May 2010 - Boost-users - lists.boost.org

[GIL] Some questions about 2D iteration
by Brett Gmoser 07 Jun '10

07 Jun '10

Hello everybody, I've been toying with GIL a bit for the last few days in my spare time, and I've got a few questions that I can't seem to find answers to in the documentation. The following function which I've written works, but I'd like to get a better understanding of how to use iterators to accomplish the same task (the documentation says that view(x, y) = z is inefficient), as well as a few other things. The job of this function is to crop an image, discarding any white background. In other words, find the top-most, left-most, right-most, and bottom-most pixel that isn't the background color (white), and crop the image around those coordinates. template <typename ViewType, typename ImageType> void crop_image(ViewType& src, boost::scoped_ptr<ImageType>& dst) const { int top = 0; int right = 0; int bottom = 0; int left = 0; // Find the top, right, bottom, and left extents. First, find the top. for(int y=0; y < src.height(); ++y) { for(typename ViewType::x_iterator xpos = src.row_begin(y); xpos != src.row_end(y); ++xpos) { if(rgb8_pixel_t(255,255,255) != *xpos) { top = y; break; } } if(top) break; } // ... Do similar things for finding the right, bottom, and left extents. // We should now have the rectangle of the image, and it should be easy to copy everything over. dst.reset(new ImageType(1 + (right - left), 1 + (bottom - top))); for(int x = left; x <= right; ++x) for(int y = top; y <= bottom; ++y) view(*dst)((x - left), (y - top)) = src(x, y); } So, my first question is - is there a better way to do that first chore of finding each extent? Such as a way to use std::find or std::find_if, keeping in mind that I have to do two of the four sides in reverse (bottom-to-top to find bottom-most, right-to-left to find right-most)? The next question is about the final loop which copies from the source view to the newly created destination image. It seems that using iterators, I could do something like this, assuming that I had the above loops find iterator positions for all four extents (left, top, bottom, and right are iterators): // Start the copying process by creating a column iterator for the destination table beginning at column 0. // The top iterator starts at the top-most extent - and we cycle through the source image in a top-to-bottom, // left-to-right fashion. for(typename ImageType::view_t::y_iterator dst_y = view(*dst).col_begin(0); top <= bottom; ++dst_y,++top) { // The left iterator needs to be initialized with whatever row we're on - this appears to be impossible. There doesn't // seem to be any suitable way to do this, so consider it pseudocode. typename ViewType::x_iterator src_x = left.y(); // The destination X iterator also needs to know what Y position to begin on. You'd think you would be able to initialize // it with an iterator position, but it doesn't seem that you can. Again, this doesn't actually compile (it's the // view(*dst).row_begin(y) part). for(typename ImageType::view_t::x_iterator dst_x = view(*dst).row_begin(y); // Here, I need to be able to tell if src_x has hit the right extent. src_x <= right doesn't seem it will work, because // again right doesn't know it's Y position. src_x != right; // And increment both src and destination iterators. ++src_x,++dst_x ) { // Finally do the should-be-simple task of assigning the source pixel to the destination pixel. *dst_x = *src_x; } } If you read the comments, most of this doesn't seem to be possible. I've seen that there is a "transform_pixel_positions" algorithm that may do something close to what I want, but unfortunately I cannot find any documentation on it (other than mentioning that it exists, and a short example that really doesn't help - one of the few examples actually provided by the documentation). It seems to pass a unary argument to a functor, but that doesn't seem useful if I don't have both the X and Y coordinates, or an X iterator and Y iterator. Also, nowhere can I find where it says what the argument passed to the functor actually /is/. So after I figure these few things out, I'm hoping to be able to speed up my program a little bit. Right now everything is pretty slow using that view(x, y) method. I'd appreciate any help that anybody can give, thanks! Brett

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[phoenix][lambda] lambda to phoenix translation: get sig (signature) from actor
by alfC 05 Jun '10

05 Jun '10

hi, what is the translation of boost.lambda typename LambdaExp::template sig<tuple<InputType> >::type in Boost.Phoenix? my best try was: actor<LambdaExp>::result< vector<InputType> >::type but it doesn't compile. Thank you, Alfredo

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[Test] Tests execution order and depends_on method.
by Maria Kozinska 04 Jun '10

