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Member Since 27 Feb 2010
Offline Last Active Jul 02 2015 11:51 AM

Topics I've Started

Applying alpha gradient to an image

02 July 2015 - 07:48 AM

I have an image that's dynamically generated. I need to apply an alpha gradient to this image so that it 'fades out' at the edges. How can I do this?

To illustrate things a bit here are some pictures. Let's say I have an image and a background;





I need to fade in the edges of the image, and then draw it onto the background. If the background were, say, entirely white, then I could 'fake' this fading effect by drawing a pre-created white quad with a linear gradient over the image at each end, like this:



If the background is another image however then I obviously can't do this, because I end up with something like this:





What I'd like to be ending up with is this:




...but I don't know how I can manage that. I'm using LibGDX if that matters, but I suppose this is a framework-agnostic question really?

Boost serialization exception problems

05 August 2014 - 07:16 PM

Hi all, I'm trying to implement serialization, using boost, into a new game. To start, I built a small tester app with a main object which contained a vector of pointers to sub-objects, and everything worked fine; I could save and load states of the main object and all containers and such, no problem.

Moving on, I tried to implement something similar in my current project. The object I am trying to serialize is a Level; it contains a bunch of assorted variables, and an array of Tile objects representing the map. I added serialize functions to the class declarations as follows:

class Level {
    Level (int _y, int _x);

    Tile tmap[256][256];
    int AREA_W, AREA_H;

    // ...functions removed for clarity

    friend class boost::serialization::access;
    template<class Archive>
    void serialize(Archive & ar, const unsigned int version) {
        ar & AREA_H;
        ar & AREA_W;
        ar & tmap;
class Tile {
    Tile ();

    int ac, rot, lmap[4];
    bool w, s, l, b, a, c;
    string tc, name;

	// ...functions removed for clarity
    friend class boost::serialization::access;
    template<class Archive>
    void serialize(Archive & ar, const unsigned int version) {
        ar & ac;
        ar & rot;
        ar & lmap;
        ar & w;
        ar & s;
        ar & l;
        ar & b;
        ar & a;
        ar & c;
        ar & tc;
        ar & name;

However, If I actually try to deserialize the Level object while the app is running (serializing works fine as far as I can tell from looking at the output file), I get this rather intimidating call stack:





I don't know what I've done wrong as serializing simple types like this seems pretty standard? If I catch the exception that Boost throws and examine it:

    std::ifstream ifs("savegame1.sav");
    boost::archive::text_iarchive ia(ifs);
    try {
        // blah
    } catch (const boost::archive::archive_exception e) {
        cout << "boost::archive exception code is "<< e.code << endl;

...I get the console output  boost::archive exception code is 8. This confuses me a bit, because according to the boost docs (http://www.boost.org/doc/libs/1_54_0/libs/serialization/doc/exceptions.html), exception code enum 8 is...

        unregistered_cast,      // base - derived relationship not registered with
                                // void_cast_register

... and I'm not serializing or deserializing any derived objects.

I would welcome any help as I am pretty lost. Thanks in advance.

Segfault upon exiting application

26 July 2014 - 02:31 PM

I'm working on a project at the moment in CodeBlocks using SDL2. At some point during development (I'm not exactly sure when tongue.png) the application began to segfault upon closing down.


If I set a breakpoint somewhere and step through the code line-by-line, the crash doesn't happen until after the app successfully executes return 0; at the end of int main, so I don't know where I should be looking for the error.


When the segfault occurs, the console output usually reads "Process returned 255 (0xFF)", although I infrequently get "Process returned -1073741819 (0xC0000005)" instead, seemingly at random. 0xC0000005 as far as I can tell means it is accessing memory it shouldn't?


Here's a screenshot of the call stack:




I would post some code, but I have no idea what code might actually be relevant and the project is very large. I would welcome any insight here as I have no idea what is going on! Thanks in advance for any replies.


edited to add: apologies if this question would be better suited for a different subforum, I didn't really know where to put it

Management of game objects

23 March 2014 - 12:08 PM

Hi all,


I'm trying to implement a sensible management system for all the objects in a basic platform game. There are various types of objects - projectiles, the player, enemies, powerups, moving platforms, that type of thing.


Initially I thought for the sake of simplicity I'd have all objects derive from a base class Entity, and then stick them all in a vector<Entity*>. Then when it's time for them to act or to be drawn to the screen, I can just make a single pass over the vector and call it->update() or it->display(). This saves a bunch of code duplication from having them all in separate vectors for each type and having to iterate over each vector separately to call display functions and so on.


Difficulties arise with this method when I need to do something like collision-test all Projectiles against all Enemies - I have to iterate over the vector, testing if each element is a Projectile, and if it is, iterate over the vector again testing to see if each element is an Enemy, and then testing for a collision if so. I have to do this a bunch of times for, say, enemies vs projectiles, player vs projectiles, player vs powerups, enemies vs player, and it turns into a huge tangled mess of code and nested iterations over the vector, and the vast majority of element comparisons don't need to be made as the entities are of the wrong type.


I'm looking for an elegant way to simplify this, so that Type A vs Type B checks are as simple as possible, and also so that mass updating and displaying of objects is equally simple.


Is there an accepted best practice for this kind of thing? Thanks in advance for any replies.

C++ vector of boost::weak_ptr troubles

03 August 2013 - 07:48 PM

I have a game that has a number of ships in space. The ships exist in the gameworld as boost::shared_ptr<Ship>. These ships can attack one another. The Ship class has a boost::weak_ptr<Ship> called target, which holds a reference to whatever it's currently firing at, and it also has a vector of weak_ptr<Ship> called aggro_vector. The aggro vector records all ships that have attacked this Ship, so that when it kills its current target, it can consult its aggro vector to see which other ships it is currently in combat with, and pick a new target.


This is the Ship class, sanitised:

class Ship
    Ship (int x, int y);

    boost::weak_ptr<Ship> target;
    std::vector< boost::weak_ptr<Ship> > aggro_vector;

    void determine_next_target();
    void set_as_target (boost::shared_ptr<Ship> next_target);


What I'm having trouble with is getting the Ship to correctly look through its aggro_vector and choose a new target.


Here is the function that picks the next target from the aggro vector. It iterates through all the weak_ptrs, and if .lock() successfully returns a shared_ptr, it hands this shared_ptr off to the member function that registers the next target. Otherwise the weak_ptr refers to a ship that has since been destroyed and is removed from the aggro vector:

void Ship::determine_next_target()
    std::vector< boost::weak_ptr<Ship> >::iterator it = aggro_vector.begin();
    while(it != aggro_vector.end()) {
        if (boost::shared_ptr<Ship> this_ship = (*it).lock()) {
        } else {
            it = aggro_vector.erase(it);


When this code is compiled and run, it crashes at the line set_as_target(this_ship). If this line is commented out, the remainder of the code works as it should (although obviously the determine_next_target() function now does nothing other than remove expired weak_ptrs).


To be clear, the set_as_target(boost::shared_ptr<Ship>) function works fine and is called from a bunch of other places in the code with no problem. It doesn't appear to be that function, but rather the fact that I'm attempting to call that function in this way.


Why am I having problems passing a shared_ptr to an outside function in this fashion? Is there something about iterating over a vector of weak_ptr that I'm missing, or am I just failing to grasp, in the broader sense, some aspect of how these pointers should be used?


Thanks in advance for any help.