full example for building opencv libraries; upgrade box demo SDL version

README.md

@@ -317,6 +317,31 @@ To link to the WASM libraries, add the `*.a` files from the build directory to E
 
     $(wildcard $(addprefix $(WASM_BUILD_DIR)/lib/,*.a)) $(wildcard $(addprefix $(WASM_BUILD_DIR)/3rdparty/lib/,*.a))
 
+#### Full example
+
+This builds both the local and WASM libraries by downloading OpenCV 4.7.0 and the contributed modules source packages. See [Gunkiss][] for an example of using these libraries in a project.
+
+    $ wget https://github.com/opencv/opencv/archive/4.7.0.zip
+    $ unzip 4.7.0.zip
+    $ tar -xf opencv_contrib-4.7.0.tar.gz
+    $ mkdir opencv_contrib-4.7.0-subset
+    $ cp -r opencv_contrib-4.7.0/modules/barcode/ opencv_contrib-4.7.0-subset/
+    $ rm -rf ~/local/opencv/
+    $ cd opencv-4.7.0/
+    $ mkdir build_linux
+    $ cd build_linux/
+    $ cmake -D CMAKE_INSTALL_PREFIX=$HOME/local/opencv -D OPENCV_EXTRA_MODULES_PATH="../../opencv_contrib-4.7.0-subset/" ..
+    ...
+    --   OpenCV modules:
+    --     To be built: barcode calib3d core dnn features2d flann gapi highgui imgcodecs imgproc java ml objdetect
+                        photo python3 stitching ts video videoio
+    ...
+    $ make -j3 && make install
+    $ cd ..
+    $ source ~/ext/software/emsdk/emsdk_env.sh
+    $ python3 platforms/js/build_js.py --cmake_option="-DOPENCV_EXTRA_MODULES_PATH=../../opencv_contrib-4.7.0-subset/" \
+        --emscripten_dir ~/ext/software/emsdk/upstream/emscripten build_wasm_contrib --build_wasm
+
 ### ZBar
 
 #### Linux

demo/box/Makefile

@@ -120,7 +120,7 @@ box_demo.js: $(addprefix $(SDLGFX2_DIR),SDL2_rotozoom.o SDL2_gfxPrimitives.o) $(
 # <https://imagemagick.org/>. The project source files are assumed to be just `$(SRC_DIR)/*.cpp`. If that is not the case, the revise_skeleton.sh script
 # must be edited.
 
-SDL_SRC := $(HOME)/ext/software/SDL2-2.24.0-android
+SDL_SRC := $(HOME)/ext/software/SDL2-2.26.3
 SDL_IMAGE_SRC := $(HOME)/ext/software/SDL2_image-2.6.2-android
 SDL_MIXER_SRC := $(HOME)/ext/software/SDL2_mixer-2.6.2-android
 SDL_TTF_SRC := $(HOME)/ext/software/SDL2_ttf-2.20.1-android