04 Jun '10

Hello, I have couple of question to Boost.Test framework. I'd like to ask why test_unit::depends_on(test_unit*) method is not described in User's guide. Is it only for internal use by framework or I can use it freely? I assume, that it can be used to determine precedence of one test before another. What happens if there is a cycle in dependence graph? If a test case fails, are dependant on it tests executed? In what order are executed automatically registered test suites and test cases? And in what order when they are manually registered (without any depends_on)? How this order is affected by depends_on method? Thanks for answer. Greetings, Maria

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[lamda] smart self binding of a function object
by alfC 02 Jun '10

02 Jun '10

Hi, I usually have function-classes that take two arguments. class f{ double operator()(double x, double y) const { x*y+y; } }; called as: f myf; cout << myf(4.,3.); for some purposes I need a function of only one variable (parametrized in the first or second argument. So I end up doing: using boost::function; using namespace boost::lambda; function<double(double)> myf_at5 = bind(&f::operator(), myf, 5., _1); // myf_at5 is myf(5.,*) So, now I can pass myf_at5 to something that accepts a function of one variable. root_finder( myf_at5, ...); since I have access to f I would like to make f smart enough to handle its own binding. Eg. class f{ ... // same as before function<double(double)> operator()(double x_value, YYYY y_arg){ return bind( &f::operator(), //maybe needs cast to double(*) (double,double) *this, x_value, y_arg ); } function<double(double)> operator()(XXXX x_arg, double y_value){ return bind( &f::operator(), //maybe needs cast *this, x_arg, y_value ); } }; The question is: What should be the type of XXXX and YYYY? in such a way that I can just call root_finder( myf(5., _1) , ... ); or more complicated things like root_finder( myf(5., _1 * 2.), ...); in other words, myf(5.,_1) is the same as the old bind(&f::operator(), myf, 5., _1); Thank you, Alfredo answers using other libraries are also accepted :)

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Boost 1.43 - Exception before main() is called
by Rob Yull 01 Jun '10

01 Jun '10

I recently upgrade from boost 1.42 to 1.43. All of a sudden, the project I was working on seems to be throwing an exception before main() is called. I was able to reproduce this error in the example below. Using 1.42, it executes just fine. Using 1.43, an exception is thrown before main is reached. In some of my classes, I have a static mutex variable that I lock before I manipulate other static variables in the class (as seen below). main.cpp - Start #include "Test.hpp" int main() { Test test; return 0; } main.cpp - End Test.hpp - Start #ifndef DEFINE_TEST_HPP #define DEFINE_TEST_HPP #include <boost/thread.hpp> class Test { public: Test(); ~Test(); private: typedef boost::mutex StaticMutex; static StaticMutex s_Mutex; typedef boost::lock_guard<StaticMutex> StaticMutexLock; }; #endif // DEFINE_TEST_HPP Test.hpp - End Test.cpp - Start #include "Test.hpp" Test::StaticMutex Test::s_Mutex; Test::Test() { const StaticMutexLock Lock(s_Mutex); } Test::~Test() { } Test.cpp - End The code above, when using boost 1.43, produces the following error in both VC2010 Express and VC2005 Pro. First-chance exception at 0x7569b727 (KernelBase.dll) in BoostTest.exe: Microsoft C++ exception: boost::exception_detail::clone_impl<boost::exception_detail::bad_alloc_> at memory location 0x0030f6e4.. The call stack (>>> is where the exception is caught). KernelBase.dll!_RaiseException@16() + 0x58 bytes msvcr100d.dll!_CxxThrowException(void * pExceptionObject, const _s__ThrowInfo * pThrowInfo) Line 157 C++ >>>BoostTest.exe!boost::copy_exception<boost::exception_detail::bad_alloc_>( const boost::exception_detail::bad_alloc_ & e) Line 47 C++ BoostTest.exe!boost::exception_detail::get_bad_alloc<42>() Line 80 + 0x9f bytes C++ BoostTest.exe!`dynamic initializer for 'boost::exception_detail::exception_ptr_bad_alloc<42>::e''() Line 94 + 0x28 bytes C++ msvcr100d.dll!_initterm(void (void)* * pfbegin, void (void)* * pfend) Line 873 C BoostTest.exe!__tmainCRTStartup() Line 473 + 0xf bytes C BoostTest.exe!mainCRTStartup() Line 371 C kernel32.dll!@BaseThreadInitThunk@12() + 0x12 bytes ntdll.dll!___RtlUserThreadStart@8() + 0x27 bytes ntdll.dll!__RtlUserThreadStart@8() + 0x1b bytes Did something change between 1.42 and 1.43 that would cause this? -Rob Yull

