I am working in visual c++, usually I do it on .NET, because I need a method which is available only on this language. What I want to do is obtain the frames per second of a video file. The best I could make was creating a project with this main() method, in which (after Debug) I could see the result is saving fine in the res variable.
void main()
{
// initialize the COM library
CoInitialize(NULL);
// get a property store for the video file
IPropertyStore* store = NULL;
SHGetPropertyStoreFromParsingName(L"C:\\Users\\Public\\Videos\\Sample Videos\\Wildlife.wmv",
NULL, GPS_READWRITE, __uuidof(IPropertyStore), (void**)&store);
// get the frame rate
PROPVARIANT variant;
store->GetValue(PKEY_Video_FrameRate, &variant);
int res = variant.intVal;
store->Release();
}
Now, I want to create this method generic, in order to obtain the frameRate of any video. For example, if the method's name is frameRate:
char* path = "C:\\Users\\Public\\Videos\\Sample Videos\\Wildlife.wmv";
int fps = frameRate(path);
Thanks
Does this not work?
int getFrameRate(std::wstring path)
{
// initialize the COM library
CoInitialize(NULL);
// get a property store for the video file
IPropertyStore* store = NULL;
SHGetPropertyStoreFromParsingName(path.c_str(),
NULL, GPS_READWRITE, __uuidof(IPropertyStore), (void**)&store);
// get the frame rate
PROPVARIANT variant;
store->GetValue(PKEY_Video_FrameRate, &variant);
int res = variant.intVal;
store->Release();
return res;
}
The assumption here is that SHGetPropertyStoreFromParsingName takes a string as its first parameter. In C++ I recommend staying away from char*, std::string is preferable in almost all situations. The only difficulty I see is making sure path is the correct type.
If you don't want to recompile your code for every video path, then you can read the path from the program parameters. To do that, modify you main() as follows:
int main(int argc, char* argv[])
{
if (argc != 2)
{
std::cout << "You have to specify the video path!" << std::endl;
return 1;
}
const char* path = arg[1];
// Rest of the program logic
return 0;
}
You can pass more than one parameter, if you want to. Note that there is always at least 1 argument (arg[0] is the program name). For further reading on the topic go here.
Related
I coded an Arduino project for my son and learned about C in the process. All works fine but after dividing up the code into ten files and grouping the variables into structs in each file I'm not able to solve one wish for clarity. We need to empirically determine the best size of an array for storing and averaging port reads so this is what I want:
struct Alarms {
// Configurable parameters
const unsigned int number_of_reads = 24;
// State variables
int reads[number_of_reads]; // Error: invalid use of non-static data member 'Alarms::num_of_reads'
};
It’s simple but doesn't work. I tried flexible array members until I found that that feature is not supported in C++. Arduino compiles with C++. I tried many examples of the 'struct hack' but they all returned errors like this one:
struct Alarms {
// Configurable parameters
int number_of_reads = 24;
// State variables
int reads[];
} ar;
void setup_alarm() {
ar.reads = malloc(sizeof(int) * ar.number_of_reads); // Error: incompatible types in assignment of 'void*' to 'int [0]'
}
That looked promising but I suspect my ignorance is glowing brightly. Most struct hack examples call for declaring the struct and later initializing the struct variables. I’m hoping to not duplicate the struct.
I considered splitting the struct but that would be error prone and, well, another compile error:
struct Alarms2 {
int reads[ar.num_of_reads]; // Error: array bound is not an integer constant before ']' token
} ar2;
An alternative is to size the array and get the size later but it needs an explanation:
struct Alarms {
// Configurable parameters
int reads[ 24 ]; // Put number of reads to average between brackets
// State variables
int number_of_reads;
};
void setup_alarm() {
ar.number_of_reads = sizeof(ar.reads) / sizeof(ar.reads[0]); // this works
}
Is there a way to work the struct hack or some similar solution in Arduino to like achieve the first example?
The size of the struct must be known at compilation time. Const data types in structs can change per instance of the structure, that is why you are getting the invalid use of non-static data member 'Alarms::num_of_reads' when you try to initialize your array. The best way to solve this is to have an init_alarm and destroy_alarm functions. Like so ...
