Alpha Blending Not Working

1
Hi!

I'm just now switching to the OSX PTB, and I've noticed that on our G5 analysis machine,
the alpha blending works just fine (with the screen DrawDots function), but only if I draw
'online'. If I choose to draw offscreen windows, then the alpha blending works only on the
first presentation of all of the frames. Also, I'm attempting to use our experimental
computer (Dual 1.25GHz, PowerPC G4, OSX 10.3.9, a machine that we've been using OS9
PTB on for a few years now), but the alpha blending does not work at all on this machine.

I have the following questions:
1) Is the alpha blending necessary for every offscreen window, or is it only needed once?
When redrawing on a frame, why isn't alpha-blending enabled?
2) Could the latter of my problems be a result of the hardware? operating system?

Thanks!
javi
2
Hi,

1) Two answers: First, i found a small bug in the offscreen window
switching code that could create some funky effects when using
offscreen windows in conjunction with alpha blending, but only under
very special circumstances, so i don't think that's your problem -
Thanks for pointing me to it anyway ;-) -- will be fixed in next update.

Second, alpha-blending can be a bit mind-bending, so i don't know what
you want to do, or what you do, but as a starter if you just want to
store images of your anti-aliased dots in offscreen windows and later
just blit those images to the screen, you would need to disable alpha
blending after you've drawn your offscreen windows and before blitting
them to the onscreen window, ie, Screen('Blendfunction', win, GL_ONE,
GL_ZERO);

2) Some features are dependent on OS version, how modern your gfx-hw
is, type of gfx-card and how much video ram it has, so this could be a
reason. E.g., reg. point anti-aliasing as in DotDemo.m: NVidia cards
support point-sizes between 1 and 64 pixels, whereas many older ATI
cards only support a range of 1 to 10 pixels. PTB would abort with
some error message if you tried to draw bigger dots than that diameter.

-mario


--- In psychtoolbox@yahoogroups.com, "ejavidotcom" <javi@...> wrote:
>
> Hi!
>
> I'm just now switching to the OSX PTB, and I've noticed that on our
G5 analysis machine,
> the alpha blending works just fine (with the screen DrawDots
function), but only if I draw
> 'online'. If I choose to draw offscreen windows, then the alpha
blending works only on the
> first presentation of all of the frames. Also, I'm attempting to
use our experimental
> computer (Dual 1.25GHz, PowerPC G4, OSX 10.3.9, a machine that we've
been using OS9
> PTB on for a few years now), but the alpha blending does not work at
all on this machine.
>
> I have the following questions:
> 1) Is the alpha blending necessary for every offscreen window, or is
it only needed once?
> When redrawing on a frame, why isn't alpha-blending enabled?
> 2) Could the latter of my problems be a result of the hardware?
operating system?
>
> Thanks!
> javi
>