[PD] GEM: texture on transparent sphere visible from opposite side

Matthias Kronlachner m.kronlachner at gmail.com
Sun Jun 9 08:48:11 CEST 2013

On 6/8/13 5:20 PM, Claude Heiland-Allen wrote:
> Hi,
> On 08/06/13 14:48, Matthias Kronlachner wrote:
>> i have a texture on a sphere and slightly add transparency to the sphere.
>> i can now see objects behind the sphere, but is it possible to make the
>> texture of the opposite side of the sphere visible? (through the
>> (semitransparent) front side of the sphere)
> Short answer: no.
> Long answer: no, because transparency and OpenGL don't quite get along
> as nicely as you might expect.  Rendering order (of every triangle that
> makes up every object) matters a great deal.  Transparency involves
> blending the current triangle with whatever is behind it. OpenGL uses
> its depth buffer to keep track of what's been drawn and how far away it
> is at each pixel - keeping only the nearest value - whether it was
> rasterized before or after the previous triangle.  You need to draw
> non-transparent stuff first, then sort every transparent triangle into
> depth order, possiby splitting up any transparent triangles that
> intersect each other into smaller triangles, then render them in order
> urthest from camera first, nearest to camera last.  I'm pretty sure that
> Gem doesn't support depth sorting.
> You might be able to hack it and get something that looks mostly ok some
> of the time by rotating the sphere to change the depth order vs render
> order of the triangles making up the sphere, but it will be very fragile...
> Claude
thanks, i was expecting something like this.
i guess i will think about an alternative approach to that instead of 
hacking around with depth sorting and rendering order within a single 
geometric object.


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