I'm trying to call JavaCV's groupRectangles function. Here is the method signature
#Namespace("cv") public static native void groupRectangles(#StdVector("CvRect,cv::Rect") CvRect rectList,
#StdVector IntPointer weights, int groupThreshold, double eps/*=0.2*/);
From the OpenCV documentation, parameters one and two should be std::vector<>'s. I would assume I should pass in some form of Java array (native array, List, etc.), however, the method only has an annotation for a #StdVector, and accepts just a single object.
Can someone please provide a very small snippet of code showing how to properly call this method by passing in two Java lists or arrays or whatever.
I have found the answer through the mailing list here. The only modification I made is below:
final CvRect rectsPointer = new CvRect(faces.size());
for (int i=0; i<faces.size(); ++i)
rectsPointer.position(i).put(faces.get(i));
Related
I was wondering if it was possible to access the values of fields in an object with their names in a manner analogous to accessing values in a map with the key names. For example, something like this
void main() {
MyData d=MyData();
List<String> fieldNames=['a','b','c'];
for(var name in fieldNames){
print('This is the value of the $name field: ${d[name]}}');
}
}
class MyData{
String a='A';
String b='B';
String c='C';
}
Of course, this doesn't work because Dart doesn't quite know what to make of d[name] because d is an object. But if d was a map, it would work. Like this.
void main() {
Map d=myData;
List<String> fieldNames=['a','b','c'];
for(var name in fieldNames){
print('This is the value of the $name field: ${d[name]}}');
}
}
Map myData={
'a':'A',
'b':'B',
'c':'C',
};
So my question is this. If I have a class, is there any way to treat it like a Map in the situations where I might want to refer to several of the field values indirectly via their names like I tried to do above? Or is this sort of trick not possible in a compiled language like Dart?
The short answer is "no". The longer answer is "noooooooo". :)
But seriously, the namespace of your program code is very separate from the data values that are managed by your code. This isn't JavaScript, where we can freely flow from code to data to code again.
I'd like to create a library, written in Java, callable from C, with simple method signatures:
int addThree(int in) {
return in + 3;
}
I know it's possible to do this with GraalVM if you do a little dance and create an Isolate in your C program and pass it in as the first parameter in every function call. There is good sample code here.
The problem is that the system I'm writing for, Postgres, can load C libraries and call functions in them, but I would have to create a wrapper function in C that would wrap every function I wanted to expose. This really limits the value of being able to slap something together in Java and use it in Postgres directly. I'd have to do something like this:
int myPublicAddThreeFunction(int in) {
graal_isolatethread_t *thread = NULL;
if (graal_create_isolate(NULL, NULL, &thread) != 0) {
fprintf(stderr, "error on isolate creation or attach\n");
return 1;
}
return SomeClassName_addThree_big_random_string_here(thread, in);
}
Is there a way, in Java alone, to expose a simple C function? I'm thinking I could create the isolate in a static method that gets loaded once on startup, somehow set it as the current isolate, and have the Java method just use it. Haven't been able to figure it out, though.
Also, it would be real nice not to have to append a big random string to every function name.
Use case: I am converting data from a very old program of mine to a database friendly format. There are parts where I have to do multiple passes over the old data, because in particular the keys have to first exist before I can reference them in relationships. So I thought why not put the incomplete parts in a vector of references during the first pass and return it from the working function, so I can easily use that vector to make the second pass over whatever is still incomplete. I like to avoid pointers when possible so I looked into std::reference_wrapper<T> which seemes like exactly what I need .. except I don't understand it's behavior at all.
I have both vector<OldData> old_data and vector<NewData> new_data as member of my conversion class. The converting member function essentially does:
//...
vector<reference_wrapper<NewData>> incomplete;
for(const auto& old_elem : old_data) {
auto& new_ref = *new_data.insert(new_data.end(), convert(old_elem));
if(is_incomplete(new_ref)) incomplete.push_back(ref(new_ref));
}
return incomplete;
However, incomplete is already broken immediately after the for loop. The program compiles, but crashes and produces gibberish. Now I don't know if I placed ref correctly, but this is only one of many tries where I tried to put it somewhere else, use push_back or emplace_back instead, etc. ..
Something seems to be going out of scope, but what? both new_data and old_data are class members, incomplete also lives outside the loop, and according to the documentation, reference_wrapper is copyable.
