alpha maps and luma maps

[colin]

since a luminance map can be used in the transparency channel i end up using that, instead of an alpha texture map. so - i was wondering if and when fellow octaners are using alpha maps...

[abstrax]

since a luminance map can be used in the transparency channel i end up using that, instead of an alpha texture map. so - i was wondering if and when fellow octaners are using alpha maps...

You mean an alphaimagetexture? You use it if you want to use the alpha channel of an RGBA image for alpha. This texture loads the RGBA image, but discards everything except the alpha channel.

Cheers,
Marcus

[colin]

thanx abstrax. my mistake though. i wasn't asking the HOW, i was simply curious about the WHEN and IF? more of a workflow question if you know what i mean.
me personally, i'm used to output a couple of passes anyways. rgb, bump, spec, luma. seems like a hastle to me to make sure my rgb got an alpha. (for instance, a luma matte is easier to spot...).

EDIT: plus i was wondering if i actually did miss something. is there any situation where i HAVE to use an alpha image texture instead of a luma? (where i can't reach a certain result with a luma? because i can't think of any).

[abstrax]

thanx abstrax. my mistake though. i wasn't asking the HOW, i was simply curious about the WHEN and IF? more of a workflow question if you know what i mean.
me personally, i'm used to output a couple of passes anyways. rgb, bump, spec, luma. seems like a hastle to me to make sure my rgb got an alpha. (for instance, a luma matte is easier to spot...).

EDIT: plus i was wondering if i actually did miss something. is there any situation where i HAVE to use an alpha image texture instead of a luma? (where i can't reach a certain result with a luma? because i can't think of any).

If you have your alpha channel stored seperate, you don't need the alphaimagetexture (there is nothing magic in it). But sometimes textures come as RGBA images (e.g. logos, cutouts and other stuff) and using the alphaimagetexture allows you to use the same texture file for RGB and for opacity, without having to split the file into two. I hope that makes sense.

Cheers,
Marcus

[colin]

fair enough. it doesn't make much sense to generate a luma from a decal from, let's say, cg-textures, if there's an alpha already. makes sense.
well, it's good to have several options anyhow
thanx marcus.