alpha channels support for 8-bit tiffs

modules/highgui/src/grfmt_tiff.cpp

@@ -101,6 +101,11 @@ bool TiffDecoder::checkSignature( const string& signature ) const
         memcmp(signature.c_str(), fmtSignTiffMM, 4) == 0);
 }
 
+int TiffDecoder::normalizeChannelsNumber(int channels) const
+{
+    return channels > 4 ? 4 : channels;
+}
+
 ImageDecoder TiffDecoder::newDecoder() const
 {
     return new TiffDecoder;
@@ -133,10 +138,11 @@ bool TiffDecoder::readHeader()
                 (ncn != 1 && ncn != 3 && ncn != 4)))
                 bpp = 8;
 
+            int wanted_channels = normalizeChannelsNumber(ncn);
             switch(bpp)
             {
                 case 8:
-                    m_type = CV_MAKETYPE(CV_8U, photometric > 1 ? 3 : 1);
+                    m_type = CV_MAKETYPE(CV_8U, photometric > 1 ? wanted_channels : 1);
                     break;
                 case 16:
                     m_type = CV_MAKETYPE(CV_16U, photometric > 1 ? 3 : 1);
@@ -185,6 +191,7 @@ bool  TiffDecoder::readData( Mat& img )
         TIFFGetField( tif, TIFFTAG_SAMPLESPERPIXEL, &ncn );
         const int bitsPerByte = 8;
         int dst_bpp = (int)(img.elemSize1() * bitsPerByte);
+        int wanted_channels = normalizeChannelsNumber(img.channels());
 
         if(dst_bpp == 8)
         {
@@ -248,9 +255,20 @@ bool  TiffDecoder::readData( Mat& img )
 
                             for( i = 0; i < tile_height; i++ )
                                 if( color )
+                                {
+                                    if (wanted_channels == 4)
+                                    {
+                                        icvCvt_BGRA2RGBA_8u_C4R( buffer + i*tile_width*4, 0,
+                                                             data + x*4 + img.step*(tile_height - i - 1), 0,
+                                                             cvSize(tile_width,1) );
+                                    }
+                                    else
+                                    {
                                         icvCvt_BGRA2BGR_8u_C4C3R( buffer + i*tile_width*4, 0,
                                                              data + x*3 + img.step*(tile_height - i - 1), 0,
                                                              cvSize(tile_width,1), 2 );
+                                    }
+                                }
                                 else
                                     icvCvt_BGRA2Gray_8u_C4C1R( buffer + i*tile_width*4, 0,
                                                               data + x + img.step*(tile_height - i - 1), 0,

modules/highgui/src/grfmt_tiff.hpp

@@ -103,6 +103,7 @@ public:
 
     size_t signatureLength() const;
     bool checkSignature( const string& signature ) const;
+    int normalizeChannelsNumber(int channels) const;
     ImageDecoder newDecoder() const;
 
 protected: