Opencv step by step - 基本数据类型

CvArr,CvMat,IplImage这三者是继承的关系。

打开opencv 3.0的源码：

cvArr

/* CvArr* is used to pass arbitrary

* array-like data structures

* into functions where the particular

* array type is recognized at runtime:

*/

typedef void CvArr;

oh god，它是可变类型的数据

cvMat

再看一下cvMat。

typedef struct CvMat

{

    int type;

    int step;

    /* for internal use only */

    int* refcount;

    int hdr_refcount;

    union

    {

        uchar* ptr;

        short* s;

        int* i;

        float* fl;

        double* db;

    } data;

#ifdef __cplusplus

    union

    {

        int rows;

        int height;

    };

    union

    {

        int cols;

        int width;

    };

#else

    int rows;

    int cols;

#endif

#ifdef __cplusplus

    CvMat() {}

    CvMat(const CvMat& m) { memcpy(this, &m, sizeof(CvMat));}

    CvMat(const cv::Mat& m);

#endif

}

CvMat;

这里先不管C++了，纯c的做法是：

typedef struct CvMat

{

    int type;

    int step;

    /* for internal use only */

    int* refcount;

    int hdr_refcount;

    union

    {

        uchar* ptr;

        short* s;

        int* i;

        float* fl;

        double* db;

    } data;

    int rows;

    int cols;

}

CvMat;

这里，cvMat有几个属性，到底是干什么的呢？

可以看看它的初始化函数：

CV_INLINE CvMat cvMat( int rows, int cols, int type, void* data CV_DEFAULT(NULL))

{

    CvMat m;

    assert( (unsigned)CV_MAT_DEPTH(type) <= CV_64F );

    type = CV_MAT_TYPE(type);

    m.type = CV_MAT_MAGIC_VAL | CV_MAT_CONT_FLAG | type;

    m.cols = cols;

    m.rows = rows;

    m.step = m.cols*CV_ELEM_SIZE(type);

    m.data.ptr = (uchar*)data;

    m.refcount = NULL;

    m.hdr_refcount = 0;

    return m;

}

看了之后恍然大悟：

type是矩阵中每一个数据点的类型。

cols是矩阵的列数。

rows当然是行数。

step是每一行的数据大小(应该是byte数吧）。

data是一个union的类型，里面是不同类型的指针，这样做的目的应该是可以接收不同的数据类型吧。

比如你传了一个float*的指针进去，他就是float*了，反正这个联合体的大小就是sizeof(pointer) = 4了

internal use的先不管了。

在其他版本中把rows和height写在union里面的做法应该是做变量的兼容性吧。

IplImage

下面来重点看一下这个结构体:

typedef struct

#ifdef __cplusplus

  CV_EXPORTS

#endif

_IplImage

{

    int  nSize;             /* sizeof(IplImage) */

    int  ID;                /* version (=0)*/

    int  nChannels;         /* Most of OpenCV functions support 1,2,3 or 4 channels */

    int  alphaChannel;      /* Ignored by OpenCV */

    int  depth;             /* Pixel depth in bits: IPL_DEPTH_8U, IPL_DEPTH_8S, IPL_DEPTH_16S,

                               IPL_DEPTH_32S, IPL_DEPTH_32F and IPL_DEPTH_64F are supported.  */

    char colorModel[4];     /* Ignored by OpenCV */

    char channelSeq[4];     /* ditto */

    int  dataOrder;         /* 0 - interleaved color channels, 1 - separate color channels.

                               cvCreateImage can only create interleaved images */

    int  origin;            /* 0 - top-left origin,

                               1 - bottom-left origin (Windows bitmaps style).  */

    int  align;             /* Alignment of image rows (4 or 8).

                               OpenCV ignores it and uses widthStep instead.    */

    int  width;             /* Image width in pixels.                           */

    int  height;            /* Image height in pixels.                          */

    struct _IplROI *roi;    /* Image ROI. If NULL, the whole image is selected. */

    struct _IplImage *maskROI;      /* Must be NULL. */

    void  *imageId;                 /* "           " */

    struct _IplTileInfo *tileInfo;  /* "           " */

    int  imageSize;         /* Image data size in bytes

                               (==image->height*image->widthStep

                               in case of interleaved data)*/

    char *imageData;        /* Pointer to aligned image data.         */

    int  widthStep;         /* Size of aligned image row in bytes.    */

    int  BorderMode[4];     /* Ignored by OpenCV.                     */

    int  BorderConst[4];    /* Ditto.                                 */

    char *imageDataOrigin;  /* Pointer to very origin of image data

                               (not necessarily aligned) -

                               needed for correct deallocation */

#ifdef __cplusplus

    _IplImage() {}

    _IplImage(const cv::Mat& m);

#endif

}

IplImage;

可以看出来，这个结构体和cvMat很像。

下面通过实际操作来understand一下这个结构体：

#include <cv.h>

#include <highgui.h>

#define u8 unsigned char

void image_operation(IplImage *image)

{

	printf("nchannels:%d\n",image->nChannels);

	printf("depth:%d\n",image->depth);

	/* ./modules/core/include/opencv2/core/types_c.h:

	 *		#define IPL_DATA_ORDER_PIXEL  0

	 */

	printf("dataOrder:%d\n",image->dataOrder);

	printf("width:%d\n",image->width);

	printf("height:%d\n",image->height);

	printf("origin:%d\n",image->origin);

	printf("widthStep:%d\n",image->widthStep);

	for (int y = 0; y<image->height ; y++) {

		u8* row = (u8*)(image->imageData + y*image->widthStep);

		for(int x = 0; x<image->width; x++) {

			row[3*x + 1] = 0;

			row[3*x + 2] = 0;

		}

	}

}

int main(int argc,char **argv)

{

	printf("this is image transforming plus\n");

        IplImage *image;

        image = cvLoadImage(argv[1]);

        if(argc != 2)

        {

            std::cout << "No image data\n";

            return -1;

        }

	cvNamedWindow("image input");

	cvNamedWindow("image output");

	cvShowImage("image input", image);

	image_operation(image);

	cvShowImage("image output", image);

	cvReleaseImage(&image);

	cvWaitKey(0);

	cvDestroyWindow("image input");

	cvDestroyWindow("image output");

	return 0;

}

这里读取一个图像，并且将两个通道变成0，结果如下：

tan@ubuntu:~/cv$ ./TransformImage image/1.jpg

this is image transforming plus

nchannels:3

depth:8

dataOrder:0

width:400

height:300

origin:0

widthStep:1200

从结果来看，这里把蓝色通道提取了，再次验证一下就可以发现opencv的存储是 BGR 顺序存储的。

重要的一个变量是image->widthStep，因为opencv会把图像数据4字节对齐，每过一行需要加上这个值而不是自己进行处理。

巴特西

Opencv step by step - 基本数据类型

cvArr

cvMat

IplImage

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