CGTalk - Alpha Is Luminance

maNtodEa

07-18-2008, 02:25 PM

hey everybody,
i have a question about alphas. i tried to paint some texture, stripes for example, and then to use layerd shader to combine that texture with some material, but i wanted texture to appear on top of that other shader. so i figure out i could use alpha channel (btw, i'm pretty new in all this). so in photoshop, i selected my stripes and then saved selection, so i created alpha channel (stripes were white colored and everything else was black). then in maya i connected that texture to color of lambert shader and then also to transparency. and then i saw this Alpha Is Luminance checkbox, and when is not checked i got what i wanted. i have my stripes on top of that other shader. so i was wondering if someone could explain me what that option actually means. i hope i wasn't to confused. thanks in advanced.

maNtodEa

07-19-2008, 05:50 PM

anyone? can i post this topic in general discussion?

DanBroughton

07-19-2008, 07:30 PM

I think if the alpha channel is missing then this switch uses luminance as alpha but I might be wrong.

Hope this is some help

Dan

EricDigital

07-20-2008, 12:46 AM

What type of file did you save your texture as? It's possible the file type you chose simply doesn't support an alpha channel. I tend to use 32bit targas myself.

I believe in regards to alpha is luminance what they both share in common is an alpha and luminance are both greyscale, so when you have an image plugged into trans you select it to use as lum, it converts it to greyscale to use as a trans.

I found this on another forum.

"1- In 2D file texture node, Out Alpha = Alpha Channel ( Mask ) of Image.
TGA, IFF and other formats supports 24 RGB + 8 Alpha = 32 bit per pixel.

2- In other texture nodes, Out Alpha = Grayscale of the resulting color.

3- In both cases when "Alpha Is Luminance = ON"
Out Alpha = Luminance of the resulting color = ( 0.30*Red ) + ( 0.59*Green ) + ( 0.11*Blue )."

maNtodEa

07-20-2008, 06:50 PM

so when alpha is luminance is on, the luminance of an image is used as alpha, like you said? it makes sense :) thanks for the replies.

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