OMG CMake/OpenCV3 can you be more difficult? Linking order problems with OpenNI2...

So I just spent 1.5 hours figuring this out.
Compiling an example on Ubuntu 16.04 with OpenCV built from scratch with OpenNI2 support.
(OpenNI2 is from Orbbec, but that doesn't make any difference: https://orbbec3d.com/develop/)

When using this straightforward CMake script for compilation - it doesn't work:

cmake_minimum_required(VERSION 3.2)
project(MyApp)

find_package(OpenCV 3 REQUIRED)

set(OPENNI2_LIBS "OpenNI2")
link_directories("/home/user/Downloads/2-Linux/OpenNI-Linux-x64-2.3/Redist")

add_executable(myapp main.cpp)
target_link_libraries(myapp ${OpenCV_LIBS} ${OPENNI2_LIBS})

Complains of undefined references:

/usr/bin/c++   -g   CMakeFiles/myapp.dir/main.cpp.o  -o myapp  -L/home/user/Downloads/2-Linux/OpenNI-Linux-x64-2.3/Redist -rdynamic -lOpenNI2 /usr/local/lib/libopencv_shape.so.3.2.0 /usr/local/lib/libopencv_stitching.so.3.2.0 /usr/local/lib/libopencv_superres.so.3.2.0 /usr/local/lib/libopencv_videostab.so.3.2.0 /usr/local/lib/libopencv_objdetect.so.3.2.0 /usr/local/lib/libopencv_calib3d.so.3.2.0 /usr/local/lib/libopencv_features2d.so.3.2.0 /usr/local/lib/libopencv_flann.so.3.2.0 /usr/local/lib/libopencv_highgui.so.3.2.0 /usr/local/lib/libopencv_ml.so.3.2.0 /usr/local/lib/libopencv_photo.so.3.2.0 /usr/local/lib/libopencv_video.so.3.2.0 /usr/local/lib/libopencv_videoio.so.3.2.0 /usr/local/lib/libopencv_imgcodecs.so.3.2.0 /usr/local/lib/libopencv_imgproc.so.3.2.0 /usr/local/lib/libopencv_core.so.3.2.0 -Wl,-rpath,/home/user/Downloads/2-Linux/OpenNI-Linux-x64-2.3/Redist:/usr/local/lib 
/usr/local/lib/libopencv_videoio.so.3.2.0: undefined reference to `oniStreamGetProperty'
/usr/local/lib/libopencv_videoio.so.3.2.0: undefined reference to `oniRecorderDestroy'
/usr/local/lib/libopencv_videoio.so.3.2.0: undefined reference to `oniDeviceIsCommandSupported'
/usr/local/lib/libopencv_videoio.so.3.2.0: undefined reference to `oniDeviceSetProperty'

You'll notice that -lOpenNI2 does indeed appear for correct linking.
The linker doesn't complain that lib was not found - it just misses the references.
This lead me to understand it's a linking order problem (after ~45 minutes of banging my head vs. the keyboard and swearing profusely).

Some more swearing and head banging got me to understand that CMake is messing around with the link order.
So even if try:

target_link_libraries(myapp ${OpenCV_LIBS} ${OPENNI2_LIBS} ${OpenCV_LIBS} ${OPENNI2_LIBS})

i.e. making the order effectively meaningless -- it still doesn't work!

More swearing and head banging, another ~40 minutes passed, and I figured out a solution.
The real solution is to slap someone in CMake in the face with a trout, but here's a solution to my problem:

find_package(OpenCV 3 REQUIRED core highgui videoio) # ORDER MATTERS!!! videoio must be last!
set(OpenCV_LIBS "${OpenCV_LIBS};OpenNI2") #add openni2 at the end (although cmake doesn't keep order anyway)
target_link_libraries(myapp ${OpenCV_LIBS})

Now it compiles.

And look at the make VERBOSE=1:

/usr/bin/c++   -g   CMakeFiles/myapp.dir/main.cpp.o  -o myapp  -L/home/user/Downloads/2-Linux/OpenNI-Linux-x64-2.3/Redist -rdynamic /usr/local/lib/libopencv_highgui.so.3.2.0 /usr/local/lib/libopencv_videoio.so.3.2.0 -lOpenNI2 /usr/local/lib/libopencv_core.so.3.2.0 -Wl,-rpath,/home/user/Downloads/2-Linux/OpenNI-Linux-x64-2.3/Redist:/usr/local/lib -Wl,-rpath-link,/usr/local/lib 

Can you see how highgui and videoio are before OpenNI2, and core is after?
Why? Whhhhhhy?

The key is to get OpenNI to be linked in order after videoio.

OMG CMake, OMG OpenCV, OMG you gaiz, W-T-F?

Update:
This method breaks down as soon as more OpenCV components are added. The order goes haywire again, and OpenNI2 comes before videoio, which breaks the link.
As of now the way I can compile it is like so:

set(LINK_LIBS /usr/local/lib/libopencv_core.so.3.2
              /usr/local/lib/libopencv_highgui.so.3.2
              /usr/local/lib/libopencv_videoio.so.3.2
              /usr/local/lib/libopencv_imgproc.so.3.2
              /usr/local/lib/libopencv_calib3d.so.3.2
              OpenNI2)
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New edition to the Mastering OpenCV book - now with OpenCV3!

Mastering OpenCV 3
I'm happy to announce that the new edition of Mastering OpenCV is out!
You can get it on Amazon: Mastering OpenCV 3

It brings up most of the older OpenCV2 book projects to OpenCV3, including my Toy-SfM (or "Exploring SfM") project.

A lot has happened in the OpenCV3 APIs with respect to Structure from Motion.
It got much easier!
The book chapter on SfM is a gentle introduction to the subject, that focuses on coding and the core concepts, while abstracting on the math.

Thanks for listening!
Roy

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Android Camera2 Touch-to-Focus

Man Camera2 APIs are hard to master...

I've scanned SO for a way to get touch-to-focus to work on Android and could not find a solution that works.
These were very partial, or provided only scaffold code which I couldn't use:

Finally I was able to do it by myself this way:

Enjoy
Roy

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My first Thing on Thingiverse!

2017-02-17 09.18.33
Just wanted to share the first Thing I uploaded to Thingiverse (which is a huge open-source collection of 3D-printable or otherwise fabricateable objects).

It's a 1/4-20 (standard camera mount) holder for a smartphone using a rubber band.
I saw many designs that use screws to hold the phone in place, but I didn't have such screws and I had many loose rubber bands. Also a rubber band allows the phone to snap in and out more easily.

It's a very basic 3D design I did with FreeCAD, then made it on my FlashForge Creator Pro.

Enjoy
Roy

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WTH OpenGL 4? Rendering elements arrays with VAOs and VBOs in a QGLWidget

I spent an entire day getting OpenGL 4 to display data from a VAO with VBOs so I thought I'd share the results with you guys, save you some pain.

I'm using the excellent GL wrappers from Qt, and in particular QGLShaderProgram.
This is pretty straightforward, but the thing to remember is that OpenGL is looking for the vertices/other elements (color? tex coords?) to come from some bound GL buffer or from the host. So if your app is not working and nothing appears on screen, just make sure GL has a bound buffer and the shader locations match up and consistent (see the const int I have on the class here).

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