IRIS GL and OpenGL Programming Tips and Pitfalls

IRIS GL

• If your GL/X11 app crashes with "error 93 ERR_WMANIPC " it's probably caused by a bad X window ID being passed to an Xlib function.
• Whenever lmbind() is called the position of the light is re-transformed by the current modelview matrix. So, enabling/disabling lighting can have the unwanted side effect of moving your lights.
• IBM's implementation of GL on the GTO has a few problems:
  • Polygons are not point sampled correctly; zero-area polygons appear as speckled lines.
  • Many useful GL functions in SGI's implementation are missing.
  • The lighting arithmetic seems to be a bit different than SGI's.
• More to come...

OpenGL

• Problem: OpenGL rendering is just slower than expected.
  Solution: Make sure you're using a direct GLX rendering context if displaying locally. See glXCreateContext().
• Problem: scaling your model causes the intensity of lighting to change.
  Solution: try glEnable(GL_NORMALIZE)
• Problem: you've defined a texture with glTexImage2d() and have enabled texturing but don't see the texture on your model.
  Solution: the default texture minification filter is GL_NEAREST_MIPMAP_LINEAR but you haven't defined the mipmaps. Try glTexParameter( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR )
• Problem: glDrawPixels() is much slower than GL's lrectwrite.
  Solution: Make sure you disable all raster operations that aren't needed such as GL_DEPTH_TEST, etc. Use the GL_RGB / GL_UNSIGNED_BYTE image format as it's usually the fastest. Some implementations support the GL_ABGR_EXT format which is faster on old systems.
• Problem: glRasterPos() doesn't position bitmaps where I want.
  Solution: glRasterPos accepts model coordinates, not window coordinates. The coordinate given to glRasterPos is transformed by the modelview and projection matrices just like a vertex.
• More to come...