[Kstars-devel] Re: Planet textures with OpenGL

[Alexey Khudyakov]

On 09.04.2011 05:45, Akarsh Simha wrote:
>> IMHO the best way to deal with the problem is to aquire texture ID
>> when they are used for first time. So they will be generated after
>> transfer of ownership.
>
> There might be a better way. Looking at the code,
> QContextPrivate::bindTexture() is supposed to check if the texture
> already has a texture ID and then bind it. It does maintain a cache of
> textures. If my understanding is correct, ideally, it should
> invalidate that cache as well when the context is reset. So maybe it
> is best to solve this in QGLContextPrivate itself. I need to ask on
> #qt or something and I plan to do it this weekend.
>
Indeed. QGLContext states:

The texture that is generated is cached, so multiple calls to 
bindTexture() with the same QImage will return the same texture id.

So you propose to avoid caching texture IDs at all and rely on Qt to do 
that, am I right?


>> Maybe TextureManager should not own context at all and have it
>> passed in as parameter. This way we can avoid transfering of context
>> at all.
>
> That sounds good to me! We have a Q_ASSERT( m_context ) whenever we
> try to bind a texture and we can have a TextureManager::setContext(
> QGLContext * ), and we can pass the context of the QGLWidget.
>
I think setContext is bad idea. TextureManager do not own context so it 
should not hold pointer to it. If context is passed as parameter than 
it's not possible to have incoherence when manager holds one context and 
caller expect another.

Also I think Texture class isn't really required. It and TextureManager 
rely on implementation of each other. There is no point in separating 
these classes.

Following API will do the trick I assume. Anyway texture cannot do 
anything but return QImage and bind texture. Also there is no need to 
make TextureManager a Q_OBJECT.

class TextureManager {
public:
   // Return image and tries load it if not already loaded
   static const QImage& getImage(const QString& name);
   // Binds texture
   static bool bindTexture(const QString& name )
   {
     const QImage& = getImage(name);
     // bind texture here
   }
...
}