Subject:
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Re: LDGlite and LPub (and maybe ldview?
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Newsgroups:
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lugnet.cad.dev
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Date:
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Fri, 15 Oct 2004 15:38:40 GMT
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Viewed:
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2335 times
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In lugnet.cad.dev, Don Heyse wrote:
> In lugnet.cad, Travis Cobbs wrote:
> > As for solving the problem, I'm not sure what would be best. You
> > could always go back and draw the lines a second time with blend func
> > set to GL_ONE, GL_ONE, depth test disabled and glColorMask set to
> > GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE. This should get rid of the
> > unwanted transparency in the middle of the image, but it would also
> > remove the antialiasing of the alpha channel around the edges.
>
> Let's see if I understand what's going on here. When I antialias
> the edge lines, the RGB colors buffer gets blended, and so does the
> alpha channel. If alpha was all zeros, now instead of all ones, it
> gets something in between. I think what I really want is to run
> a filter over the alpha channel and convert any nonzero numbers
> to all ones before dumping the transparent png. But you're right, it
> would be nice to leave the alpha bits for outer edge lines fuzzy so
> they blend in with whatever background the transparent png is laid
> over. I think there's an opengl halo trick that might help out here.
> Perhaps something to do with the stencil buffer?
>
> Ack! More reading...
Ok i did a quick once over of glBlendFunc manpage and it looks like
switching line antialiasing from:
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
to
glBlendFunc(GL_SRC_ALPHA_SATURATE, GL_ONE_MINUS_SRC_ALPHA);
eliminates the problem without extra passes. I still gotta look into
that halo thingy for the outer edges though. Or maybe I'm still
missing something in my understanding of the blend fns. It seems
like there oughta be some way to blend that insures the destination
alpha value never decreases. Then the polys drawn first would
bump the alpha values to 1.0 everywhere but at the extreme outer
antialiased edges.
Don
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Message has 1 Reply:
 | | Re: LDGlite and LPub (and maybe ldview?
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| (...) Ahh, nevermind. That didn't really work either. Applying the opaque/transparent only filter right before dumping the png is simple, and looks good enough for me. I'm going with that and forget about the icky sharp outer edges for now. They (...) (20 years ago, 15-Oct-04, to lugnet.cad.dev)
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Message is in Reply To:
| | Re: LDGlite and LPub (and maybe ldview?
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| (...) Thanks, that did it. Yuck! (...) Let's see if I understand what's going on here. When I antialias the edge lines, the RGB colors buffer gets blended, and so does the alpha channel. If alpha was all zeros, now instead of all ones, it gets (...) (20 years ago, 15-Oct-04, to lugnet.cad.dev)
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