Stencil by Discard-ing pixels through a shader

Drawing alpha objects tends to be very expensive. If you want to draw

a lot of alpha objects to simulate grass or plants, it quickly becomes

TOO expensive, lowering dramatically your FPSs.

This happens because in order to render alpha objects, the renderer

has to sort them by distance first, you end up having to sort 300

objects.

There is another problem with alpha objects: when an object is very large,

for example a plane, it cannot be SORTED by distance correctly because

a vertex in the end of the plane would have another distance pass than

the object itself; you end up having artifacts, where a pixel that should

be behind another object, is rendered in front.

As a hobby game programmer i used to have this issues with the alpha objects

a lot, but did not know how to fix them, until, when I started working as

CTO in Hollow Games, i finally found the ultimate fix.

I came to the answer, because I kept thinking, that a plant, for example

is not REALLY an Alpha, but some sort of stencil, where the renderer

has got to either write the pixel or dont. There is a function in CG

(obviously there should be one in GLSL, GLSLES and HLSL as well, but

havent had to use them) called discard that does exactly that.

So I wrote a new shader (its an UberShader in my engine) that

when it reads the texture (tex2D) it checks if the alpha byte is less

than say 0.9, if it is so, the renderer should not write the pixel.

With this DISCARD-stencil technique, the renderer is not sorting

anything, and for objects like plants, it works like a charm. If your 

texture is in DDS format (honestly it should be) there is an option

for 1 bit alpha, instead of explicit alpha, that way, the texture

is lighter than an actual alpha, and 1 bit is all you need to decide

if to render or not to a pixel.

There are some other advantages to this technique, for example

if you are using deferred lightning, alphas are a problem, but

discard-stencils are not, they can be rendered as any other object.

Another advantage is that discarding is completely friendly with

bump mapping, which can seriously upgrade your plants, also, 

discarding tends to be neater.

Oh, by the way, most game programmers in the industry probably

knew about this, but me as a hobby game programmer did not

have a clue.

 Alphas instead of stencils, look dirty

 We can see here, how some grasses are not being rendered in the right order

 Stencils look much more clean

Stencils with bump in the leaves

Have a good one!