#include <stdio.h>
#include <stdlib.h>
#define DEFAULT_NUM_OF_READS (24)
struct alarm {
// Configurable parameters
const int number_of_reads;
// State variables
int *reads;
};
void init_alarm(struct alarm *alarm)
{
alarm->reads = (int *) malloc(alarm->number_of_reads * sizeof(int));
}
void destroy_alarm(struct alarm *alarm)
{
free(alarm->reads);
}
int main(int argc, char **argv)
{
// When we create our struct, set number_of_reads to default
struct alarm alarm = {.number_of_reads = DEFAULT_NUM_OF_READS, .reads = NULL};
init_alarm(&alarm);
alarm.reads[0] = 13;
alarm.reads[23] = 100;
printf("alarm.reads[0] = %d, alarm.reads[23] = %d\n", alarm.reads[0], alarm.reads[23]);
destroy_alarm(&alarm);
return 0;
}
Note: Inorder to use the designated initializer to initialize a structure you must compile with ANSI (C99) like so ...
gcc --std=c99 test.c -o test
As part of a WinRT C++cx component, what's the most efficient way to convert an unmanaged buffer of bytes (say expressed as a std::string) back and forth with a Windows::Web::Http::HttpBufferContent?
This is what I ended up with, but it doesn't seem very optimal:
std::string to HttpBufferContent:
std::string m_body = ...;
auto writer = ref new DataWriter();
writer->WriteBytes(ArrayReference<unsigned char>(reinterpret_cast<unsigned char*>(const_cast<char*>(m_body.data())), m_body.length()));
auto content = ref new HttpBufferContent(writer->DetachBuffer());
HttpBufferContent to std::string:
HttpBufferContent^ content = ...
auto operation = content->ReadAsBufferAsync();
auto task = create_task(operation);
if (task.wait() == task_status::completed) {
auto buffer = task.get();
size_t length = buffer->Length;
if (length > 0) {
unsigned char* storage = static_cast<unsigned char*>(malloc(length));
DataReader::FromBuffer(buffer)->ReadBytes(ArrayReference<unsigned char>(storage, length));
auto m_body = std::string(reinterpret_cast<char*>(storage), length);
free(storage);
}
} else {
abort();
}
UPDATE: Here's the version I ended up using (you can trivially create a HttpBufferContent^ from an Windows::Storage::Streams::IBuffer^):
void IBufferToString(IBuffer^ buffer, std::string& string) {
Array<unsigned char>^ array = nullptr;
CryptographicBuffer::CopyToByteArray(buffer, &array); // TODO: Avoid copy
string.assign(reinterpret_cast<char*>(array->Data), array->Length);
}
IBuffer^ StringToIBuffer(const std::string& string) {
auto array = ArrayReference<unsigned char>(reinterpret_cast<unsigned char*>(const_cast<char*>(string.data())), string.length());
return CryptographicBuffer::CreateFromByteArray(array);
}
I think you are making at least one unnecessary copy of your data in your current approach for HttpBufferContent to std::string, you could improve this by accessing the IBuffer data directly, see the accepted answer here: Getting an array of bytes out of Windows::Storage::Streams::IBuffer
I think it's better to use smart pointer (no memory management needed) :
#include <wrl.h>
#include <robuffer.h>
#include <memory>
using namespace Windows::Storage::Streams;
using namespace Microsoft::WRL;
IBuffer^ buffer;
ComPtr<IBufferByteAccess> byte_access;
reinterpret_cast<IInspectable*>(buffer)->QueryInterface(IID_PPV_ARGS(&byte_access));
std::unique_ptr<byte[]> raw_buffer = std::make_unique<byte[]>(buffer->Length);
byte_access->Buffer(raw_buffer.get());
std::string str(reinterpret_cast<char*>(raw_buffer.get())); // just 1 copy
In my project, I use to load textures by specifying its file name. Now, I made this function const char* app_dir(std::string fileToAppend); that returns the mains argv[0] and change the application name by the fileToAppend. Since I cannot make the string manipulation easy with a char*, I use the std::string. My texture loader takes a const char* for file name so need to switch back to c_str(), now it generates a sequence of ASCII symbol characters (bug). I already fix the problem by changing the return type of the app_dir() to std::string. But why is that happening?
EDIT
sample code:
//in main I did this
extern std::string app_filepath;
int main(int argc, char** arv) {
app_filepath = argv[0];
//...
}
//on other file
std::string app_filepath;
void remove_exe_name() {
//process the app_filepath to remove the exe name
}
const char* app_dir(std::string fileToAppend) {
string str_app_fp = app_filepath;
return str_app_fp.append(fileToAppend).c_str();
//this is the function the generates the bug
}
I already have the functioning one by changing its return type to std::string as I said earlier.
