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  OpenGL Alpha Blending - issue.. looking for advice  (Read 618 times)
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Offline jmguillemette
« Posted 2013-07-25 03:54:08 »

Im pretty sure im making a newb mistake but after trying to solve this for a while i thought i would ask the community..

Im trying to implement alpha blending but im getting a weird result.

I make two spheres and give them both 50% blending.
The first sphere doesnt appear to have any blending.
The 2nd one appears to have perfect blending...
but only when seen from the perspective of inside the 2nd sphere..
from outside the sphere both spheres appear like they are drawing every other triangle..

Here is a visual example of my exact situation. Any hints / tips would be greatly appreciated.

Ideally i would like transparency to work from both inside and outside the spheres as i hope to use this technique to create planets with atmosphere.

<a href="http://www.youtube.com/v/1qxGuH1l7Mw?version=3&amp;hl=en_US&amp;start=" target="_blank">http://www.youtube.com/v/1qxGuH1l7Mw?version=3&amp;hl=en_US&amp;start=</a>

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Offline jmguillemette
« Reply #1 - Posted 2013-07-25 12:39:17 »

adjusted youtube settings to make my demonstration video public.. sorry about that.

j.

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Offline ClickerMonkey

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« Reply #2 - Posted 2013-07-25 12:41:23 »

When rendering transparent objects in OpenGL (with alpha other than 1 or 0) you manually need to sort them based on what's closer to the viewer.

Yeah, seriously.

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Offline jmguillemette
« Reply #3 - Posted 2013-07-25 14:15:34 »

by manual ordering.. do you mean "I" have to ensure i draw the models in opengl from closest to furthest in relation to the camera ?

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Offline ClickerMonkey

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« Reply #4 - Posted 2013-07-25 15:08:36 »

Yes:

Opaque (or Transparent) object = object consisting of pixels where the alpha is 0.0 or 1.0
Translucent (alpha/blended) object = object consisting of pixels where the alpha is between 0.0 and 1.0 exclusively

Drawing:
1. Draw all opaque objects
2. Sort translucent objects by their distance from the camera
3. Draw translucent objects in determined order


If you have complex translucent objects that can cross over each other like this:



Good luck! Not possible!

Offline jmguillemette
« Reply #5 - Posted 2013-07-25 16:21:28 »

k thank explains why one sphere doesnt appear to have any alpha blending.

any ideas why only one side of the sphere (in this case the inside) is blended.. where the outside draws like a mesh of triangles?

When i dont use alpha the sphere is drawn properly both inside and out and the normals interact with the lighting as i would expect.

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Offline ClickerMonkey

JGO Coder


Medals: 20


Game Engineer


« Reply #6 - Posted 2013-07-25 17:44:44 »

The one that doesn't appear to have any alpha blending is the one being drawn first on the black background... it looks fine when it's the one further in the distance... but when it's the closest it's still being drawn behind the other one, even though it's in front (hence the need to order the drawing).

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