Merge pull request #6839 from thierrylamarre:better_png_transparency

modules/imgcodecs/src/grfmt_png.cpp

@@ -190,16 +190,14 @@ bool  PngDecoder::readHeader()
                         switch(color_type)
                         {
                             case PNG_COLOR_TYPE_RGB:
-                                m_type = CV_8UC3;
-                                break;
                             case PNG_COLOR_TYPE_PALETTE:
-                                png_get_tRNS( png_ptr, info_ptr, &trans, &num_trans, &trans_values);
-                                //Check if there is a transparency value in the palette
-                                if ( num_trans > 0 )
+                                png_get_tRNS(png_ptr, info_ptr, &trans, &num_trans, &trans_values);
+                                if( num_trans > 0 )
                                     m_type = CV_8UC4;
                                 else
                                     m_type = CV_8UC3;
                                 break;
+                            case PNG_COLOR_TYPE_GRAY_ALPHA:
                             case PNG_COLOR_TYPE_RGB_ALPHA:
                                 m_type = CV_8UC4;
                                 break;
@@ -255,12 +253,13 @@ bool  PngDecoder::readData( Mat& img )
                  * stripping alpha..  18.11.2004 Axel Walthelm
                  */
                  png_set_strip_alpha( png_ptr );
-            }
+            } else
+                png_set_tRNS_to_alpha( png_ptr );
 
             if( m_color_type == PNG_COLOR_TYPE_PALETTE )
                 png_set_palette_to_rgb( png_ptr );
 
-            if( m_color_type == PNG_COLOR_TYPE_GRAY && m_bit_depth < 8 )
+            if( (m_color_type & PNG_COLOR_MASK_COLOR) == 0 && m_bit_depth < 8 )
 #if (PNG_LIBPNG_VER_MAJOR*10000 + PNG_LIBPNG_VER_MINOR*100 + PNG_LIBPNG_VER_RELEASE >= 10209) || \
     (PNG_LIBPNG_VER_MAJOR == 1 && PNG_LIBPNG_VER_MINOR == 0 && PNG_LIBPNG_VER_RELEASE >= 18)
                 png_set_expand_gray_1_2_4_to_8( png_ptr );
@@ -268,7 +267,7 @@ bool  PngDecoder::readData( Mat& img )
                 png_set_gray_1_2_4_to_8( png_ptr );
 #endif
 
-            if( CV_MAT_CN(m_type) > 1 && color )
+            if( (m_color_type & PNG_COLOR_MASK_COLOR) && color )
                 png_set_bgr( png_ptr ); // convert RGB to BGR
             else if( color )
                 png_set_gray_to_rgb( png_ptr ); // Gray->RGB