这段代码是什么意思

int iXOff, iYOff, iSample;

iSample = 0;

for( iYOff = 0; iYOff < nYSize; iYOff++ )
{
for( iXOff = 0; iXOff < nXSize; iXOff++ )
{
unsigned char *pabyData;

pabyData = pabySrc + iYOff * nLineOffset + iXOff * nPixelOffset;

if( nSampleFormat == SAMPLEFORMAT_UINT && nPixelBytes == 1 )
{
padfSamples[iSample++] = *pabyData;
}
else if( nSampleFormat == SAMPLEFORMAT_UINT && nPixelBytes == 2 )
{
padfSamples[iSample++] = ((uint16 *) pabyData)[0];
}
else if( nSampleFormat == SAMPLEFORMAT_UINT && nPixelBytes == 4 )
{
padfSamples[iSample++] = ((uint32 *) pabyData)[0];
}
else if( nSampleFormat == SAMPLEFORMAT_INT && nPixelBytes == 2 )
{
padfSamples[iSample++] = ((int16 *) pabyData)[0];
}
else if( nSampleFormat == SAMPLEFORMAT_INT && nPixelBytes == 32 )
{
padfSamples[iSample++] = ((int32 *) pabyData)[0];
}
else if( nSampleFormat == SAMPLEFORMAT_IEEEFP && nPixelBytes == 4 )
{
padfSamples[iSample++] = ((float *) pabyData)[0];
}
else if( nSampleFormat == SAMPLEFORMAT_IEEEFP && nPixelBytes == 8 )
{
padfSamples[iSample++] = ((double *) pabyData)[0];
}
}

pabySrc 是形参传过来的值，为unsigned char*，padfSamples为double*。(（xx*）pabyData)[0]是什么意思啊？