test ImageMagick to see if two non-identical images are considered identical.

on failure, fall back to diffpng comparison (diffpng.cpp)

tests/CMakeLists.txt

@@ -504,12 +504,7 @@ add_definitions(${HARFBUZZ_CFLAGS})
 # Image comparison - expected test image vs actual generated image
 
 if (DIFFPNG)
-  # How to set cflags to optimize the executable?
-  set(IMAGE_COMPARE_EXECUTABLE ${CMAKE_CURRENT_BINARY_DIR}/diffpng)
-  set(COMPARATOR "diffpng")
-  add_executable(diffpng diffpng.cpp ../src/lodepng.cpp)
   set(SKIP_IMAGEMAGICK 1)
-  message(STATUS "using diffpng for image comparison")
 endif()
 
 if (SKIP_IMAGEMAGICK)
@@ -537,8 +532,28 @@ else()
     message(STATUS "ImageMagick: OpenMP bug workaround - setting MAGICK_THREAD_LIMIT=1")
     set(CTEST_ENVIRONMENT "${CTEST_ENVIRONMENT};MAGICK_THREAD_LIMIT=1")
   endif()
+
+  message(STATUS "Comparing magicktest1.png with magicktest2.png")
+  set(IM_TEST_FILES "${CMAKE_CURRENT_SOURCE_DIR}/magicktest1.png" "${CMAKE_CURRENT_SOURCE_DIR}/magicktest2.png")
+  set(COMPARE_ARGS ${IMAGE_COMPARE_EXECUTABLE} ${IM_TEST_FILES} -alpha Off -compose difference -composite -threshold 10% -morphology Erode Square -format %[fx:w*h*mean] info:)
+  # compare arguments taken from test_cmdline_tool.py
+  execute_process(COMMAND ${COMPARE_ARGS} OUTPUT_VARIABLE IM_OUT )
+  if ( ${IM_OUT} MATCHES "0" )
+    message(STATUS "magicktest1.png and magicktest2.png were incorrectly detected as identical")
+    message(STATUS "Using alternative image comparison")
+    set(DIFFPNG 1)
+  endif()
 endif()
 
+if (DIFFPNG)
+  # How to set cflags to optimize the executable regardless of CMAKE_BUILD_TYPE?
+  set(IMAGE_COMPARE_EXECUTABLE ${CMAKE_CURRENT_BINARY_DIR}/diffpng)
+  set(COMPARATOR "diffpng")
+  add_executable(diffpng diffpng.cpp ../src/lodepng.cpp)
+  message(STATUS "Image comparison program: diffpng")
+endif()
+
+
 # Internal includes
 include_directories(../src)