OpenCV剖析(2)

imread探究

imread是opencv用于图片读取的一个函数

Mat imread( const String& filename, int flags )

Mat imread( const String& filename, int flags )
{
    CV_TRACE_FUNCTION();

    /// create the basic container
    Mat img;

    /// load the data
    imread_( filename, flags, LOAD_MAT, &img );

    /// optionally rotate the data if EXIF' orientation flag says so
    if( !img.empty() && (flags & IMREAD_IGNORE_ORIENTATION) == 0 && flags != IMREAD_UNCHANGED )
    {
        ApplyExifOrientation(filename, img);
    }

    /// return a reference to the data
    return img;
}

对imread的解读可以参见博文

【opencv】opencv源码分析（一）：imread、cvLoadImage、waitKey、imshow函数

通过对比可以发现对比多了一个对EXIF旋转信息参数的读取，并且对图片进行一个旋转操作

if( !img.empty() && (flags & IMREAD_IGNORE_ORIENTATION) == 0 && flags != IMREAD_UNCHANGED )
{
    ApplyExifOrientation(filename, img);
}

imread_

static void*
imread_( const String& filename, int flags, int hdrtype, Mat* mat=0 )
{
    IplImage* image = 0;
    CvMat *matrix = 0;
    Mat temp, *data = &temp;

    /// Search for the relevant decoder to handle the imagery
    ImageDecoder decoder;

#ifdef HAVE_GDAL
    if(flags != IMREAD_UNCHANGED && (flags & IMREAD_LOAD_GDAL) == IMREAD_LOAD_GDAL ){
        decoder = GdalDecoder().newDecoder();
    }else{
#endif
        decoder = findDecoder( filename ); //解读1
#ifdef HAVE_GDAL
    }
#endif

    /// if no decoder was found, return nothing.
    if( !decoder ){
        return 0;
    }

    int scale_denom = 1;
    if( flags > IMREAD_LOAD_GDAL )
    {
    if( flags & IMREAD_REDUCED_GRAYSCALE_2 )
        scale_denom = 2;
    else if( flags & IMREAD_REDUCED_GRAYSCALE_4 )
        scale_denom = 4;
    else if( flags & IMREAD_REDUCED_GRAYSCALE_8 )
        scale_denom = 8;
    }

    /// set the scale_denom in the driver
    decoder->setScale( scale_denom );

    /// set the filename in the driver
    decoder->setSource( filename );

    CV_TRY
    {
        // read the header to make sure it succeeds
        if( !decoder->readHeader() )
            return 0;
    }
    CV_CATCH (cv::Exception, e)
    {
        std::cerr << "imread_('" << filename << "'): can't read header: " << e.what() << std::endl << std::flush;
        return 0;
    }
    CV_CATCH_ALL
    {
        std::cerr << "imread_('" << filename << "'): can't read header: unknown exception" << std::endl << std::flush;
        return 0;
    }


    // established the required input image size
    Size size = validateInputImageSize(Size(decoder->width(), decoder->height()));

    // grab the decoded type
    int type = decoder->type();
    //type是通过readHeader函数读取出来的

    if( (flags & IMREAD_LOAD_GDAL) != IMREAD_LOAD_GDAL && flags != IMREAD_UNCHANGED )
    {
        if( (flags & CV_LOAD_IMAGE_ANYDEPTH) == 0 )
            type = CV_MAKETYPE(CV_8U, CV_MAT_CN(type));

        if( (flags & CV_LOAD_IMAGE_COLOR) != 0 ||
           ((flags & CV_LOAD_IMAGE_ANYCOLOR) != 0 && CV_MAT_CN(type) > 1) )
            type = CV_MAKETYPE(CV_MAT_DEPTH(type), 3);
        else
            type = CV_MAKETYPE(CV_MAT_DEPTH(type), 1);
    }

    if( hdrtype == LOAD_CVMAT || hdrtype == LOAD_MAT )
    {
        if( hdrtype == LOAD_CVMAT )
        {
            matrix = cvCreateMat( size.height, size.width, type );
            temp = cvarrToMat( matrix );
        }
        else
        {
            mat->create( size.height, size.width, type );
            data = mat;
        }
    }
    else
    {
        image = cvCreateImage( size, cvIplDepth(type), CV_MAT_CN(type) );
        temp = cvarrToMat( image );
    }

    // read the image data
    bool success = false;
    CV_TRY
    {
        if (decoder->readData(*data))
            success = true;
    }
    CV_CATCH (cv::Exception, e)
    {
        std::cerr << "imread_('" << filename << "'): can't read data: " << e.what() << std::endl << std::flush;
    }
    CV_CATCH_ALL
    {
        std::cerr << "imread_('" << filename << "'): can't read data: unknown exception" << std::endl << std::flush;
    }
    if (!success)
    {
        cvReleaseImage( &image );
        cvReleaseMat( &matrix );
        if( mat )
            mat->release();
        return 0;
    }

    if( decoder->setScale( scale_denom ) > 1 ) // if decoder is JpegDecoder then decoder->setScale always returns 1
    {
        resize( *mat, *mat, Size( size.width / scale_denom, size.height / scale_denom ), 0, 0, INTER_LINEAR_EXACT);
    }

    return hdrtype == LOAD_CVMAT ? (void*)matrix :
        hdrtype == LOAD_IMAGE ? (void*)image : (void*)mat;
}

解读1

static ImageDecoder findDecoder( const String& filename ) {

    size_t i, maxlen = 0;

    /// iterate through list of registered codecs
    for( i = 0; i < codecs.decoders.size(); i++ )
    {
        size_t len = codecs.decoders[i]->signatureLength();
        maxlen = std::max(maxlen, len);
    }

    /// Open the file
    FILE* f= fopen( filename.c_str(), "rb" );

    /// in the event of a failure, return an empty image decoder
    if( !f )
        return ImageDecoder();

    // read the file signature
    String signature(maxlen, ' ');
    maxlen = fread( (void*)signature.c_str(), 1, maxlen, f );
    //通过maxlen提取signature

    fclose(f);
    signature = signature.substr(0, maxlen);

    /// compare signature against all decoders
    for( i = 0; i < codecs.decoders.size(); i++ )
    {
        if( codecs.decoders[i]->checkSignature(signature) )
            return codecs.decoders[i]->newDecoder();
        //通过比较signature来判断解码器
    }

    /// If no decoder was found, return base type
    return ImageDecoder();
}

struct ImageCodecInitializer
{
    /**
     * Default Constructor for the ImageCodeInitializer
    */
    ImageCodecInitializer()
    {
        /// BMP Support
        decoders.push_back( makePtr<BmpDecoder>() );
        encoders.push_back( makePtr<BmpEncoder>() );

        decoders.push_back( makePtr<HdrDecoder>() );
        encoders.push_back( makePtr<HdrEncoder>() );
    #ifdef HAVE_JPEG
        decoders.push_back( makePtr<JpegDecoder>() );
        encoders.push_back( makePtr<JpegEncoder>() );
    #endif
    #ifdef HAVE_WEBP
        decoders.push_back( makePtr<WebPDecoder>() );
        encoders.push_back( makePtr<WebPEncoder>() );
    #endif
        decoders.push_back( makePtr<SunRasterDecoder>() );
        encoders.push_back( makePtr<SunRasterEncoder>() );
        decoders.push_back( makePtr<PxMDecoder>() );
        encoders.push_back( makePtr<PxMEncoder>(PXM_TYPE_AUTO) );
        encoders.push_back( makePtr<PxMEncoder>(PXM_TYPE_PBM) );
        encoders.push_back( makePtr<PxMEncoder>(PXM_TYPE_PGM) );
        encoders.push_back( makePtr<PxMEncoder>(PXM_TYPE_PPM) );
    #ifdef HAVE_TIFF
        decoders.push_back( makePtr<TiffDecoder>() );
        encoders.push_back( makePtr<TiffEncoder>() );
    #endif
    #ifdef HAVE_PNG
        decoders.push_back( makePtr<PngDecoder>() );
        encoders.push_back( makePtr<PngEncoder>() );
    #endif
    #ifdef HAVE_GDCM
        decoders.push_back( makePtr<DICOMDecoder>() );
    #endif
    #ifdef HAVE_JASPER
        decoders.push_back( makePtr<Jpeg2KDecoder>() );
        encoders.push_back( makePtr<Jpeg2KEncoder>() );
    #endif
    #ifdef HAVE_OPENEXR
        decoders.push_back( makePtr<ExrDecoder>() );
        encoders.push_back( makePtr<ExrEncoder>() );
    #endif

    #ifdef HAVE_GDAL
        /// Attach the GDAL Decoder
        decoders.push_back( makePtr<GdalDecoder>() );
    #endif/*HAVE_GDAL*/
        decoders.push_back( makePtr<PAMDecoder>() );
        encoders.push_back( makePtr<PAMEncoder>() );
    }

    std::vector<ImageDecoder> decoders;
    std::vector<ImageEncoder> encoders;
};

static ImageCodecInitializer codecs;

codecs是个静态变量，其构造函数将不同的解码器指针压到vector中

typedef Ptr< BaseImageDecoder> ImageDecoder;

ImageDecoder是一个虚基类指针

class BaseImageDecoder
{
public:
    BaseImageDecoder();
    virtual ~BaseImageDecoder() {}

    int width() const { return m_width; }
    int height() const { return m_height; }
    virtual int type() const { return m_type; }

    virtual bool setSource( const String& filename );
    virtual bool setSource( const Mat& buf );
    virtual int setScale( const int& scale_denom );
    virtual bool readHeader() = 0;
    virtual bool readData( Mat& img ) = 0;

