[GEM-dev] "auto 1, 1"--[alpha] with double [gemhead] idiom

[Claude Heiland-Allen]

Hi Matthias,

I hope you don't mind me sending this reply to the list too.

Matthias Neuenhofer wrote:
> Hi Claude,
> 
> it´s not a bug in Gem the depth sorting follows the render order. Normal 
> openGL behaviour.

That's what I suspected, and in that case maybe the wording of [alpha]'s 
help patch could be better:

""
Inlet 1: message "auto 1" | "auto 0" turn on/off automatic depth detection
""

I thought this would cache all objects following the [alpha] and sort 
them by depth automatically, or something.  Now I'm not sure what the 
"auto $1" message does at all.

Reading carefully the large paragraph I see it says:

""
Be aware that the rendering order matters, so you probably want to set 
the gemhead order number high so that the object is rendered after all 
of the non-alpha blended ones.
""

Maybe this could be made more explicit, saying render order of 
alpha-blended objects matters as well as alpha vs non-alpha objects.

> In the second way you render your patch it´s well working.

Sure, but naturally the "real patch" has much more complicated geometry, 
it would be a major pain in Pd as far as I can tell to do some kind of 3 
pass rendering, where the first pass gets the depth of each object, the 
second pass sorts by depth, and the third pass actually draws them.

I guess I'm too ambitious :)


Thanks,


Claude

> http://www.opengl.org/resources/faq/technical/transparency.htm
> When using depth buffering in an application, you need to be careful 
> about the order in which you render primitives. Fully opaque primitives 
> need to be rendered first, followed by partially opaque primitives in 
> back-to-front order. If you don't render primitives in this order, the 
> primitives, which would otherwise be visible through a partially opaque 
> primitive, might lose the depth test entirely.
> 
> In a double gemhead patch i see no chance to change. Otherwise with set 
> messages to gemhead.
> 
> lg
> Matthias
> 
> 
> Am 02.07.2008 um 19:37 schrieb Claude Heiland-Allen:
> 
>> Hi,
>>
>> The attached patch "double-gemhead-auto-alpha.pd" doesn't work as I 
>> expect (I don't get any automatic depth sorting).  Is there any 
>> suggested workaround for repetition with auto alpha depth sorting or 
>> is this a bug in Gem?
>>
>> Thanks,
>>
>>
>> Claude
>> -- 
>> http://claudiusmaximus.goto10.org