View Full Version : Rendering in a part of window
01-09-2003, 11:29 PM
The information in my window is logically splitted into two parts. The upper half of window is drawn by GDI, the lower - by OpenGL. I restrict OpenGL output by glScissor and glViewport - fine. But nevertheless OpenGL clears entire window. So if I make OpenGL drawing after GDI, OpenGL paints erases GDI part of window. I tried to make GDI drawing after OpenGL, it works but produces flickering of GDI part.
My question is: does it possible to fully restrict OpenGL by given rectangular area of window?
Create a sub-window in your window for OpenGL.
01-10-2003, 01:33 AM
- OpenGL clears are restricted by glScissor, you must be doing something wrong when the glClear erases the whole window.
- Defualt window handling for WM_ERASEBKGND is to clear the whole window. Add a WM_ERASEBKGND message handler which only returns non-zero.
- Restrict GDI drawing to the window area you want to render to with a clipping region.
- GDI does only know single buffering. If you render double buffered, the SwapBuffers call swaps the whole window area.
Conclusion: Use a different sub-window for OpenGL like evil suggested.
A common mistake is to set the scissor box but not enable the scissor test.
01-10-2003, 05:16 AM
If I use glViewport without glScissor entire window is filled with clear color. If I use glScissor only the area specified in glScissor is cleared with clear color but other area (GDI) is filled with gray color even if double buffering is turned off. So I suppose that this effect is Win32 or my OpenGL implementation feature (or a bug).
01-10-2003, 07:22 AM
The grey GDI area is probably the erase background handling I mentioned above. Implement the handler as
case WM_ERASEBKGND: return 1;
and that should stop.
Though it's a lost case, better use a separate or child window for OpenGL if the renderings don't overlap anyway. Clipping is handled by the windows subsystem then and you don't need tricks with scissor and viewports.
01-11-2003, 03:35 AM
Originally posted by airatsa:
If I use glScissor only the area specified in glScissor is cleared with clear color but other area (GDI) is filled with gray color even if double buffering is turned off.
Sorry, I was mistaken. GDI area is filled with gray color if double buffering is turned on. If double buffer is turned off all works fine (with flickering of course).
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