adding support for 16-bit 4 channel tiffs

- when IMREAD_UNCHANGED is set in flags, the tiff is read with 4 channels
- adding test for read/write

modules/imgcodecs/src/grfmt_tiff.cpp

@@ -158,7 +158,7 @@ bool TiffDecoder::readHeader()
                     m_type = CV_MAKETYPE(CV_8U, photometric > 1 ? wanted_channels : 1);
                     break;
                 case 16:
-                    m_type = CV_MAKETYPE(CV_16U, photometric > 1 ? 3 : 1);
+					m_type = CV_MAKETYPE(CV_16U, photometric > 1 ? wanted_channels : 1);
                     break;
 
                 case 32:
@@ -326,6 +326,21 @@ bool  TiffDecoder::readData( Mat& img )
                                                                (ushort*)(data + img.step*i) + x*3, 0,
                                                                cvSize(tile_width,1) );
                                     }
+									else if (ncn == 4)
+									{
+										if (wanted_channels == 4)
+										{
+											icvCvt_BGRA2RGBA_16u_C4R(buffer16 + i*tile_width0*ncn, 0,
+												(ushort*)(data + img.step*i) + x * 4, 0,
+												cvSize(tile_width, 1));
+										}
+										else
+										{
+											icvCvt_BGRA2BGR_16u_C4C3R(buffer16 + i*tile_width0*ncn, 0,
+												(ushort*)(data + img.step*i) + x * 3, 0,
+												cvSize(tile_width, 1), 2);
+										}
+									}
                                     else
                                     {
                                         icvCvt_BGRA2BGR_16u_C4C3R(buffer16 + i*tile_width0*ncn, 0,

modules/imgcodecs/test/test_grfmt.cpp

@@ -139,9 +139,6 @@ public:
 
                     string filename = cv::tempfile(".jpg");
                     imwrite(filename, img);
-                    img = imread(filename, IMREAD_UNCHANGED);
-
-                    filename = string(ts->get_data_path() + "readwrite/test_" + char(k + 48) + "_c" + char(num_channels + 48) + ".jpg");
                     ts->printf(ts->LOG, "reading test image : %s\n", filename.c_str());
                     Mat img_test = imread(filename, IMREAD_UNCHANGED);
 
@@ -160,8 +157,9 @@ public:
 #endif
 
 #ifdef HAVE_TIFF
-                for (int num_channels = 1; num_channels <= 3; num_channels+=2)
+                for (int num_channels = 1; num_channels <= 4; num_channels++)
                 {
+					if (num_channels == 2) continue;
                     // tiff
                     ts->printf(ts->LOG, "image type depth:%d   channels:%d   ext: %s\n", CV_16U, num_channels, ".tiff");
                     Mat img(img_r * k, img_c * k, CV_MAKETYPE(CV_16U, num_channels), Scalar::all(0));