Set Ghostscript -sColorConversionStrategy the way old/new versions expect

docs/release_notes.rst

@@ -16,7 +16,7 @@ v6.1.4
 
 -   The test cache was updated to reflect the change above.
 
--   Change test suite to accommodate Ghostscript 9.23's new ability to insert JPEGs into PDFs without transcoding.  An apparent `regression in Ghostscript 9.23 <https://bugs.ghostscript.com/show_bug.cgi?id=699216>`_ will cause some ocrmypdf output files to become invalid in rare cases; the workaround for the moment is to set ``--force-ocr``.
+-   Change test suite to accommodate Ghostscript 9.23's new ability to insert JPEGs into PDFs without transcoding.
 
 -   XMP metadata in PDFs is now examined using ``defusedxml`` for safety.
 
@@ -24,6 +24,11 @@ v6.1.4
 
 -   qpdf 7.0.0 or newer is now required as the test suite can no longer pass without it.
 
+Notes
+~~~~~
+
+-   An apparent `regression in Ghostscript 9.23 <https://bugs.ghostscript.com/show_bug.cgi?id=699216>`_ will cause some ocrmypdf output files to become invalid in rare cases; the workaround for the moment is to set ``--force-ocr``.
+
 
 v6.1.3
 ------

src/ocrmypdf/exec/ghostscript.py

@@ -127,6 +127,10 @@ def generate_pdfa(pdf_pages, output_file, compression, log,
             "-dAutoFilterGrayImages=true",
         ]
 
+    # Older versions of Ghostscript expect a leading slash, newer ones should
+    # not have it. See Ghostscript git commit fe1c025d.
+    strategy = 'RGB' if ghostscript.version() >= '9.19' else '/RGB'
+
     with NamedTemporaryFile(delete=True) as gs_pdf:
         args_gs = [
             "gs",
@@ -137,7 +141,7 @@ def generate_pdfa(pdf_pages, output_file, compression, log,
             "-dNumRenderingThreads=" + str(threads),
             "-sDEVICE=pdfwrite",
             "-dAutoRotatePages=/None",
-            "-sColorConversionStrategy=RGB",
+            "-sColorConversionStrategy=" + strategy,
             "-sProcessColorModel=DeviceRGB"
         ] + compression_args + [
             "-dJPEGQ=95",