B4A Library OpenCV 3.x


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OpenCV (Open Source Computer Vision Library) is a really huge project/framework actively developed, mainly written in C++ . It is released under a BSD license.
Read more here: http://opencv.org/
OpenCV versions: https://opencv.org/releases/

OpenCV library for B4A: wraps the official OpenCV 3.x release for Android (in fact not all, about 95% of it)
  • Feel free to test and use it. You can even
  • License: You can use it for your projects, but you are not allowed to distribute nor sell this library. Of course you can distribute apps that use it (remember that OpenCV itself has BSD license as stated before)
  • Supported hardware is: armeabi-v7a and arm64-v8
  • Versions
    • 1.04 (2020/05/17)
      • This version wraps OpenCV 3.4.1 Android release and fixes some bugs (mostly some instance methods and some not exposed classes) of the previous version.
      • One of the major additions is the DNNmodule and related classes.
      • Link to the B4A library files: HERE :)
    • 1.00 (2017/09/27)
      • First B4A library wrapper (OpenCv320forB4A V1.00) which replicates (95%) the official OpenCV 3.20 Java API for Android.
      • (removed link. Use 1.04)

  • Please note that my support will be limited to issues with the wrapper itself, not to help translating OpenCV code from other languages to B4A 🤷‍♂️


(There may be some inaccuracies in what I expose in this post, related to the OpenCV project, since I am relatively new to it and don't know all about its internals. If you find any, please let me know and I will correct it)

A bit of explanation
There exist 'official' OpenCV wrappers for different languages and platforms. Android is one of them.
The official OpenCV 3.20 for Android API includes a lot of classes, organized in modules. But it does not include "all" the original OpenCV modules (since there are other 'experimental', non-free, or platform specific modules which may be present in other platforms but not for Android). Also, there are build options to "tune" it...

I have played quite a lot with it this last year, with a huge project which I started with inline Java, and also translating examples or testing features. But what I have used is just a small percent of the classes and methods exposed. So, there may be some (let's hope not too many) things to fix.

How to learn OpenCVforB4A
If you have worked before with OpenCV, the learning curve will be easy.
If it was using Java with OpenCV for Android, then it will be immediate, since all the methods have exactly the same syntax (except for initializers, polimorphism, and some special cases where simply I did my best).
Anyhow, the ways that I can think of, are (will add links later, also suggestions as online tutorials,.. are welcome)
  • Attached examples.
  • B4A OpenCV Tutorials. I will write a couple of them with what I consider the most important building pieces (for instance the Mat class, which in B4A is OCVMat) and modules
  • Internet examples: there are A LOT of examples over there, written in C++, Java, Python, JavaCV. I would look for examples in the language that is easier to understand for you and then try to translate. Some tips about it (based on my experience)
  • OpenCV syntax has changed as new versions. So there is an 'old' syntax in which nearly everything started with "cv...". Since version 3.X, there was 'cleaner' organization (project was written in C++ instead of C), and there were major syntax changes.
  • JavaCV: Translating from JavaCV to OpenCV should be quite easy but not always direct. JavaCV uses a mix of the old OpenCV syntax with some of its own, and at the beginning it can be a bit confusing, but then it is also easy.
  • Python: there is a lot of material...

First steps. Prepare for some crashes...
  • In OpenCV nearly everything takes part in the native code.
  • When we call a Sub/method/algorithm, it performs some internal checks to see if all the input data is correct. This check is perfomed in the native side. If something is not correct (wrong OCVMat dimensions, some incoherent parameters,...) it throws an exception and crashes. If we are lucky, perhaps we see in the log some clues about the check that made it crash.
  • On the good side, it is very easy to achieve results with OpenCV (check the examples). The real difficult part, as with many other things, is to fine-tune it: OpenCV has a collection of really powerful 'primitive' objects and operations, and really complex algorithms that can do many things. But it is the user who has to glue all of them to achieve the desired results.