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[Random] random_device and windows - multiply defined symbols
by Tore Halvorsen 01 Jun '10

01 Jun '10

The release notes for Boost 1.43 says "Put the implementation of random_device in a compiled lib, and added Windows support." Trying to use this library does not seem to work for me, under visual studio 2005, with static libraries. #include <cstdlib> #include <boost/nondet_random.hpp> int main(int argc, char* argv[]) { boost::random_device d; return EXIT_SUCCESS; } 1>libboost_random-vc80-mt-sgd-1_43.lib(random_device.obj) : error LNK2005: "public: static bool const boost::random_device::has_fixed_range" (?has_fixed_range@random_device@boost@@2_NB) already defined in main.obj 1>libboost_random-vc80-mt-sgd-1_43.lib(random_device.obj) : error LNK2005: "public: static unsigned int const boost::random_device::min_value" (?min_value@random_device@boost@@2IB) already defined in main.obj 1>libboost_random-vc80-mt-sgd-1_43.lib(random_device.obj) : error LNK2005: "public: static unsigned int const boost::random_device::max_value" (?max_value@random_device@boost@@2IB) already defined in main.obj 1>Debug\Win32\random_device.exe : fatal error LNK1169: one or more multiply defined symbols found -- Eld på åren og sol på eng gjer mannen fegen og fjåg. [Jøtul] <demo> 2010 Tore Halvorsen || +052 0553034554

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[preprocessor] add pointer to T..
by Manfred Doudar 31 May '10

31 May '10

Hello, Thinking maybe little to late in the night at my end, but can see how to generate the following: T0* T0*, T1* T0*, T1*, T2* ..and so on Some variant on the below maybe..? Of course, below, you'd get T*0, T*1, etc if tried BOOST_PP_ENUM_PARAMS(n, T*) which doesn't produce a valid token. #include <boost/preprocessor/iteration/local/hpp> #include <boost/preprocessor/repetition.hpp> #define OPERATOR(z, n, _) \ \ BOOST_PP_ENUM_PARAMS(n, T) /**/ #define BOOST_PP_LOCAL_MACRO(n) OPERATOR(_, n, _) #define BOOST_PP_LOCAL_LIMITS(1, 3) #include BOOST_PP_LOCAL_ITERATE() #undef BOOST_PP_LOCAL_MACRO #undef BOOST_PP_LOCAL_LIMITS #undef OPERATOR All help appreciated. Cheers, -- Manfred

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BGL property maps
by Arne Schwabe 31 May '10

31 May '10

Hi, I trying to wrap my head about the usage of iterator_property_map<RandomAccessIterator, OffsetMap, T, R> What I am trying to do is the following: create a map like color with key a graph_node and value type of a struct like struct foo { int numbervists; int color; } so I can use that in a breadth first like algorithm. The example shows me how to do it for an int but do not know how to change typedef boost::graph_traits<Graph>::edge_descriptor Edge; typedef boost::property_map<Graph, boost::id_tag>::type EdgeID_PMap; EdgeID_PMap edge_id = get(boost::edge_index(), G); boost::iterator_property_map<int*, EdgeID_PMap, int, int&> capacity_pa(capacity, edge_id), flow_pa(flow, edge_id); to use foo instead of int. In my understanding, I would have to change int* to a RandomAccessIterator to to Iterator that has foo as value type. But I am kind of lost how to construct such a thing. Arne

2 1

[preprocessor] variable repetition
by Manfred Doudar 31 May '10

31 May '10

Hello, Not having a good time with the PP lately, and am struggling over the following - would like to generate: void foo(T1) { bar<T1>(a[0]); } void foo(T1, T2) { bar<T1, T2>(a[0]); bar<T1, T2>(a[1]); } void foo(T1, T2, T3) { bar<T1, T2, T3>(a[0]); bar<T1, T2, T3>(a[1]); bar<T1, T2, T3>(a[2]); } ...and so on. All help most welcome. Cheers, -- Manfred

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GIL YUV color layout
by mailinglist＠squarepx.com 31 May '10

31 May '10

I've been searching for hours to find how to convert from and to yuv color format with gil. Without much luck that is. Can someone give an example how to do this. Another question would be whether gil is the right tool to reconvert image frames for video streaming / playback purposes. Regards Viktor

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