继续opencv源码的研读，这里还是imread函数的跟进，昨天遇到了ImageDecoder这个类，这个类其实就是一个图像数据的解析类。且看下面的源代码：
class BaseImageDecoder //这就是我们要找的ImageDecoder类
{
public:
BaseImageDecoder();
virtual ~BaseImageDecoder() {};
int width() const { return m_width; };
int height() const { return m_height; };
int type() const { return m_type; };
virtual bool setSource( const string& filename );
virtual bool setSource( const Mat& buf );
virtual bool readHeader() = 0;
virtual bool readData( Mat& img ) = 0;
virtual size_t signatureLength() const;//（1）
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;
string m_filename;
string m_signature;//（2）
Mat m_buf;
bool m_buf_supported;
};
下面定义了一些类型，
typedef Ptr<BaseImageDecoder> ImageDecoder; //这里typedef了一个类型ImageDecoder，这个最原始的类型为Ptr，猜想是一个指针，但是在源代码中没能找到其声明和定义的地方。但这不妨碍源码的阅读。
在findDecoder中用到了：static vector<ImageDecoder> decoders;
且在代码中有：decoders[i]->signatureLength();的调用，且可以看到（1）处就有这个函数。
size_t BaseImageDecoder::signatureLength() const
{
return m_signature.size();
}
上面这个函数其实返回的就是其保存的图像签名（signature）（2）。
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;
}
ImageDecoder BaseImageDecoder::newDecoder() const
{
return ImageDecoder();//这里其实就是一个直接的调用了一个构造函数
}
BaseImageDecoder::BaseImageDecoder()
{
m_width = m_height = 0;
m_type = -1;
m_buf_supported = false;
}
这里看到的只是一个Base类，那么要真真的执行就是涉及到特定的图像类别了。
这里不看别的，就看一个bmp类型的吧：
class BmpDecoder : public BaseImageDecoder
{
public:
BmpDecoder();
~BmpDecoder();
bool readData( Mat& img );//很明显这是读取数据
bool readHeader();
void close();
ImageDecoder newDecoder() const;//这里重新的给了一个声明，表明在调用的时候调用的是子类的东西。
protected:
RLByteStream m_strm;
PaletteEntry m_palette[256];//调试版(palette)
int m_origin;
int m_bpp;
int m_offset;
BmpCompression m_rle_code;
/*压缩的格式
enum BmpCompression
{
BMP_RGB = 0,
BMP_RLE8 = 1,
BMP_RLE4 = 2,
BMP_BITFIELDS = 3
};
*/
};
bmpDecoder类里面的东西很多，这里简要的给看看：
ImageDecoder BmpDecoder::newDecoder() const
{
return new BmpDecoder;
}
BmpDecoder::BmpDecoder()
{
m_signature = fmtSignBmp;//static const char* fmtSignBmp = "BM"; 设定了自己标签名
m_offset = -1;
m_buf_supported = true;//buf设定为true，表示可以读取数据
}
这里就完全的看到了底层的代码了。这就需要完全的理解bmp图像的保存格式了。还没完全的了解bmp，暂且把这个代码贴在这吧。
bool BmpDecoder::readData( Mat& img )
{
uchar* data = img.data;
int step = (int)img.step;
bool color = img.channels() > 1;
uchar gray_palette[256];
bool result = false;
int src_pitch = ((m_width*(m_bpp != 15 ? m_bpp : 16) + 7)/8 + 3) & -4;
int nch = color ? 3 : 1;
int y, width3 = m_width*nch;
if( m_offset < 0 || !m_strm.isOpened())
return false;
if( m_origin == IPL_ORIGIN_BL )
{
data += (m_height - 1)*step;
step = -step;
}
AutoBuffer<uchar> _src, _bgr;
_src.allocate(src_pitch + 32);
if( !color )
{
if( m_bpp <= 8 )
{
CvtPaletteToGray( m_palette, gray_palette, 1 << m_bpp );
}
_bgr.allocate(m_width*3 + 32);
}
uchar *src = _src, *bgr = _bgr;
try
{
m_strm.setPos( m_offset );
switch( m_bpp )
{
/************************* 1 BPP ************************/
case 1:
for( y = 0; y < m_height; y++, data += step )
{
m_strm.getBytes( src, src_pitch );
FillColorRow1( color ? data : bgr, src, m_width, m_palette );
if( !color )
icvCvt_BGR2Gray_8u_C3C1R( bgr, 0, data, 0, cvSize(m_width,1) );
}
result = true;
break;
/************************* 4 BPP ************************/
case 4:
if( m_rle_code == BMP_RGB )
{
for( y = 0; y < m_height; y++, data += step )
{
m_strm.getBytes( src, src_pitch );
if( color )
FillColorRow4( data, src, m_width, m_palette );
else
FillGrayRow4( data, src, m_width, gray_palette );
}
result = true;
}
else if( m_rle_code == BMP_RLE4 ) // rle4 compression
{
uchar* line_end = data + width3;
y = 0;
for(;;)
{
int code = m_strm.getWord();
int len = code & 255;
code >>= 8;
if( len != 0 ) // encoded mode
{
PaletteEntry clr[2];
uchar gray_clr[2];
int t = 0;
clr[0] = m_palette[code >> 4];
clr[1] = m_palette[code & 15];
gray_clr[0] = gray_palette[code >> 4];
gray_clr[1] = gray_palette[code & 15];
uchar* end = data + len*nch;
if( end > line_end ) goto decode_rle4_bad;
do
{
if( color )
