Reading out Pixel Values, png++, C++ - c++

I am trying to load the pixel rgb/ga information of a png image into a matrix, using the library png++, to do some computations with them.
My Code (which does not work at the moment):
#include <iostream>
#include <png++/image.hpp>
#include <png++/rgb_pixel.hpp>
int main(int argc, const char * argv[]) {
const std::string path="img_03.png";
png::image< png::basic_rgb_pixel <unsigned char> > pic(path);
pixel=pic.get_pixel(0, 0);
pixelp = &pixel;
std::cout << "value=" << pic[10][10].red << std::endl; //output: '?'
std::cout << "value=" << pixel.red << std::endl; //nothing
std::cout << "pointer=" << pixelp << std::endl; //delivers adress
pic.read(path);
std::cout << "value=" << pic[10][10].red << std::endl; //nothing
pic.write("picOutput.png"); //same picture
return 0;
}
However, none of those methods work to get the rgb values of each pixel.
If there is another way to get rgb/ga information of each pixel, please mention it.
The line pic.write("picOutput.png"); delivers the same png i loaded in the line pic.read(path). This is a personal exercise for me to get more used to C++, criticise my code as much as you can.
Thanks!

Here comes the solution:
change line:
std::cout << "value=" << pic[10][10].red << std::endl; //nothing
with:
std::cout << "value=" << (int) pic[10][10].red << std::endl; //nothing
because std::cout can't output types of unsigned char.
Thanks to Alex!
For in-depth explanation, look here:
cout not printing unsigned char

Related

stringstream::seekp not functioning on Visual Studio 2015

I want to read a chunk of data from file into stringstream, which later will be used to parse the data (using getline, >>, etc). After reading the bytes, I set the buffer of the stringstream, but I cant make it to set the p pointer.
I tested the code on some online services, such as onlinegdb.com and cppreference.com and it works. However, on microsoft, I get an error - the pointers get out of order.
Here's the code, I replaced the file-read with a char array.
#include <sstream>
#include <iostream>
int main()
{
char* a = new char [30];
for (int i=0;i<30;i++)
a[i]='-';
std::stringstream os;
std::cout << "g " << os.tellg() << " p " << os.tellp() << std::endl;
os.rdbuf()->pubsetbuf(a,30);
os.seekp(7);
std::cout << "g " << os.tellg() << " p " << os.tellp() << std::endl;
}
the output I get when it works
g 0 p 0
g 0 p 7
the output I get on visual studio 2015
g 0 p 0
g -1 p -1
any ides?
thanks
std::sstream::setbuf may do nothing:
If s is a null pointer and n is zero, this function has no effect.
Otherwise, the effect is implementation-defined: some implementations do nothing, while some implementations clear the std::string member currently used as the buffer and begin using the user-supplied character array of size n, whose first element is pointed to by s, as the buffer and the input/output character sequence.
You are better off using the std::stringstream constructor to set the data or call str():
#include <sstream>
#include <iostream>
int main()
{
std::string str( 30, '-' );
std::stringstream os;
std::cout << "g " << os.tellg() << " p " << os.tellp() << std::endl;
os.str( str );
os.seekp(7);
std::cout << "g " << os.tellg() << " p " << os.tellp() << std::endl;
}

TIFF files garbled by ArrayFire (C++)

I notice that this simple ArrayFire program is causing loaded TIFF images to be heavily distorted:
#include <iostream>
#include <arrayfire.h>
int main( int argc, char** argv ) {
af::array img = af::loadImage( argv[1] );
double mn, mx;
unsigned idxn, idxx;
af::min( &mn, &idxn, img );
af::max( &mx, &idxx, img );
std::cout << "Image size = " << img.dims()[0] << ", " << img.dims()[1] << '\n';
std::cout << "Data type = " << img.type() << '\n';
std::cout << "Min = " << mn << " (at " << idxn << ")\n";
std::cout << "Max = " << mx << " (at " << idxx << ")\n";
af::saveImage( argv[2], img );
return 0;
}
I then compile and run on a simple (monochrome) image:
./a.out orig.tif out.tif
with the following output:
Image size = 256, 256
Data type = 0
Min = 0 (at 65535)
Max = 81.5025 (at 31356)
When I visualize these images I get the following result:
which of course is not what ArrayFire is expected to do; I would expect it to dump the exact same image out since I didn't make any changes to it. Unfortunately I don't know enough about the TIFF image format or the graphics backend of ArrayFire to understand what is going on. Am I doing something wrong while loading the image? (I followed the ArrayFire documentation for loadImage and saveImage).
I also tried using loadImageNative and saveImageNative alternatively, but the latter returns a 4-layer TIFF image while the original image is only a 1-layer TIFF.
Any help at all from ArrayFire experts would be great.
Thanks!

