B4A Library OpenCV 3.x

JordiCP · May 22, 2017

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 example (too large to be uploaded here)
    - Result video
  - 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

2D-FFT

Color Blob detection

cesarcm · Dec 5, 2020

JordiCP,

Probably my last question (I hope so) ...

I have to migrate these lines as well:

Mat X = Mat::zeros(13,6,CV_32FC1);
Mat mat43= Mat::eye(4,3,CV_32FC1); //this is a submatrix at position (4,4)

Mat aux = X.colRange(4,7).rowRange(4,8); // you are pointing to submatrix 4x3 at X(4,4)
mat43.copyTo(aux);

Any tip helps a lot

Thanks
Regards,
Cear

JordiCP · Dec 5, 2020

cesarcm said:
Mat X = Mat::zeros(13,6,CV_32FC1);
Mat mat43= Mat::eye(4,3,CV_32FC1); //this is a submatrix at position (4,4)

Mat aux = X.colRange(4,7).rowRange(4,8); // you are pointing to submatrix 4x3 at X(4,4)
mat43.copyTo(aux);

Untested, but should be something similar to this

B4X:

    Dim mType As OCVCvType

    Dim X as OCVMat 
    X = X.zeros(13,6,mType.CV_32FC1)
    Dim mat43 As OCVMat = X.submat(4,8,4,7) 
    Dim aux As OCVMat
    aux = aux.eye(4,3, mType.CV_32FC1)
    mat43.copyTo(aux)

I'd recommend to add a code module to your project with some static helper methods. These become very useful to make the final code more readable

B4X:

Sub CreateScalar(X As Double) As OCVScalar
    Dim sc As OCVScalar
    sc.Initialize(X)
    Return sc
End Sub

Sub CreateScalar2(X As Double, Y As Double) As OCVScalar
    Dim sc As OCVScalar
    sc.Initialize2(X, Y)
    Return sc    
End Sub

Sub CreateScalar3(X As Double, Y As Double, Z As Double) As OCVScalar
    Dim sc As OCVScalar
    sc.Initialize3(X, Y, Z)
    Return sc    
End Sub

Sub CreateSize(W As Double, H As Double) As OCVSize
    Dim sz As OCVSize
    sz.Initialize2(Array As Double(W,H))
    Return sz
End Sub

Sub CreateMat(rows As Int, cols As Int, cType As Int) As OCVMat
    Dim m As OCVMat
    m.Initialize2(rows, cols, cType)
    Return m
End Sub

Sub CreateEyeMat(rows As Int, cols As Int, cType As Int) As OCVMat
    Dim m As OCVMat 
    m = m.eye(rows, cols, cType)
    Return m
End Sub

Sub CreateZeroMat(rows As Int, cols As Int, cType As Int) As OCVMat
    Dim m As OCVMat 
    m = m.zeros(rows, cols, cType)
    Return m
End Sub

cesarcm · Dec 5, 2020

JordiCP,

Perfect!

Please, one more tip ...

// from Javascript
let means_c3 = cv.matFromArray(1, 1, cv.CV_32FC3, [mean.data64F[0], mean.data64F[1], mean.data64F[2]]);

PS: I guess that I have to use OCVMatOfDouble .. Am I right?

Thanks
Regards,
Cesar

JordiCP · Dec 5, 2020

Yes, but I guess that you can also work with OCVMat.
(Untested)

B4X:

Dim mm As OCVMatOfDouble
mm.Initialize3(Array As Double(  firstElement, secondElement, ...    ))

'or

Dim mm As OCVMat
mm.initialize2(1,1, mType.CV_32FC3)
mm.put(0,0, Array as Double( firsElement, secondElement, tihrdElement))

cesarcm · Dec 5, 2020

YOU ROCK!

But how can I get mean.data64F? or is it not necessary?

let mean = new cv.Mat();
let stdDev = new cv.Mat();
let t1 = new cv.Mat();
cv.meanStdDev(signal, mean, stdDev, t1);

let means_c3 = cv.matFromArray(1, 1, cv.CV_32FC3, [mean.data64F[0], mean.data64F[1], mean.data64F[2]]);

Thanks for all.
Regards,
Cesar

JordiCP · Dec 5, 2020

It's a bit difficult to follow without seeing the whole code.

