntGraph(const int *arraySrc, int nArrayLength, IplImage *imageDst DEFAULT(0), int minV DEFAULT(0), int maxV DEFAULT(0), int width DEFAULT(0), int height DEFAULT(0), char *graphLabel DEFAULT(0), bool showScale DEFAULT(true));// Draw the graph of an array of uchars into imageDst or a new image, between minV & maxV if given.// Remember to free the newly created image if imageDst is not given.IplImage* drawUCharGraph(const uchar *arraySrc, int nArrayLength, IplImage *imageDst DEFAULT(0), int minV DEFAULT(0), int maxV DEFAULT(0), int width DEFAULT(0), int height DEFAULT(0), char *graphLabel DEFAULT(0), bool showScale DEFAULT(true));// Display a graph of the given float array.// If background is provided, it will be drawn into, for combining multiple graphs using drawFloatGraph().// Set delay_ms to 0 if you want to wait forever until a keypress, or set it to 1 if you want it to delay just 1 millisecond.void showFloatGraph(const char *name, const float *arraySrc, int nArrayLength, int delay_ms DEFAULT(500), IplImage *background DEFAULT(0));// Display a graph of the given int array.// If background is provided, it will be drawn into, for combining multiple graphs using drawIntGraph().// Set delay_ms to 0 if you want to wait forever until a keypress, or set it to 1 if you want it to delay just 1 millisecond.void showIntGraph(const char *name, const int *arraySrc, int nArrayLength, int delay_ms DEFAULT(500), IplImage *background DEFAULT(0));// Display a graph of the given unsigned char array.// If background is provided, it will be drawn into, for combining multiple graphs using drawUCharGraph().// Set delay_ms to 0 if you want to wait forever until a keypress, or set it to 1 if you want it to delay just 1 millisecond.void showUCharGraph(const char *name, const uchar *arraySrc, int nArrayLength, int delay_ms DEFAULT(500), IplImage *background DEFAULT(0));// Simple helper function to easily view an image, with an optional pause.void showImage(const IplImage *img, int delay_ms DEFAULT(0), char *name DEFAULT(0));// Call 'setGraphColor(0)' to reset the colors that will be used for graphs.void setGraphColor(int index DEFAULT(0));// Specify the exact color that the next graph should be drawn as.void setCustomGraphColor(int R, int B, int G);//------------------------------------------------------------------------------// Color conversion functions//------------------------------------------------------------------------------// Return a new image that is always greyscale, whether the input image was RGB or Greyscale.// Remember to free the returned image using cvReleaseImage() when finished.IplImage* convertImageToGreyscale(const IplImage *imageSrc);// Create an RGB image from the YIQ image using an approximation of NTSC conversion(ref: "YIQ" Wikipedia page).// Remember to free the generated RGB image.IplImage* convertImageYIQtoRGB(const IplImage *imageYIQ);// Create a YIQ image from the RGB image using an approximation of NTSC conversion(ref: "YIQ" Wikipedia page).// Remember to free the generated YIQ image.IplImage* convertImageRGBtoYIQ(const IplImage *imageRGB);// Create an RGB image from the HSV image using the full 8-bits, since OpenCV only allows Hues up to 180 instead of 255.// ref: "http://cs.haifa.ac.il/hagit/courses/ist/Lectures/Demos/ColorApplet2/t_convert.html"// Remember to free the generated RGB image.IplImage* convertImageHSVtoRGB(const IplImage *imageHSV);// Create a HSV image from the RGB image using the full 8-bits, since OpenCV only allows Hues up to 180 instead of 255.// ref: "http://cs.haifa.ac.il/hagit/courses/ist/Lectures/Demos/ColorApplet2/t_convert.html"// Remember to free the generated HSV image.IplImage* convertImageRGBtoHSV(const IplImage *imageRGB);// Do the color conversion of a single pixel, from RGB to HSV using Hue values between 0 to 255, whereas OpenCV only allows Hues up to 180 instead of 255.// ref: "http://cs.haifa.ac.il/hagit/courses/ist/Lectures/Demos/ColorApplet2/t_convert.html"inline void convertPixelRGBtoHSV_256(int bR, int bG, int bB, int &bH, int &bS, int &bV);// Do the color conversion of a single pixel, from HSV to RGB using Hue values between 0 to 255, whereas OpenCV only allows Hues up to 180 instead of 255.