Jpeg2k color to greyscale conversion on non-Windows is done post decoding because system libjasper segfaults when decoding color images as greyscale.

modules/imgcodecs/src/grfmt_jpeg2000.cpp

@@ -45,6 +45,7 @@
 #ifdef HAVE_JASPER
 
 #include "grfmt_jpeg2000.hpp"
+#include "opencv2/imgproc.hpp"
 
 #ifdef WIN32
 #define JAS_WIN_MSVC_BUILD 1
@@ -159,6 +160,21 @@ bool  Jpeg2KDecoder::readData( Mat& img )
     jas_stream_t* stream = (jas_stream_t*)m_stream;
     jas_image_t* image = (jas_image_t*)m_image;
 
+#ifndef WIN32
+    // At least on some Linux instances the
+    // system libjasper segfaults when
+    // converting color to grey.
+    // We do this conversion manually at the end.
+    Mat clr;
+    if (CV_MAT_CN(img.type()) < CV_MAT_CN(this->type()))
+    {
+        clr.create(img.size().height, img.size().width, this->type());
+        color = true;
+        data = clr.ptr();
+        step = (int)clr.step;
+    }
+#endif
+
     if( stream && image )
     {
         bool convert;
@@ -171,7 +187,7 @@ bool  Jpeg2KDecoder::readData( Mat& img )
         else
         {
             convert = (jas_clrspc_fam( jas_image_clrspc( image ) ) != JAS_CLRSPC_FAM_GRAY);
-            colorspace = JAS_CLRSPC_SGRAY; // TODO GENGRAY or SGRAY?
+            colorspace = JAS_CLRSPC_SGRAY; // TODO GENGRAY or SGRAY? (GENGRAY fails on Win.)
         }
 
         // convert to the desired colorspace
@@ -256,6 +272,13 @@ bool  Jpeg2KDecoder::readData( Mat& img )
 
     close();
 
+#ifndef WIN32
+    if (!clr.empty())
+    {
+        cv::cvtColor(clr, img, COLOR_BGR2GRAY);
+    }
+#endif
+
     return result;
 }
 

modules/imgcodecs/src/loadsave.cpp

@@ -374,15 +374,8 @@ imreadmulti_(const String& filename, int flags, std::vector<Mat>& mats)
                 type = CV_MAKETYPE(CV_MAT_DEPTH(type), 1);
         }
 
-        // established the required input image size.
-        CvSize size;
-        size.width = decoder->width();
-        size.height = decoder->height();
-
-        Mat mat;
-        mat.create(size.height, size.width, type);
-
         // read the image data
+        Mat mat(decoder->height(), decoder->width(), type);
         if (!decoder->readData(mat))
         {
             break;