(from the previous Beta announcement)
  • IMPORTANT: you must take this into account:
  • OpenCV (the included binary modules) is a free(*) project, but subject to license terms as described here: http://opencv.org/
    • (*): There are some modules in the OpenCV project which are on-free, but here I am refering to the ones included in the library
  • My work: (the B4A library) is free to test and use, but you can for it :). I'll keep donators updated with "advanced" material and examples
  • If you are interested, please PM me with your mail address and I will send you a link with the library and some basic examples. (be patient if you don't receive it immediately, I'll do it as soon as possible).
  • There is no documentation. In short, the syntax is nearly-exactly the same as the OpenCV3.20 Java API, adding "OCV" prefix and only the minimum modifications to adapt to B4A, For reference (taking into account described syntax changes) you can look at http://docs.opencv.org/java/3.1.0/ (which is not the latest one, but the API is nearly the same).
  • It would be preferable if you have worked before with OpenCV and/or can translate examples from Java/C++ and/or simply are interested in it.
  • I recommend starting with the examples and try to understand what is done. Just experimenting can lead to crash after crash of the native libraries with nearly no useful information, and can be very discouraging.
  • I forgot, the included binaries are for ameabi-v7a and arm64-v8 devices
Some screenshots taken from the examples
Canny operator - Features2D - Color space conversion
Color Blob detection


Last edited:


Well-Known Member
Licensed User
New real-time image processing example which already makes use of JavaCameraView (and dramatically decreases code complexity.:))
Needs Beta3: library and example already available for beta-testers

The result

The code ;)
Sub Process_Globals
   'These global variables will be declared once when the application starts.
   'These variables can be accessed from all modules.
   Dim ocl As OCVOpenCVLoader
End Sub

Sub Globals
   'These global variables will be redeclared each time the activity is created.
   'These variables can only be accessed from this module.
   Dim loaded As Boolean = ocl.initDebug
   Dim P As Panel
   Dim mOpenCvCameraView As OCVJavaCameraView
   Dim mCore As OCVCore
   Dim mImgProc As OCVImgproc   
End Sub

Sub Activity_Create(FirstTime As Boolean)
   'Do not forget to load the layout file created with the visual designer. For example:
End Sub

Sub Activity_Resume
End Sub

Sub Activity_Pause (UserClosed As Boolean)
End Sub

' This Sub will be called from another thread.
' Here we process myMat and put results in the same OCVMat object.
' Its contents will be drawn on the camera sufaceView
Public Sub frameprocessor_newFrame(myMat As OCVMat)
   'Log("processing..." )
End Sub


Licensed User
first beta is available under request.
hi @JordiCP. i am interested to try this great lib out.
what is the amount for the donation that is needed to get all updates?

thank you


Well-Known Member
Licensed User
Thanks a lot for your interest @ilan ! I will PM you in some minutes with the link

There's no minimum amount, donate only if you find it useful or like it very much ;). This will encourage me to develop more examples and perhaps custom builds which I will share with donators as gratitude. Anyway, "regular" updates (bug fixes an similar) will keep on being free.

In fact it has been a big effort and I even paused other customer projects for some weeks (which for a freelance is not good) , but the main motivation was not money (I would have made it chargeable), it was a mix of personal challenge and also my way to give something back to the community.


Active Member
Licensed User
Hello JordiCP
i also would have intrest to test you libary.

Many thanks
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New Member
Licensed User
Very interesting library. I was looking for an OpenCV solution to test the speed of objects recognition in visible space...
Dontation done ... thank you so much.


Active Member
Licensed User
Dear JordiCP,
It would indeed be a great augmentation to B4A to have the OpenCv library. Please count me in!
Thanks for the efforts.


Well-Known Member
Licensed User
This example is a bit more complex and must be still optimized (so not released yet). I was "trying" to translate a camera calibration example, when I realised that I haven't much idea about cameras, focal length, ...
So, starting from the same example I jumped onto another and got something more funny

Must say that for "real time", if processing becomes a bit complex, Android will suffer. OpenCV algorithms are not so much optimized for mobile devices as for other plattforms (desktop). Also, some of these optimisations are only accessible from C++ but not from the standard OpenCV Android API. Hope this will be better as OpenCV is continuously evolving...;)