getting wrong answer by logarithm in c++

Well i was creating a program in c++ .Then i noticed that there were some error due to which my program was not functioning correctly
compiler used:- Dev c++
Error was this:-
Suppose n1=274877906944 which is 2^38. so log(n1) should be 38 which is correct. now let n2=274877906942 which is smaller than n1.
So that means if we calculate log(n2) then it must give me less than log(n1).But log(n2) gave me the same result as log(n1) which is 38.
Which is wrong!!!
Someone please explain this stuff..
You see the result that you do because of rounding. If you increase your precision, it'll be okay (note that the log2 function in <cmath> will cast your integers to double):
std::cout << std::fixed;
std::cout << std::setprecision(16);
std::cout << static_cast<double>(274877906944) << std::endl;
std::cout << static_cast<double>(274877906942) << std::endl;
std::cout << static_cast<double>(274877906948) << std::endl;
std::cout << log2(274877906944) << std::endl;
std::cout << log2(274877906942) << std::endl;
std::cout<< log2(274877906948) << std::endl;
Produces:
274877906944.0000000000000000
274877906942.0000000000000000
274877906948.0000000000000000
38.0000000000000000
37.9999999999895053
38.0000000000209965
Demo

how to iterate through boost unordered_map?

I just want to iterate through the members of an unordered map.
There are many simple examples on the web, including on this site, and yet none of them will compile. Apparently some examples are from a previous non-standard STL version, some are just old, and some are so new that my gcc 4.7.2 can't handle them. Please do not suggest the new auto iterator from C++11. I will get there some day when all my libraries are validated for that. Until then, I just want the old one to work. (see below for what I have tried)
Here is my test code:
#include <iostream>
#include <boost/unordered_map.hpp>
#include <string>
int main(int argc,char *argv[]) {
boost::unordered::unordered_map<std::string,int> umap;
//can't get gcc to accept the value_type()...
//umap.insert(boost::unordered_map::value_type("alpha",1));
//umap.insert(boost::unordered_map::value_type("beta",2));
//umap.insert(boost::unordered_map::value_type("gamma",3));
umap["alpha"]=1; //this works
umap["beta"]=2;
umap["gamma"]=3;
//can't get gcc to compile the iterator
//for (boost::unordered_map::iterator it=umap.begin();it!=umap.end();++it)
// std::cout << it->first <<", " << it->second << std::endl;
//gcc does not like it this way either
//for (int x=0;x<umap.size();x++)
// std::cout << x << " : " << umap[x].first << " = " << umap[x].second << std::endl;
//will gcc take this? No it does not
//for (int x=0;x<umap.size();x++)
// std::cout << x << " : " << umap[x] << std::endl;
//this does not work
//boost::unordered::unordered_map::iterator<std::string,int> it;
//this does not work
//boost::unordered::unordered_map::iterator it;
//for (it=umap.begin();it!=umap.end();++it)
// std::cout << it->first <<", " << it->second << std::endl;
//this does not work
//BOOST_FOREACH(boost::unordered_map::value_type value, umap) {
// std::cout << value.second;
// }
//std::cout << std::endl;
//this does not work either
//BOOST_FOREACH(boost::unordered_map::value_type<std::string,int> value, umap) {
// std::cout << value.second;
// }
//std::cout << std::endl;
std::cout << "umap size: " << umap.size() << std::endl;
std::cout << "umap max size: " << umap.max_size() << std::endl;
std::cout << "find alpha: " << (umap.find("alpha")!=umap.end()) << std::endl;
std::cout << "count beta: " << umap.count("beta") << std::endl;
}
Most of the errors are a variation of this:
error: 'template<class K, class T, class H, class P, class A> class boost::unordered::unordered_map' used without template parameters
Here is my build command:
g++ -I..\boost umap.cpp
I should be embarrassed for getting stuck on such a beginner's question, but from the volume of similar questions I am finding, this is just hard enough to stop a lot of people in their tracks. I have written hash containers before (back when it was recommended NOT to use STL) and I am very tempted to just write my own... but the right thing to do is learn to use as many existing tools as possible... help!
I've looked at the following questions on stackoverflow where I haven't found an answer:
iterate through unordered_map using boost_foreach
I tried:
BOOST_FOREACH(boost::unordered_map::value_type& value, umap) {
but it gives the same error I show below.
Unordered_map iterator invalidation
This one is close, but not quite my issue:
Iterator invalidation in boost::unordered_map
This one uses auto
and I can't switch compilers at this time.
C++ some questions on boost::unordered_map & boost::hash
This one is mostly about the theory of maps:
how to use boost::unordered_map
This is a rather complicated example, but you will see in my code I am already trying to declare iterators... they just won't compile.
How to use BOOST_FOREACH with an Unordered_map?
This is a nice example, but
it just does not compile. I tried a version of this in my code.
IT WORKS !
Here is the working code:
#include <iostream>
#include <boost/unordered_map.hpp>
#include <string>
int main(int argc,char *argv[]) {
boost::unordered::unordered_map<std::string,int> umap;
umap["alpha"]=1;
umap["beta"]=2;
umap["gamma"]=3;
boost::unordered::unordered_map<std::string,int>::iterator it;
for (it=umap.begin();it!=umap.end();++it)
std::cout << it->first <<", " << it->second << std::endl;
std::cout << "umap size: " << umap.size() << std::endl;
std::cout << "umap max size: " << umap.max_size() << std::endl;
std::cout << "find alpha: " << (umap.find("alpha")!=umap.end()) << std::endl;
std::cout << "count beta: " << umap.count("beta") << std::endl;
}
It was a syntax error. I was putting the type in the wrong place when declaring the iterator.
Thanks everyone for your responses.
try changing boost::unordered::unordered_map::iterator it; it to boost::unordered::unordered_map<std::string,int>::iterator it;
NOTE:
It is also possible, and a good idea in more complex situations, to create a typedef of it, such as typedef boost::unordered::unordered_map<std::string,int>::iterator UMapStringIntIt;, or whatever you may call it.
The answer is in the question, but the simple solution is here for your convenience:
#include <iostream>
#include <boost/unordered_map.hpp>
#include <string>
int main(int argc,char *argv[])
{
boost::unordered::unordered_map<std::string,int> umap;
umap["alpha"]=1;
umap["beta"]=2;
umap["gamma"]=3;
for ( auto it= umap.begin(); it != umap.end(); ++it )
std::cout << it->first <<", " << it->second << std::endl;
}

