$blendtintbybasealpha

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$blendtintbybasealpha is a VMT parameter that allows tinting of a mesh'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.

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

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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?