A big no no :) returning pointer to local objects
return str_app_fp.append(fileToAppend).c_str();
Change your function to
std::string app_dir(const std::string& fileToAppend) {
string str_app_fp = app_filepath + fileToAppend;
return str_app_fp;
}
And on the return value use c_str()
When you using function const char* app_dir(std::string fileToAppend); you get pointer to the memory that allocated on the stack and already deleted when the function ends.
I am trying to put number of frames which are taken from a video in a loop.I want to display that frames in sequence an after that I want to subtract it using opencv 2.3.
My problem is that I am not able to know where function is not called.
here is my code below:
using namespace cv;
void loadImage(IplImage *image, int number)
{
// Store path to directory
char filename[100];
strcpy(filename, "S:\FINAL PROJECT ABSTRACT\images 1");
char *frame;
// Convert integer to char
char frameNo[10];
//sprintf(frame, "%0.3i", number);
// Combine to generate path
strcat(filename, frameNo);
strcat(filename, ".jpg");
// Use path to load image
image = cvLoadImage(filename);
}
int _tmain(int argc, _TCHAR* argv[]){
IplImage *im=0;
int nImages = 6;
for (int i = 0; i < nImages; ++i)
{
loadImage(im, i);
char filename[100];
strcpy(filename, "images 1");
char frameNo[10];
sprintf(frameNo, "%03i", i);
strcat(filename, frameNo);
strcat(filename, ".jpg");
IplImage *im = cvLoadImage(filename,CV_LOAD_IMAGE_COLOR);
cvNamedWindow("pic");
cvShowImage("pic",im);
cvWaitKey();
}
}
//}
I am not getting any error in build.bt while debugging it shows:-
Unhandled exception at 0x77db15de in loop of frames.exe: 0xC0000005: Access violation.
At
strcat(filename, frameNo);
strcat(filename, ".jpg");
this point some error is there..
Your string handling is pretty confused. It's not clear exactly what path you're trying to generate.
You can replace most of your path generation with a single sprintf()
char filename[100];
sprintf(filename, "S:\\FINAL PROJECT ABSTRACT\\images 1%03i.jpg",number);
image = cvLoadImage(filename);
(100 chars seems somewhat arbitrary - and potentially a bit short)
No idea if that's the right string for your image paths, you were trying to insert a '.' in there, so I don't know what your real file paths look like.
However having loaded the image inside the loadImage() function, you then seem to do the exact same thing in main() (throw away the image you've just loaded, generate another path, and then load that instead). So I doubt this is going to work even when you fix the string handling.
I had a perfectly working OpenCV code (having the function cvCaptureFromCAM(0)). But when I modified it to run in a separate thread, I get this "Video Source" selection dialog box and it asks me to choose the Webcam. Even though I select a cam, it appears that the function cvCaptureFromCAM(0) returns null. I also tried by passing the values 0, -1,1, CV_CAP_ANYto this function. I have a doubt that this dialog box causes this issue. Is there any way to avoid this or does anyone have any other opinion?
I've followed the following posts when debugging:
cvCreateCameraCapture returns null
OpenCV cvCaptureFromCAM returns zero
EDIT
Code structure
//header includes
CvCapture* capture =NULL;
IplImage* frame = NULL;
int main(int argc, char** argv){
DWORD qThreadID;
HANDLE ocvThread = CreateThread(0,0,startOCV, NULL,0, &qThreadID);
initGL(argc, argv);
glutMainLoop();
CloseHandle(ocvThread);
return 0;
}
void initGL(int argc, char** argv){
//Initialize GLUT
//Create the window
//etc
}
DWORD WINAPI startOCV(LPVOID vpParam){
//capture = cvCaptureFromCAM(0); //0 // CV_CAP_ANY
if ((capture = cvCaptureFromCAM(1)) == NULL){ // same as simply using assert(capture)
cerr << "!!! ERROR: vCaptureFromCAM No camera found\n";
return -1;
}
frame = cvQueryFrame(capture);
}
//other GL functions
Thanks.
Since this is a problem that only happens on Windows, an easy fix is to leave cvCaptureFromCAM(0) on the main() thread and then do the image processing stuff on a separate thread, as you intented originally.
Just declare CvCapture* capture = NULL; as a global variable so all your threads can access it.
Solved. I couldn't get rid of the above mentioned dialog box, but I avoided the error by simply duplicating the line capture = cvCaptureFromCAM(0);
capture = cvCaptureFromCAM(0);
capture = cvCaptureFromCAM(0);
It was just random. I suspect it had something to do with behavior of Thread. What's your idea?
Thanks all for contributing.