How to use RabinMillerTest() in CryptoPP?

Can anyone share an example of RabinMillerTest() that works? My googlefu is sadly lacking.
Here is my test code:
#include "integer.h"
#include "nbtheory.h"
#include "cryptlib.h"
#include "osrng.h"
#include <iostream>
int main(int argc,char *argv[])
{
CryptoPP::RandomNumberGenerator rng;
CryptoPP::Integer a("123456789");
CryptoPP::Integer b;
std::cout << a << std::endl;
std::cout << "is prime: " << IsPrime(a) << std::endl;
b=a+CryptoPP::Integer::Two();
std::cout << b << std::endl;
std::cout << "is prime: " << IsPrime(b) << std::endl;
int r=RabinMillerTest(rng,&b,2);
std::cout << "RabinMiller: " << r <<std::endl;
}
Here is the build command:
g++ bignum.cpp -I. -fpermissive libcryptopp.a
The only example I could find is here:
http://www.cryptopp.com/wiki/Diffie-Hellman
My code crashes every time I run it, and one crash was so bad I had to cold boot to recover. If I remove the call to RabinMillerTest() then it runs fine. I suspect a problem with C++ syntax is the culprit, but I just don't see it. Since I can't use it properly, I am currently stripping the guts of the RabinMillerTest() function to use separately.
You're using the wrong signature for MillerRabinTest. Here's what it is (from nbtheory.h):
CRYPTOPP_DLL bool CRYPTOPP_API RabinMillerTest(RandomNumberGenerator &rng,
const Integer &w, unsigned int rounds);
Instead of:
int r=RabinMillerTest(rng,&b,2);
Try:
bool b=RabinMillerTest(rng, b, 2);
There's also a discussion of Miller-Rabin round counts on the Cryptography Stack Exchange at Trial divisions before Miller-Rabin checks?. It was written with OpenSSL and Crypto++ in mind.
This was the answer edited into the question. Adding as a separate answer block. Credit goes to user3920315.
UPDATE: I got it to work. Here is the working code:
#include "integer.h"
#include "nbtheory.h"
#include "cryptlib.h"
#include "osrng.h"
#include <iostream>
int main(int argc,char *argv[])
{
//CryptoPP::RandomNumberGenerator rng;
CryptoPP::NonblockingRng rng;
CryptoPP::Integer a("123456789");
CryptoPP::Integer b;
std::cout << a << std::endl;
std::cout << "is prime: " << IsPrime(a) << std::endl;
b=a+CryptoPP::Integer::Two();
std::cout << b << std::endl;
std::cout << "is prime: " << IsPrime(b) << std::endl;
bool r=RabinMillerTest(rng,b,2);
std::cout << "RabinMiller: " << r <<std::endl;
}
I changed the call to RabinMillerTest() but the resulting executable would still crash.
When I changed the random number generator to a class that inherits from RandomNumberGenerator, then it works.