Pixel Perfect Collision Image Problem.

[Paijan]

Hi.
I'm writing a code, which performs pixel perfect collision check.
It creates sf::Rect which is rectangle of collision.
Then it transforms its coordinates to sprites' local (referring it to 0,0 points of both sprites).
Then I can use those coordinates to choose pixels in sf::Image entities which are created from the same texture as sprites (it has to be sf::Image because spirtes don't have getPixel method) and check their alpha channels.

And it works perfectly fine until i try to scale my sprites in main loop. Then coordinates in sprite don't match coordinates in image, because one of them is scaled.

How do I manage this? Should I somehow scale sprite back to it's original size at the beginning of function?

(click to show/hide)

bool PixelPerfect(MyClass A, MyClass B)
// MyClass::getGlobalBounds(); just means the same as MyClass::sprite::getGlobalBounds();
// MyClass::GetImg(); returns a reference to static sf::Image variable in class (loaded at the beginning of app)
{
        if( A.getGlobalBounds().intersects(B.getGlobalBounds()) )       //if sprites intersect, do pixel perfect test
        {
                int RectLeft   = static_cast<int> ( max(A.getGlobalBounds().left , B.getGlobalBounds().left) );
                int RectRight  = static_cast<int> ( min(A.getGlobalBounds().left + A.getGlobalBounds().width , B.getGlobalBounds().left + B.getGlobalBounds().width) );
                int RectBottom = static_cast<int> ( min(A.getGlobalBounds().height + A.getGlobalBounds().top , B.getGlobalBounds().height + B.getGlobalBounds().top) );
                int RectTop    = static_cast<int> ( max(A.getGlobalBounds().top , B.getGlobalBounds().top) );

                sf::IntRect Rect(RectLeft,                                      //collision rectangle
                                                RectTop,
                                                RectRight - RectLeft,
                                                RectBottom - RectTop);         

                int LocalLeftA = RectLeft - static_cast<int> (A.sprite.getPosition().x);        //local coordinates of Rect (referring to A and B),
                int LocalTopA  = RectTop  - static_cast<int> (A.sprite.getPosition().y);        //they will be used to refer to pixel positions in sf::Image
                int LocalLeftB = RectLeft - static_cast<int> (B.sprite.getPosition().x);        //as they are the same (if not scaled)
                int LocalTopB  = RectTop  - static_cast<int> (B.sprite.getPosition().y);

                for(int i=0; i<Rect.width; i++)
                {
                        for(int j=0; j<Rect.height; j++)
                        {
                                if (A.GetImg().getPixel(i+LocalLeftA , j+LocalTopA).a > 0 && B.GetImg().getPixel(i+LocalLeftB , j+LocalTopB).a > 0)
                                {
                                        return 1;  //pixel intersection
                                }
                        }
                }
                return 0; //only bounding boxes intersection, no pixel
        }
        else return 0; //no bounding boxes intersection
}

[Ixrec]

The short answer is you need to work with the transforms (positions, rotations, scales, etc) of the objects whose pixels/coordinates you're testing, rather than trying to "untransform" them.

Getting all the details right is a moderately difficult task and we're not going to walk you through all of it, but conveniently it's already been done on the wiki: https://github.com/SFML/SFML/wiki/Source:-Simple-Collision-Detection-for-SFML-2