// ref: "http://cs.haifa.ac.il/hagit/courses/ist/Lectures/Demos/ColorApplet2/t_convert.html"inline void convertPixelHSVtoRGB_256(int bH, int bS, int bV, int &bR, int &bG, int &bB);// Do the color conversion of a single pixel, from RGB to HSV using Hue values between 0 to 179 just like OpenCV does, instead of Hues upto 255.void convertPixelRGBtoHSV_180(int bR, int bG, int bB, int &bH, int &bS, int &bV);// Do the color conversion of a single pixel, from HSV to RGB using Hue values between 0 to 179 just like OpenCV does, instead of Hues upto 255.void convertPixelHSVtoRGB_180(int bH, int bS, int bV, int &bR, int &bG, int &bB);//------------------------------------------------------------------------------// 2D Point functions//------------------------------------------------------------------------------// Return (pointA + pointB).CvPoint2D32f addPointF(const CvPoint2D32f pointA, const CvPoint2D32f pointB);// Return (pointA - pointB).CvPoint2D32f subtractPointF(const CvPoint2D32f pointA, const CvPoint2D32f pointB);// Return (point * scale).CvPoint2D32f scalePointF(const CvPoint2D32f point, float scale);// Return the point scaled relative to the given origin.CvPoint2D32f scalePointAroundPointF(const CvPoint2D32f point, const CvPoint2D32f origin, float scale);// Return (p * s), to a maximum value of maxValfloat scaleValueF(float p, float s, float maxVal);#define scaleValueFI(p, s, maxVal) scaleValueF(p, s, (float)maxVal)// Return (p * s), to a maximum value of maxValint scaleValueI(int p, float s, int maxVal);// Return the point rotated around its origin by angle (in degrees).CvPoint2D32f rotatePointF(const CvPoint2D32f point, float angleDegrees);// Return the point rotated around the given origin by angle (in degrees).CvPoint2D32f rotatePointAroundPointF(const CvPoint2D32f point, const CvPoint2D32f origin, float angleDegrees);// Calculate the distance between the 2 given (floating-point) points.float findDistanceBetweenPointsF(const CvPoint2D32f p1, const CvPoint2D32f p2);// Calculate the distance between the 2 given (integer) points.float findDistanceBetweenPointsI(const CvPoint p1, const CvPoint p2);// Calculate the angle between the 2 given integer points (in degrees).float findAngleBetweenPointsF(const CvPoint2D32f p1, const CvPoint2D32f p2);// Calculate the angle between the 2 given floating-point points (in degrees).float findAngleBetweenPointsI(const CvPoint p1, const CvPoint p2);// Draw a crossbar at the given position.void drawCross(IplImage *img, const CvPoint pt, int radius, const CvScalar color );// Print the label and then the rect to the console for easy debuggingvoid printPoint(const CvPoint pt, const char *label);// Print the label and then the rect to the console for easy debuggingvoid printPointF(const CvPoint2D32f pt, const char *label);//------------------------------------------------------------------------------// Rectangle region functions//------------------------------------------------------------------------------// Enlarge or shrink the rectangle size & center by a given scale.// If w & h are given, will make sure the rectangle stays within the bounds if it is enlarged too much.// Note that for images, w should be (width-1) and h should be (height-1).CvRect scaleRect(const CvRect rectIn, float scaleX, float scaleY, int w DEFAULT(0), int h DEFAULT(0));// Enlarge or shrink the rectangle region by a given scale without moving its center, and possibly add a border around it.// If w & h are given, will make sure the rectangle stays within the bounds if it is enlarged too much.// Note that for images, w should be (width-1) and h should be (height-1).CvRect scaleRectInPlace(const CvRect rectIn, float scaleX, float scaleY, float borderX DEFAULT(0.0f), float borderY DEFAULT(0.0f), int w DEFAULT(0), int h DEFAULT(0));// Return a new rect that the same as rectA when shifted by (rectB.x, rectB.y).CvRect offsetRect(const CvRect rectA, const CvRect rectB);// Return a new rect that the same as rectA when shifted by (pt.x, pt.y).CvRect offsetRectPt(const CvRect rectA, const CvPoint pt);// Draw a rectangle around the given object. (Use CV_RGB(255,0,0) for red color)void drawRect(IplImage *img, const CvRect rect, const CvScalar color DEFAULT(CV_RGB(220,0,0)));// Draw a filled rectangle around the given object.void drawRectFilled(IplImage *img, const CvRect rect, const CvScalar color DEFAULT(CV_RGB(220,0,0)));// Print the label and then the rectangle information to the console for easy debuggingvoid printRect(const CvRect rect, const char *label DEFAULT(0));// Make sure the given rectangle is completely within the given image dimensions.CvRect cropRect(const CvRect rectIn, int w, int h);//------------------------------------------------------------------------------// Image transforming functions//------------------------------------------------------------------------------// Returns a new image that is a cropped version of the original image.// Remember to free the new image later.IplImage* cropImage(const IplImage *img, const CvRect region);// Creates a new image copy that is of a desired size. The aspect ratio will be kept constant if desired, by cropping the desired region.