If I understood correctly

B4X:

Dim mean, stdDev, t1 as OCVMat
Dim mCore as OCVCore
mCore.meanStdDev( signal, mean, stdDev, t1)  

Dim meanVals() as Double = means.get5(0,0)     '<-- each 'getX' suffix corresponds to a different data type and must match the OCV Type

Dim means_c3 as OCVMat    '<-- now the code needs a Mat of floats. There is a "convertTo' method that casts element, but we can extract them and build a new OCVMat. Need to test it,
means_c3.initialize2(1,1, mType.CV_32FC3)
means_c3.put1(0,0, Array as float( meanVals(0), meanVals(1), meanVals(2) ))

cesarcm · Dec 5, 2020

JordiCP,

Thanks ?

Probably my last question (I hope so)..

Any idea to convert these lines? (in blue color)

For (var i = 1; i < signal.rows; i++) {
' If (lrescan == True) {
' let adjV = new cv.MatVector();

' let adjR = cv.matFromArray(signal.rows, 1, cv.CV_32FC1,
' new Array(signal.rows).fill(0).fill(diff.data32F[(i-1)*3], i, signal.rows));

' let adjG = cv.matFromArray(signal.rows, 1, cv.CV_32FC1,
' new Array(signal.rows).fill(0).fill(diff.data32F[(i-1)*3+1], i, signal.rows));

' let adjB = cv.matFromArray(signal.rows, 1, cv.CV_32FC1,
' new Array(signal.rows).fill(0).fill(diff.data32F[(i-1)*3+2], i, signal.rows));

I guess that I have to create the array manually ... I do think that B4A has a function like Javascript for "new Array" like that

Thanks for all
Regards,
Cesar

JordiCP · Dec 5, 2020

I guess that 'signal' and 'diff' are OCVMat with depth=3 (cv_32FC3) and contains R,G,B channels

If you are accessing single channel elements from diff, you can first split it into 3 channels, it will be easier to manage. As it is done HERE

Regarding the code

B4X:

Dim channels As List
channels.Initialize
mCore.split( diff, channels)     '<-- Statically declare mCore as OCVCore 
Dim redChannel  As OCVMat = channels.Get(0)
Dim greenChannel  As OCVMat = channels.Get(1)
Dim blueChannel As OCVMat = channels.Get(2)

  Dim myReadData( signal.rows) as Float    'Only declare it once. Will be reused.

For i=1 to signal.rows()-1

  '---- Red channel ----
  Dim adjR as OCVMat
  adjR.initialize( signal.rows, 1, mType.CV_32F1)
  Dim valData(1) as Float
  redChannel.get3( i-1, 1, valData) 
  for j=0 to i-1
    myReadData(j) = 0
  Next
  For j=i to signal.rows-1
    myReadData(j) = valData(0)
  Next 
  adjR.put1(0,0, myReadData)

  '---- Green channel ----
  Dim adjG as OCVMat
  adjG.initialize( signal.rows, 1, mType.CV_32F1)
  Dim valData(1) as Float
  greenChannel.get3( i-1, 1, valData) 
  Dim myReadData( signal.rows) as Float
  for j=0 to i-1
    myReadData(j) = 0
  Next
  For j=i to signal.rows-1
    myReadData(j) = valData(0)
  Next 
  adjG.put1(0,0, myReadData)

 ' Do the same with the Blue channel

Next

Pls PM me if you have more issues

cesarcm · Dec 5, 2020

(clap clap clap)

Simple and beauty! Congrats!
Thanks for all.
Kind regards,
Cesar

PS: I am trying to convert all source-code to B4A until next Monday since I will have an presentation

cesarcm · Dec 5, 2020

Last question for today (I promisse it)

C++ code:

// New values
Scalar means = mean(frameRGB, mask);

// Add new values to raw signal buffer
double values[] = {means(0), means(1), means(2)};

signal.push_back(Mat(1, 3, CV_64F, values));

Thanks.
Regards,
Cesar

JordiCP · Dec 5, 2020

Java OpenCV only allows to push_back Mats, so the scalar values must be converted to a single row Mat.

Test if this works (code is assuming that signal.cols equals 3), may need some fine tuning.

