RE: [Algorithms] Alpha Channel

[Pallister, K. <kim...@in...>]

And if you only care about on/off transparency, alpha test will also save
you on having to depth sort your trees. All the better if you are going to
have 2x-3x the number of trees. The alpha-test early-outs the z-write (hows
that for a hyper-hyphenated sentence) and so you don't have the
need-to-sort-transparent-objects problem.

The demo we showed at IDF shows this off nicely. Source code to be released
really soon. promise.

Kim Pallister

We will find a way or we will make one.
                     - Hannibal


> -----Original Message-----
> From: Discoe, Ben [mailto:ben...@in...]
> Sent: Saturday, August 26, 2000 8:33 PM
> To: 'gda...@li...'
> Cc: vt...@eg...
> Subject: RE: [Algorithms] Alpha Channel
> 
> 
> 
> Pai-Hung,
> 
> It is indeed a difficult process to generate a good 8-bit 
> alpha channel for
> a 32-bit texture.  For nearly any photographic data source, 
> it cannot be
> done procedurally.  It requires a human artist to produce 
> usable results.
> Here is a tutorial i wrote on how to produce the alpha using 
> PhotoShop:
> http://vterrain.org/Plants/Create/
> 
> Part of why it is so difficult is that along the perimeter of 
> the shape
> (e.g. tree) there are pixels with alpha between 0 and 1, so 
> bits of the
> surrounding (non-tree) texel color will appear on the output. 
>  This is very
> difficult to fix.  No matter what color you choose (black is a common
> choice), you will get a "fringe" of that color around the 
> resulting texture
> billboard, unless you hand-edit on a nearly pixel basis.
> 
> One popular simplification is to use single-bit alpha (alpha 
> testing, not
> alpha blending).  The advantages are that the art task is 
> easier, and you
> don't have to draw the texture billboards in back-to-front 
> order, which can
> be a significant performance hit since it is faster to sort 
> by texture to
> avoid context switches.  The drawbacks are harsh, unnatural 
> aliased edges
> for your texture billboards.
> 
> In the VTP software, we currently use 8-bit alpha blending 
> for quality, but
> i'm considering switching to alpha-testing for speed.  A 
> forest may be more
> convincing with 2-3x as many trees even if the trees have 
> jittery edges.
> 
> Good luck,
> Ben
> http://vterrain.org/
> 
> > -----Original Message-----
> > From: Pai-Hung Chen [mailto:pa...@ac...]
> > Sent: Saturday, August 26, 2000 1:09 PM
> > To: gda...@li...
> > Subject: [Algorithms] Alpha Channel
> > 
> > Hi,
> > 
> > I want to use billboard in my program but cannot find a good 
> > way to create 32-bit bitmap with alpha channel of appropriate
> > values.  Basically I want to create a bitmap of tree with leaves
> > in various green colors (with alpha = 255) and all non-green
> > colors transparent (alpha = 0) so that I can use it
> > as the texture for my billboarded trees.
> 
> 
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