Texture errors in multiple wxGLCanvas windows

Graphics and GPU Programming Programming OpenGL

Started by Oranda December 11, 2006 08:20 PM

3 comments, last by Oranda 17 years, 4 months ago

122

Author

December 11, 2006 08:20 PM

Hi all. I'm currently building a quick utility to help me layout some scenes, and I have two window classes, both of which extend wxGLCanvas. Each class has its own event handler for painting. If I run the utility with only one or the other windows enabled, they work fine, but if I try to enable both of them at the same time, each texture in the scene will only work for one or the other window. i.e, if I can render a quad in one window with a certain texture, I will only get a null texture in the window. I don't think there are any race conditions present, since I am not multithreading and there is no randomness to which render target gets which textures. Both render windows get the texture ids from the same class, so I don't think that should be a problem either. FYI, in my test case, after adding a texture to the scene, exactly one quad with that texture applied to it is added to CRenderWindow, so I should see the exact same textures in both CRenderWindow and CTextureWindow (only in different locations, obviously :)). Here is the offending code, stripped down to just the OpenGL setup specific parts: This class just renders a bunch of quads and guidlines into a window


void CRenderWindow::OnPaint(wxPaintEvent& event)
{
  wxPaintDC dc(this);
  this->SetCurrent();
  
  //resize the viewport
  int width;
  int height;
  
  //--snip-- build the viewport
  
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glDisable(GL_CULL_FACE);
  glEnable(GL_DEPTH_TEST);
  
  //--snip-- build the matrices
 
  //--snip-- render a bunch of quads with textures. textures have ids
  //         which are contained in scene->GetTexture(i)->getTextureID()
  //         as done in CTextureWindow::OnPaint below
  
  //--snip -- render some other non-texture related stuff
	
  glFinish();
  this->SwapBuffers();
  event.Skip();
}

And this function renders the contains of a texture list window:


void CTextureWindow::OnPaint(wxPaintEvent& event)
{
  
  wxPaintDC dc(this);
  this->SetCurrent();
  
  //--snip-- build the viewport
  
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glDisable(GL_CULL_FACE);
  glDisable(GL_DEPTH_TEST);
  
  //--snip-- build the matrices
  
  //render all of the textures to the window 
  int numTextures = scene->getNumTextures();
  for(int i = 0; i < numTextures; i++)
  {
    //to clamp x to every n units: x - x % n
    width = (width - width % 100);
    
    int x = (i * 100) % (width);
    int y = (i * 100) / (width) * 100;
    
    CTexture* texture = scene->getTexture(i);
    
    if(texture != null) 
      glBindTexture(GL_TEXTURE_2D, texture->getTextureID());
    else
      glBindTexture(GL_TEXTURE_2D, 0);
  
    //--snip-- render the actual quad
  }
  
  glFinish();
  this->SwapBuffers();

  event.Skip();

Any help/suggestions would be very much appreciated!

vx7johmi

139

December 11, 2006 08:51 PM

For every wxGLCanvas you create you get a separate OpenGL context. When you create/upload a texture in OpenGL it is only accessible/usable in the context where you created it, UNLESS you call [wglShareLists]. So, let your main context (your main wxGLCanvas) generate the textures and then pass the rendering context to any other wxGLCanvas you use and call [wglShareLists].

I'm using this for my GL-based texture-palette:

void TextureGLCanvas::OnPaint(wxPaintEvent& event)
{
//Must always be here.
wxPaintDC dc(this);

//Do we need this every time..?
if(!GetContext())
return;

//Only need to do this once...
if(!_hasCalledShareLists)
{
//wglShareLists(HGLRC,HGLRC)
wglShareLists(Core::getInstance()->getMainOGLContext(),
m_glContext->m_glContext);

_hasCalledShareLists = true;
}

//Set current OpenGL context
SetCurrent();

//Does clear, render and swap...
Render();
}

/Mike

Oranda

122

Author

December 11, 2006 09:10 PM

Thanks, that makes a lot more sense now. I'm running Linux on my development box, though, so wglShareLists won't work for me (since it's part of the microsoft-specific OGL API).

There appears to a special constructor that allows me to share a context from within the wxWindows containers. I added it to my texture window, but it doesn't appear to be working. Here is the modified constructor:

CTextureWindow::CTextureWindow(wxWindow *parent, wxGLContext *sharedContext, wxPoint pos, wxSize size, CScene *scene): 		wxGLCanvas(parent, sharedContext, -1, pos, size, 0, 		wxString(wxT("GLCanvas")), NULL, wxNullPalette){  this->scene = scene;}

And here is where I actually construct the two windows:

        renderWindow = new CRenderWindow(this, renderPoint, renderSize, scene);                        wxDialog* dialog = new wxDialog(this, -1, wxT("Textures"), wxDefaultPosition,                                         wxDefaultSize, wxRESIZE_BORDER|wxCAPTION, wxT("dialogBox"));        textureWindow = new CTextureWindow(dialog, renderWindow->GetContext(), wxDefaultPosition, wxDefaultSize, scene);

Any ideas on why this doesn't seem to work either?

vx7johmi

139

December 11, 2006 09:41 PM

Forgot to mention that it is Windows only... Sorry!

However, I don't think it is practical to have two separate wxGLCanvas sharing the same context (drawing into one window will very much affect the other one, as you are using the same context). Could be problem with clearing and swapping (how do you restrict it to a certain window?).

The easiest workaround (unless there is som Linux-specific way to share textures between context's) would probably be to generate duplicates of the textures, one for each context. It is of course a question of memory, and depends on the amount of textures you plan to use...

The other workaround would be to use one (1) wxGlCanvas and "split" the window by use of pure OpenGL commands (restrict by [glViewport], or by using two FBO's, one for each "window" and display the results on two textured quads that will represent the two "window", using the FBO's as textures)...

/Mike

Oranda

122

Author

December 12, 2006 08:30 AM

I think you might be getting contexts and render targets mixed up. Contexts hold OGL data and state, wheras the seperate wxGLCanvas objects are seperate render targets. You can render to the one window without affecting the contents of the other window if they have the same context, the only thing you need to be careful of is restoring any changes you make to the context state (such as changing clear colors, etc) before rendering to the other target again.

Which is why I'm confused. I'm still getting separate render contexts in both windows, even though I'm passing the context for the CRenderWindow to the CTextureWindow. Which means there is either a bug in wxWindows (unlikely), or else I'm missing some other step.

Texture errors in multiple wxGLCanvas windows

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