Ensure image codecs are allocated in C heap

See #17

Author: sfe-SparkFro

src/imgcodecs.cpp

@@ -13,6 +13,25 @@ extern "C" {
 
 using namespace cv;
 
+// Fix for https://github.com/sparkfun/micropython-opencv/issues/17
+// 
+// TLDR; The `g_codecs` object (which stores all image encoders and decoders) is
+// allocated once, whenever the first OpenCV function that needs it happens to
+// be called. That will only happen from the user's code, after the GC has been
+// initialized, meaning it gets allocated on the GC heap (see `__wrap_malloc()`)
+// If a soft reset occurs, the GC gets reset and overwrites the memory location,
+// but the same memory location is still referenced for the the `g_codecs`
+// object, resulting in bogus values and subsequent `imread()` and `imwrite()`
+// calls fail
+// 
+// The solution here is to create a global variable that subsequently creates
+// the `g_codecs` object before the GC has been initialized, so it's allocated
+// on the C heap and persists through soft resets. `g_codecs` is initialized
+// when calling `getCodecs()`, which is not publicly exposed. The next best
+// option is to call `haveImageWriter()`, which calls `findEncoder()`, which
+// calls `getCodecs()`
+volatile bool haveImageWriterPNG = haveImageWriter(".png");
+
 // Helper macro to create an empty mp_map_t, derived from MP_DEFINE_CONST_MAP.
 // Primarily used for function calls with no keyword arguments, since we can't
 // just pass `NULL` or mp_const_none (crash occurs otherwise)

src/opencv_upy.cmake

@@ -23,8 +23,8 @@ target_link_libraries(usermod INTERFACE usermod_cv2)
 
 # OpenCV creates some global variables on the heap. These get created before
 # the GC is initialized, so we need to allocate some space for them on the C
-# heap. 10kB seems sufficient. TODO: See if we can get away with less.
-set(MICROPY_C_HEAP_SIZE 10240)
+# heap. 64kB seems sufficient.
+set(MICROPY_C_HEAP_SIZE 65536)
 
 # Makes m_tracked_calloc() and m_tracked_free() available. These track pointers
 # in a linked list to ensure the GC does not free them. Needed for some OpenCV