    /// Called after readData to advance to the next page, if any.
    virtual bool nextPage() { return false; }

    virtual size_t signatureLength() const;
    virtual bool checkSignature( const String& signature ) const;
    virtual ImageDecoder newDecoder() const;

protected:
    int  m_width;  // width  of the image ( filled by readHeader )
    int  m_height; // height of the image ( filled by readHeader )
    int  m_type;
    int  m_scale_denom;
    String m_filename;
    String m_signature;
    Mat m_buf;
    bool m_buf_supported;
};

按照上面的参考博文仍然以JpegDecoder为例
首先是获取文件的signature

m_signature = "\xFF\xD8\xFF";//Jpeg图片的signature

bool checkSignature(const String& signature) const

bool BaseImageDecoder::checkSignature( const String& signature ) const
{
    size_t len = signatureLength();
    return signature.size() >= len && memcmp( signature.c_str(), m_signature.c_str(), len ) == 0;
}

memcm函数说明

int memcmp ( const void * ptr1, const void * ptr2, size_t num );

比较指针指向地址中的num字节
与strcmp函数不同的是，该函数遇到’\0’字符不会停止比较
返回值说明

返回值	说明
<0	ptr1指向地址字节的ASCII码小于ptr2
=0	ptr1与ptr2指向地址字节的ASCII吗相同
0	ptr1>ptr2

opencv3源码比较与opencv2有有改动但是大体流程和上述参考博文中总结的是一致的

输入filename—>解析图片—>确定译码器—>译码函数进行信息、数据的读取—>存放于Mat容器—>返回

imwrite探究

imwrite函数定义

bool imwrite( const String& filename, InputArray _img,
              const std::vector<int>& params )
{
    CV_TRACE_FUNCTION();
    std::vector<Mat> img_vec;
    //Did we get a Mat or a vector of Mats?
    if (_img.isMat())
        img_vec.push_back(_img.getMat());
    else if (_img.isMatVector())
        _img.getMatVector(img_vec);

    return imwrite_(filename, img_vec, params, false);
}

InpuArray 通过成员变量flags判断类型，输入是单个Ma还是vector< Mat >
将img存储到vecto< Mat >中然后调用 imwrite 进行存储

imwrite_

static bool imwrite_( const String& filename, const std::vector<Mat>& img_vec,
                      const std::vector<int>& params, bool flipv )
{
    bool isMultiImg = img_vec.size() > 1; //判断是否是多个Mat变量
    std::vector<Mat> write_vec;

    ImageEncoder encoder = findEncoder( filename ); //查找解码器->解读1
    if( !encoder )
        CV_Error( CV_StsError, "could not find a writer for the specified extension" );

    for (size_t page = 0; page < img_vec.size(); page++)
    {
        Mat image = img_vec[page];
        CV_Assert( image.channels() == 1 || image.channels() == 3 || image.channels() == 4 );
        //说明只能存储1/3/4通道的图片

        Mat temp;
        if( !encoder->isFormatSupported(image.depth()) )
        {
            CV_Assert( encoder->isFormatSupported(CV_8U) );
            image.convertTo( temp, CV_8U );
            image = temp;
        }

        if( flipv )//翻转
        {
            flip(image, temp, 0);
            image = temp;
        }

        write_vec.push_back(image);
    }

    encoder->setDestination( filename );
    CV_Assert(params.size() <= CV_IO_MAX_IMAGE_PARAMS*2);
    bool code;
    if (!isMultiImg)
        code = encoder->write( write_vec[0], params );
    else
        code = encoder->writemulti( write_vec, params ); //to be implemented

    //    CV_Assert( code );
    return code;
}

解读1

以JpegEncoder为例

static ImageEncoder findEncoder( const String& _ext )
{
    if( _ext.size() <= 1 )
        return ImageEncoder();

    const char* ext = strrchr( _ext.c_str(), '.' );
    if( !ext )
        return ImageEncoder();
    int len = 0;
    for( ext++; len < 128 && isalnum(ext[len]); len++ )
        ;

    for( size_t i = 0; i < codecs.encoders.size(); i++ )
    {
        String description = codecs.encoders[i]->getDescription();
        const char* descr = strchr( description.c_str(), '(' );

        while( descr )
        {
            descr = strchr( descr + 1, '.' );
            if( !descr )
                break;
            int j = 0;
            for( descr++; j < len && isalnum(descr[j]) ; j++ )
            {
                int c1 = tolower(ext[j]);
                int c2 = tolower(descr[j]);
                if( c1 != c2 )
                    break;
            }
            if( j == len && !isalnum(descr[j]))
                return codecs.encoders[i]->newEncoder();
            descr += j;
        }
    }

    return ImageEncoder();
}

1. 获取当前文件名的后缀名
2. 将当前文件的后缀名与解码器中的m_description（getDescription）（JpegEncoder->m_description = “JPEG files (.jpeg;.jpg;*.jpe)”）成员变量进行对比
3. 对比一致返回JpegEncoder

总结一下imwrite存取图片的流程

传入文件名->获取解码器->判断图片通道数->使用解码器存储图片