WRITE_PIX( data, clr[t] );
else
*data = gray_clr[t];
t ^= 1;
}
while( (data += nch) < end );
}
else if( code > 2 ) // absolute mode
{
if( data + code*nch > line_end ) goto decode_rle4_bad;
m_strm.getBytes( src, (((code + 1)>>1) + 1) & -2 );
if( color )
data = FillColorRow4( data, src, code, m_palette );
else
data = FillGrayRow4( data, src, code, gray_palette );
}
else
{
int x_shift3 = (int)(line_end - data);
int y_shift = m_height - y;
if( code == 2 )
{
x_shift3 = m_strm.getByte()*nch;
y_shift = m_strm.getByte();
}
len = x_shift3 + ((y_shift * width3) & ((code == 0) - 1));
if( color )
data = FillUniColor( data, line_end, step, width3,
y, m_height, x_shift3,
m_palette[0] );
else
data = FillUniGray( data, line_end, step, width3,
y, m_height, x_shift3,
gray_palette[0] );
if( y >= m_height )
break;
}
}
result = true;
decode_rle4_bad: ;
}
break;
/************************* 8 BPP ************************/
case 8:
if( m_rle_code == BMP_RGB )
{
for( y = 0; y < m_height; y++, data += step )
{
m_strm.getBytes( src, src_pitch );
if( color )
FillColorRow8( data, src, m_width, m_palette );
else
FillGrayRow8( data, src, m_width, gray_palette );
}
result = true;
}
else if( m_rle_code == BMP_RLE8 ) // rle8 compression
{
uchar* line_end = data + width3;
int line_end_flag = 0;
y = 0;
for(;;)
{
int code = m_strm.getWord();
int len = code & 255;
code >>= 8;
if( len != 0 ) // encoded mode
{
int prev_y = y;
len *= nch;
if( data + len > line_end )
goto decode_rle8_bad;
if( color )
data = FillUniColor( data, line_end, step, width3,
y, m_height, len,
m_palette[code] );
else
data = FillUniGray( data, line_end, step, width3,
y, m_height, len,
gray_palette[code] );
line_end_flag = y - prev_y;
}
else if( code > 2 ) // absolute mode
{
int prev_y = y;
int code3 = code*nch;
if( data + code3 > line_end )
goto decode_rle8_bad;
m_strm.getBytes( src, (code + 1) & -2 );
if( color )
data = FillColorRow8( data, src, code, m_palette );
else
data = FillGrayRow8( data, src, code, gray_palette );
line_end_flag = y - prev_y;
}
else
{
int x_shift3 = (int)(line_end - data);
int y_shift = m_height - y;
if( code || !line_end_flag || x_shift3 < width3 )
{
if( code == 2 )
{
x_shift3 = m_strm.getByte()*nch;
y_shift = m_strm.getByte();
}
x_shift3 += (y_shift * width3) & ((code == 0) - 1);
if( y >= m_height )
break;
if( color )
data = FillUniColor( data, line_end, step, width3,
y, m_height, x_shift3,
m_palette[0] );
else
data = FillUniGray( data, line_end, step, width3,
y, m_height, x_shift3,
gray_palette[0] );
if( y >= m_height )
break;
}
line_end_flag = 0;
}
}
result = true;
decode_rle8_bad: ;
}
break;
/************************* 15 BPP ************************/
case 15:
for( y = 0; y < m_height; y++, data += step )
{
m_strm.getBytes( src, src_pitch );
if( !color )
icvCvt_BGR5552Gray_8u_C2C1R( src, 0, data, 0, cvSize(m_width,1) );
else
icvCvt_BGR5552BGR_8u_C2C3R( src, 0, data, 0, cvSize(m_width,1) );
}
result = true;
break;
/************************* 16 BPP ************************/
case 16:
for( y = 0; y < m_height; y++, data += step )
{
m_strm.getBytes( src, src_pitch );
if( !color )
icvCvt_BGR5652Gray_8u_C2C1R( src, 0, data, 0, cvSize(m_width,1) );
else
icvCvt_BGR5652BGR_8u_C2C3R( src, 0, data, 0, cvSize(m_width,1) );
}
result = true;
break;
/************************* 24 BPP ************************/
case 24:
for( y = 0; y < m_height; y++, data += step )
{
m_strm.getBytes( src, src_pitch );
if(!color)
icvCvt_BGR2Gray_8u_C3C1R( src, 0, data, 0, cvSize(m_width,1) );
else
memcpy( data, src, m_width*3 );
}
result = true;
break;
/************************* 32 BPP ************************/
case 32:
for( y = 0; y < m_height; y++, data += step )
{
m_strm.getBytes( src, src_pitch );
if( !color )
icvCvt_BGRA2Gray_8u_C4C1R( src, 0, data, 0, cvSize(m_width,1) );
else
icvCvt_BGRA2BGR_8u_C4C3R( src, 0, data, 0, cvSize(m_width,1) );
}
result = true;
break;
default:
assert(0);
}
}
catch(...)
{
}
return result;
}
