Subject:
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Re: White noise around model with colored background
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Newsgroups:
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lugnet.cad
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Date:
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Wed, 10 Oct 2007 05:40:30 GMT
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Viewed:
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2728 times
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In lugnet.cad, Don Heyse wrote:
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In lugnet.cad, Travis Cobbs wrote:
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What we need it the ability to have the blending not occur in the color
channel at all when the background is fully tranparent. As far as I
know, that’s not possible.
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Ok, I had a little time over the weekend to spend with the Red book,
and I think I may have it now. Hopefully not too late to make it into
LDView version 3.0 RC 2. ;^)
So to eliminate the halo, what we want to put around the image edges
of the destination buffer is the unblended source RGB colors and the
antialiasing coverage values in the alpha channel. I think if you
turn on antialiasing, and don’t enable blending, that’s exactly what
you get. However, we only want it on the outside edges (the halo).
Everywhere else we want to do the normal alpha blending to composite
the antialiased lines with whatever lies behind it. This sounds like
a 2 pass operation to me.
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Well, it’s certainly long past LDView 3.0 RC 2, but I did finally come back to
this. After spending a few hours getting nice alpha data into the image file
for transparent parts, I realized that the same set of steps would probably work
for the edge lines. So, I gave it a try, and sure enough, it worked. See
here:
http://www.halibut.com/~tcobbs/ldraw/private/AAAlphaDemo.html
My sequence doesn’t really match your suggestions, but it uses the overall
concept. Here’s what I did:
First of all, enable stencil writes (always pass) to set stencil to 2 while
drawing all solid geometry. After we’ve done that, we’ll draw the lines, and
then draw the transparent polygons. Do the following setup for each pass for
both lines and transparency, but draw all three passes of lines, then all three
passes of transparency.
Pass one setup:
- Enable color writes; disable alpha writes; enable stencil writes
- Enable depth test, with GL_LEQUAL as depth func.
- Enable stencil test; set to pass when stencil value == 0, and increment the stencil value when it passes and Z passes.
- Disable blending
- For lines, enable line smoothing.
Pass Two setup:
- Disable color writes; enable alpha writes.
- Set stencil to pass when stencil value == 1, and not update the stencil value ever.
- Disable depth test.
- Enable blending, and use glBlendFunc(GL_SRC_ALPHA_SATURATE, GL_ONE_MINUS_SRC_ALPHA)
Pass Three setup:
- Enable color writes; disable alpha writes
- Disable stencil test
- Enable depth test, with GL_LESS as depth func.
- glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA)
The first pass sets the color buffer in spots that were previously unset
(stencil is zero, so they haven’t been drawn to and still have background
color), and sets the stencil to 1 to prevent multiple writes to the same pixel
during this pass, and to set up a mask for the next pass. As long as the
transparent polygons are drawn back to front, that means that the furthest
transparent polygon color gets laid down. For lines, I should sort also, but
don’t. They’re usually thin enough that it hopefully won’t be visible. Note
that for the lines around the model, pretty much only the partially transparent
parts that stick out during antialiasing will get written here, since we don’t
write anywhere that the model has already drawn to.
The second pass essentially bumps the destination alpha up a little bit for each
layer, but only in the spots that were written to in pass one. So two layers
will have a higher (more opaque) alpha value than one. Note that this pass will
redraw all the geometry drawn in the first pass, and that pass didn’t touch the
destination alpha values. Since all we care about are the number of layers at
each pixel, and we’re only writing to pixels that don’t have any non-transparent
geometry, we don’t want to depth test. We also don’t need to sort, but since my
transparent geometry is already sorted, it still gets drawn sorted.
The third pass does the color blending. Since the depth test is set to
GL_LESS, the initial polygons drawn in pass one don’t get redrawn. There’s no
stencil test, so transparent shapes that come in front of opaque shapes get
drawn here.
--Travis
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Message has 1 Reply:
 | | Re: White noise around model with colored background
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| (...) I forgot to mention, but while the above page looks very similar to the page I posted recently on a lugnet.cad.ray thread, it is different in a very important way. The image on the above page was saved out of LDView at 800x600, which is the (...) (17 years ago, 10-Oct-07, to lugnet.cad, FTX)
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Message is in Reply To:
| | Re: White noise around model with colored background
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| (...) Ok, I had a little time over the weekend to spend with the Red book, and I think I may have it now. Hopefully not too late to make it into LDView version 3.0 RC 2. ;^) So to eliminate the halo, what we want to put around the image edges of the (...) (19 years ago, 20-Jun-05, to lugnet.cad)
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