+2  A: 

I'd say it's because your semi-transparent square gets added to the existing pixels, thus incrementing their intensity.

The documentation for glBlendFunc() recommends setting the second parameter to GL_ONE_MINUS_SRC_ALPHA, that is the boilerplate for implementing transparency. Try it.

unwind  2009-04-06 15:59:26
Yeah, looks like a white translucent square, I don't see the problem.
Karl  2009-04-06 16:10:56
Excellent, thanks! GL_ONE_MINUS_SRC_ALPHA fixed the problem.
nbolton  2009-04-06 16:14:01

related questions

OpenGl And Flickering
Best way to draw text with OpenGL and Cocoa?
How do I draw text using OpenGL, SDL and C++?
OpenGL or Direct3D for a new Windows game project? Or something else?
Which 3D cards support full scene antialiasing?
What could prevent OpenGL glDrawPixels from working on some video cards?
OpenGL Rotation
How to enable multisampling for a wxWidgets OpenGL program?
Is OpenGL dead?
Suggested gaming platform to learn - Direct X (C++), Open GL or XNA
Using GLEW to use OpenGL extensions under Windows
What are OpenGL extensions and what are the benefits/tradeoffs of using them?
Using OpenGL extensions On Windows
GLUT exit redefinition error
Using the mouse scrollwheel in GLUT
GLUT pop-up menus
Valid OpenGL context
Accessing OpenGL state variables in Cg
Starting off with OpenGL under Cygwin
Using GLUT bitmap fonts
Using OpenGL textures larger than window/display size
GL_FRAMEBUFFER_INCOMPLETE_DUPLICATE_ATTACHMENT_EXT errors
Using GLUT with Visual C++ Express Edition
Enabling OpenGL in wxWidgets
How to display unicode text in OpenGL?