Here's a simplified MWE that compiles, crashes, and produces gibberish:
// includes ..
using namespace std;
int main() {
int N = 2; // works correctly for N = 1 without any other changes ... ???
vector<string> strs;
vector<reference_wrapper<string>> refs;
for(int i = 0; i < N; ++i) {
string& sref = ref(strs.emplace_back("a"));
refs.push_back(sref);
}
for (const auto& r : refs) cout << r.get(); // crash & gibberish
}
This is g++ 10.2.0 with -std=c++17 if it means anything. Now I will probably just use pointers and be done, but I would like to understand what is going on here, documentation / search does not seem to help..
The problem here is that you are using vector data structure which might re-allocate memory for the entire vector any time that you add an element, so all previous references on that vector most probably get invalidated, you can resolve your problem by using list instead of vector.
I am working on a Java binding for the excellent libvips
Using this function all is fine:
VipsImage *in;
in = vips_image_new_from_file( test.jpg, NULL )
vips_image_write_to_file( in, "out.jpg", NULL )
So mapped in Java:
Pointer vips_image_new_from_file(String filename,String params);
But I have a problem when the parameter like this:
VipsImage *in;
VipsImage *out;
vips_invert( in, &out, NULL )
vips_image_write_to_file( out, "out.jpg", NULL )
I have tried:
int vips_resize(Pointer in, PointerByReference out, Double scale, String params);
Pointer in = vips_image_new_from_file("file.png",null);
PointerByReference ptr1 = new PointerByReference();
vips_invert(in, ptr1, null);
vips_image_write_to_file( ptr1.getValue(), "fileout.png", null);
But doesn't work. The ptr1.getValue() does not contains the expected result.
How can I do it?
Thanks
I'm the libvips maintainer, a Java binding would be great!
But I think you might be taking the wrong approach. I think you are trying a straight wrap of the C API, but that's going to be tricky to do well, since it makes use of a lot of C-isms that don't map well to Java. For example, in C you can write:
VipsImage *image;
if (!(image = vips_image_new_from_file("somefile.jpg",
"shrink", 2,
"autorotate", TRUE,
NULL)))
error ...;
ie. the final NULL marks the end of a varargs name / value list. Here I'm asking the jpeg loader to do a x2 shrink during load, and to apply any Orientation tags it finds in the EXIF.
libvips has a lower-level API based on GObject which is much easier to bind to. There's some discussion and example code in this issue, where someone is making a C# binding using p/invoke.
https://github.com/jcupitt/libvips/issues/558
The code for the C++ and PHP bindings might be a useful reference:
https://github.com/jcupitt/libvips/tree/master/cplusplus
https://github.com/jcupitt/php-vips-ext
That's a PHP binding for the entire library in 1800 lines of C.
I'd be very happy to help if I can. Open an issue on the libvips tracker:
https://github.com/jcupitt/libvips/issues
I'm basically reading several fields in an OpenCV FileNode, doing some calculations, and then replacing just one of the fields in the original OpenCV FileNode with the calculated data.
I can't find any methods that allow me to do this. Has anyone done this before?
Thanks for your help!
After some searching, it appears that I cannot append or add to an existing filenode or filestorage object.
However, what you can do is create a new FileStorage object, copy over any existing FileNodes you may want to keep, discard any old fields you do not want, and then save this new FileStorage object with the updated data.
I couldn't find a way to use the latest C++ interface to copy FileNodes, however, I did find a way to use the old deprecated structures to accomplish this. The following will copy mapped FileNode to a new FileStorage object.
cv::FileStorage fileStructure;
fileStructure.open("yourfile.xml", cv::FileStorage::WRITE);
CvFileStorage* fsPtr= fileStructure.operator *(); //gets the underlying pointer
cvWriteFileNode(fsPtr, "CopiedNode", fileNodeObject.operator *(),0);
I hope this helps people who are stuck like I was.
Best,
Paul
In OpenCV 4.0, there is a member function of FileNode
void setValue (int type, const void *value, int len=-1);
I haven't try, but it seems to design for this.
There seems not to be any official way to do this prior OpenCV 4.x. A workaround is to use the underlying basic element:
//type int with value of 5
cv::FileNode node;
CvFileNode* rawNode = *node;
//now the new value is 6
rawNode->data.i = 6;