B4X:

Dim means as OCVScalar = mCore.mean(frameRGB, mask)
Dim values() as Double = means.val
Dim newMat As OCVMat
newMat.Initialize2(1, signal.cols, mType.CV_64FC1)
newMat.put(0, 0, values)
signal.push_back( newMat )

Good luck with the presentation, don't forget to add me to the credits ?

cesarcm · Dec 5, 2020

JordiCP,

Credits? of course!!! ???

As like I have mentioned before: if everything goes right you will get an extra bonus (donation) or a monthly basis fee

Thanks
Regards,
Cesar

cesarcm · Dec 6, 2020

JordiCP,

Maybe a stupid question... (or not) ...

Should I always to "Initialize" an OCVMat object?

Exemple:

Dim myObj As OCVMat
myObj.initialize '???
myObj.zeros(10, 10, mType.mCvt.CV_32FC1)

Thanks
Regards,
Cesar

cesarcm · Dec 6, 2020

JordiCP,

DFT function has 4 parameters:

4th param => nonZeroRows as Int

In my libraries of DFT for Javascript / C++ / Python, DFT has only 2 o3 parameters

And I am get confused to how to convert the code below (in red):

// Javascript
timeToFrequency(signal, magnitude) {
// Prepare planes
let planes = new cv.MatVector();
planes.push_back(signal);
planes.push_back(new cv.Mat.zeros(signal.rows, 1, cv.CV_32F))
//let powerSpectrum = new cv.Mat();
cv.merge(planes, signal);

// Fourier transform
cv.dft(signal, signal, cv.DFT_COMPLEX_OUTPUT);
if (magnitude) {
cv.split(signal, planes); // planes[0] = Re(DFT(I), planes[1] = Im(DFT(I))
cv.magnitude(planes.get(0), planes.get(1), signal); // signal = magnitude
}
}

Thanks.
Regards,
Cesar

cesarcm · Dec 6, 2020

JordiCP<

Same source code as last message but in C++

void timeToFrequency(InputArray _a, OutputArray _b, bool magnitude) {

// Prepare planes
Mat a = _a.getMat();
Mat planes[] = {cv::Mat_<float>(a), cv::Mat::zeros(a.size(), CV_32F)};
Mat powerSpectrum;
merge(planes, 2, powerSpectrum);

// Fourier transform
dft(powerSpectrum, powerSpectrum, DFT_COMPLEX_OUTPUT);

if (magnitude) {
split(powerSpectrum, planes);
cv::magnitude(planes[0], planes[1], planes[0]);
planes[0].copyTo(_b);
} else {
powerSpectrum.copyTo(_b);
}
}

Thanks.
Regards,
Cesar

JordiCP · Dec 6, 2020

cesarcm said:
Should I always to "Initialize

It depends. In this case 'zeros' is a static method that returns a OCVMat.
So the original Mat does not need to be initialized, but the call should be

B4X:

myObj = myObj.zeros(10, 10, mType.mCvt.CV_32FC1)

JordiCP · Dec 6, 2020

cesarcm said:
if (magnitude) {
cv.split(signal, planes); // planes[0] = Re(DFT(I), planes[1] = Im(DFT(I))
cv.magnitude(planes.get(0), planes.get(1), signal); // signal = magnitude
}

You must declare planes as a list. Remember the code I sent yesterday with the split method.

cesarcm · Dec 6, 2020

Mr.J,

Perfect! Thanks a lot!

Regarding DFT:

DFT function has 4 parameters:

4th param => nonZeroRows as Int

In my libraries of DFT for Javascript / C++ / Python, DFT has only 2 o3 parameters

I presume that you use this 4th parameter to calculate the next power of 2, right?

Regards,
Cesar

cesarcm · Dec 6, 2020

JordiCP,

My original cvMat is NOT power2 compatible ... (signal As OCVMat)

How to copy it into a new OCVMat with next power2 allocated?

Thanks.
Regards,
Cesar

JordiCP · Dec 6, 2020

cesarcm said:
I presume that you use this 4th parameter to calculate the next power of 2, right?

Yes, it seems so.
Note also that there is a simpler method, dft1, which only needs 2 params, src and dst.

B4A Library OpenCV 3.x

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