$blendtintbybasealpha

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$blendtintbybasealpha is a material shader parameter available in all Source games since Left 4 Dead. It can be used in Source 2013.

It allows tinting of a model's material using the $basetexture's alpha channel as a mask to determine tint intensity (but not the tint's RGB color!) Applying an actual color RGB values is done elsewhere - either in-game or in Hammer with entities using the 'rendercolor' keyvalue. This technique is used in games like Left 4 Dead to allow for recoloring of the vehicle props, and in Team Fortress 2 for painting hats.

Bug:Not compatible with $alphatest or $translucent, this parameter will override both. Use the Modulate shader as a workaround.  [todo tested in ?]

Note:Only compatible with VertexLitGeneric.

Usage

The tint mask simply controls the intensity of tinting. The more alpha, the more intense tinting is.

In practice, it is a good idea to use the mask to avoid tinting parts of the model that aren't covered by paint: either elements like lights, license plates, wheels, warning stripes, or scratched off, rusted or dirty areas, where the paint lost its effect.

Alternatively, this method can be used to only paint selected elements, like labels, colored glass, parts of clothing, character hair, makeup and so on.

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  Using rendercolor on prop physics or prop dynamic entities without $blendtintbybasealpha. The tint is applied across the entire texture, which in this case is undesireable.

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  Same but with $blendtintbybasealpha enabled and the texture edited. The alpha channel of the texture, plus painting the base white, allows the trucks to be tinted in a desireable manner.

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  The example texture for the truck. The white base (top) eliminates unwanted color influences, and the alpha channel (bottom) contains the tint mask.

$blendtintcoloroverbase

$blendtintcoloroverbase, a related parameter, controls the amount of color replacement versus tinting. (replacing the color per pixel entirely, versus adding more color to make the colors closer to the tint RGB color.) A value of '0' will be full tint while a value of '1.00' will replace the albedo in the mask area with the color defined with $color2.

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  Using $blendtintbybasealpha as above plus $blendtintcoloroverbase at 1.0. The color replacement (instead of tinting) removes texture detail from the masked areas.

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  Same but with $color2 at "{255 0 0}".

Todo: Explain the practical effect of this in better detail. What's the reason for the black colours? Is this a result of opposite colours mixing?

Todo: Also explain how this is useful for creating skins by manipulating the $color2 in the vmt while reusing the same basetexture.