[rboaz]

I have an oval logo with several colors that I want to use as decal over a surface. what are the steps to create a material that is a decal such that the corners of the image file outside the logo are transparent

[Bubbaloo]

You need an alpha image.
Black for the logo and white for the "transparent part".
Place this in the transparency slot.
(Anything white in the image will be transparent.)

[rboaz]

that did the trick. thanks

[rboaz]

Well I thoguht that it worked but the corners render black when in fact a green surface should be showing. any suggestions

[Bubbaloo]

Can you post a screenshot?
I am about to leave so I won't be able to help you until tomorrow.
Hopefully someone else will!

[Mattia Sullini]

Have you checked that the Nd for your mat is 1? If not, and you have a single surface poly (i mean no thickness) the transparency will not correctly displayed. If you need a higher Nd (for reflections, highlights or whatever) you can then add a second BSDF layer masked with the same map you used for transparency, darkened as much as you need to weight down the effect.
Second solution (more practical) is to go for two BSDF, one masked with the inverted map of the other. The first one will control the transparent part (refl 0,90 0%, transparency 100%, Nd1, roughness 0%), while the other one will control the decal itself. Hope this helps.

[rboaz]

By changing nd to 1 the transparency worked. I tried your second solution (more practical) and could not get it to work. I did not follow your comment "one masked with the inverted map of the other" as to what channels to put these maps (refl, transparency, weight etc) can you give me some additional direction. Thanks

[Mattia Sullini]

When you create a BSDF you can weight it in two ways: numerical or through a map. If you click on the BSDF folder, near the field where you set the value in %, you have a greyed symbol for the texture: click on it will popup the usual texture viewer where you will be able to load the texture. Anyway all these things are described in the manual...

[Bubbaloo]

Here's a tip:
You can put the alpha map into the layer weight slot.
Create a new layer, invert the alpha map, and put it in that layers weight slot.
This way you have one layer for the background and one for the decal.