$distancealpha

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$distancealpha is a cheap edge filtering technique for raster images. It takes advantage of 3D hardware's well-established texture filtering methods by defining a series of alpha channel thresholds above, below, or between which effects can be applied. This feature was added to the Source Engine with the Orange Box engine branch.

It's great for UI elements, foliage, chain link fences, grates...

Advantages
Vector-like edges on raster images even during extreme zoom.
Far faster than adaptive AA.
Disadvantages
Effects look extremely pixellated from grazing angles in some situations (might be related to brush application?).

For (lots) more detail, see this SIGGRAPH paper.

Bug:Distance alpha materials are not rendered correctly in Hammer's 3D view.

Texture creation

Distance alpha source textures are created at a very high resolution. Valve use 4096x4096 images, but the SDK's build of VTEX will crash if you pass it anything over 2048x2048.

The image should otherwise be completely normal, except that the alpha channel should avoid gradients (you'll be able to re-construct them with the shader).

To compile the texture you must use VTEX; third party tools don't support distance alpha yet. Your .txt file should look something like this:

// Enable distance alpha compiling
alphatodistance 1

// Scale the image down to 1/nth its original size. '8' takes 2048 down to 256
reduce 8

// Size of 'spread' zone between alpha values of 0 and 1. This is the area the shader works with.
// Bear in mind that higher numbers exponentially increase texture compile time!
distancespread 4

// To prevent artefacting:
nomip 1
nolod 1
nocompress 1

Material creation (class_spy_outline)

Alpha channel of $basetexture (100% zoom).
The material in-game (50% zoom).

This material is displayed by Team Fortress 2 when a Spy player gains or loses a disguise. Despite the fact that it fills the entire screen at any resolution, its texture (seen right) is only 256 pixels square.

UnlitGeneric
{
	$basetexture	hud\class_spy_outline
	$translucent	1
	$distancealpha	1
	
	$softedges		1
	$edgesoftnessstart	.5
	$edgesoftnessend	.46
	$scaleedgesoftnessbasedonscreenres 1
	
	$glow		1
	$glowcolor	"[0 0 0]"
	$glowalpha	.4
	$glowstart	.1
	$glowend	.5
	//$glowx	.98
	//$glowy	.98

	$vertexcolor	1
	$vertexalpha	1
	$no_fullbright	1
	$ignorez	1
}

The most interesting commands here are the 'start's and 'end's: their values determine the range of alpha values across which each effect is applied.

  • $softedges is applied from 50% to 46% opacity
  • $glow is applied from 10% to 50% opacity

By setting $glowcolor to black and $glowalpha to 40%, the glow effect is made to look like a shadow.

To do: $outline

Effects

$softedges

Fades out all colour channels from $edgesoftnessstart (opaque edge) to $edgesoftnessend (transparent edge).

$scaleedgesoftnessbasedonscreenres <bool>
To do: Describe

$glow

Creates a monotone gradient from $glowstart (opaque edge) to $glowend (transparent edge).

$glowcolor <RGB>
Color of glow.
$glowalpha <normal>
Opacity of glow.
$glowx <normal>
$glowy <normal>
Offsets glow; good for shadow effects.
$glowstart <normal>
$glowend <normal>
Alpha channel thresholds. $glowstart is full intensity, $glowend is where it becomes invisible.


$outline

The same as glow, but with the added option of defining how sharp each edge appears.

$outlinecolor <RGB>
Color of outline.
$outlinealpha <normal>
Opacity of outline.
$outlinestart0 <normal>
$outlinestart1 <normal>
Defines the outer (i.e. low alpha) edge of the outline. Any gap between the two values will become a gradient.
0 is 'outer outer', while 1 is 'inner outer'.
$outlineend0 <normal>
$outlineend1 <normal>
Defines the inner (i.e. high alpha) edge of the outline.
0 is the 'outer inner', while 1 is the 'inner inner'.
Note:The two values must differ by at least 0.02 or the entire outline will disappear!
$scaleoutlinesoftnessbasedonscreenres 1
scales the outlines softness based on the users screen resolution
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