Hi All,
I'm a newb to this list, and although not quite a newb to OpenGL,
probably close enough in present company.
I'm doing a fairly straightforward transparency blend as follows:
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glColor4f((GLfloat)1.0, (GLfloat)1.0, (GLfloat)1.0, (GLfloat)_alpha);
if (!_currentFrame)
[self updateCurrentFrame];
[self renderCurrentFrame];
glDisable(GL_BLEND);
The current frame comes from a QuickTime movie, and is simply
texture mapped onto a quad.
The problem I'm trying to solve is related to the resolution of the
blending operation. On my MacBook Pro, on the built-in screen,
interpolating the _alpha value from 1 to 0 produces a smooth fade-
out of the image. However, on the attached Cinema display
(1920x1200), that same operation produces a visually erratic fade.
Namely, as the _alpha value approaches zero the image eventually
"jumps" to invisible without smoothly fading out. In this case,
anything below 0.05 for the alpha value results in a completely
invisible image.
If I create a window that spans both screens and perform a fade-out
of the image, the half on the main display works correctly and the
half on the secondary display does not.
Thinking it might be an issue of bit depth in the
NSOpenGLPixelFormat, I tried creating a pixel format explicitly
with an NSOpenGLPFAAlphaSize of 32, but that did not change the
behavior.
Any idea what I'm doing wrong here?
Best,
Christopher
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