// Remember to free the new image later.IplImage* resizeImage(const IplImage *origImg, int newWidth, int newHeight, bool keepAspectRatio);// Rotate the image clockwise and possibly scale the image. Use 'mapRotatedImagePoint()' to map pixels from the src to dst image.IplImage *rotateImage(const IplImage *src, float angleDegrees, float scale DEFAULT(1.0f));// Get the position of a pixel in the image after the rotateImage() operation.CvPoint2D32f mapRotatedImagePoint(const CvPoint2D32f pointOrig, const IplImage *image, float angleRadians, float scale DEFAULT(1.0f));//------------------------------------------------------------------------------// Image utility functions//------------------------------------------------------------------------------// Paste multiple images next to each other as a single image, for saving or displaying.// Remember to free the returned image.// Sample usage: cvSaveImage("out.png", combineImages(2, img1, img2) );// Modified by Shervin from the cvShowManyImages() function on the OpenCVWiki by Parameswaran.// 'combineImagesResized()' will resize all images to 300x300, whereas 'combineImages()' doesn't resize the images at all.IplImage* combineImagesResized(int nArgs, ...);IplImage* combineImages(int nArgs, ...);// Do Bilateral Filtering to smooth the image noise but preserve the edges.// A smoothness of 5 is very little filtering, and 100 is very high filtering.// Remember to free the returned image.IplImage* smoothImageBilateral(const IplImage *src, float smoothness DEFAULT(30));// Blend color images 'image1' and 'image2' using an 8-bit alpha-blending mask channel.// Equivalent to this operation on each pixel: imageOut = image1 * (1-(imageAlphaMask/255)) + image2 * (imageAlphaMask/255)// So if a pixel in imageAlphMask is 0, then that pixel in imageOut will be image1, or if imageAlphaMask is 255 then imageOut is image2,// or if imageAlphaMask was 200 then imageOut would be: image1 * 0.78 + image2 * 0.22// Returns a new image, so remember to call 'cvReleaseImage()' on the result.IplImage* blendImage(const IplImage* image1, const IplImage* image2, const IplImage* imageAlphaMask);// Get an 8-bit equivalent of the 32-bit Float Matrix.// Returns a new image, so remember to call 'cvReleaseImage()' on the result.IplImage* convertMatrixToUcharImage(const CvMat *srcMat);// Get a normal 8-bit uchar image equivalent of the 32-bit float greyscale image.// Returns a new image, so remember to call 'cvReleaseImage()' on the result.IplImage* convertFloatImageToUcharImage(const IplImage *srcImg);// Save the given image to a JPG or BMP file, even if its format isn't an 8-bit image, such as a 32bit image.int saveImage(const char *filename, const IplImage *image);// Store a greyscale floating-point CvMat image into a BMP/JPG/GIF/PNG image,// since cvSaveImage() can only handle 8bit images (not 32bit float images).void saveFloatMat(const char *filename, const CvMat *src);// Store a greyscale floating-point CvMat image into a BMP/JPG/GIF/PNG image,// since cvSaveImage() can only handle 8bit images (not 32bit float images).void saveFloatImage(const char *filename, const IplImage *srcImg);// Draw some text onto an image using printf() format.void drawText(IplImage *img, CvPoint position, CvScalar color, char *fmt, ...);// Print the label and then some text info about the IplImage properties, to LOG() for easy debugging.void printImageInfo(const IplImage *image_tile, const char *label DEFAULT(0));// Print the pixel values of the IplImage, to LOG() for easy debugging. maxElements of 0 will print all the data.void printImagePixels(const IplImage *image, const char *label DEFAULT(0), int maxElements DEFAULT(300));// Print the label and then contents of a cv::Mat from the C++ interface (using "LOG()") for easy debugging.void printMat(const cv::Mat M, const char *label DEFAULT(0), int maxElements DEFAULT(300));// Print the label and then contents of the Matrix (using "LOG()") for easy debugging. maxElements of 0 will print all the data.void printMatrix(const CvMat *M, const char *label DEFAULT(0), int maxElements DEFAULT(300));// Print the label and info of a cv::Mat from the C++ interface (using "LOG()") for easy debugging.void printMatInfo(const cv::Mat M, const char *label DEFAULT(0));// Print the x & y coords of the given point. If 'label' is supplied, prints that first, and also prints a newline character on the end.void printPoint32f(const CvPoint2D32f pt, const char *label DEFAULT(0));// Print the start & end coords of the given line. If 'label' is supplied, prints that first, and also prints a newline character on the end.void printLine(const CvPoint ptA, const CvPoint ptB, const char *label DEFAULT(0));// Just for debugging float images & matrices.void printDataRange(const CvArr *src, const char *msg);#if defined (__cplusplus)}#endif#endif /* IMAGEUTILS_H_ */