Can you please provide more information? Which file format are you using? PNG?
I've tried both png, tiff and psd and nothing seems to work.
I've attached a simple scene file.
Seems like a bug. It works if you move the base material after the polygon selection tag.
Hmmm.. doesn't work for me. I get white, scaled cirkels where the red i supposed to be.
I'm not sure it's Arnold's fault, I've got the same result with the standard renderer.
Working now after I removed the actual alpha from the black and white pic.
Really strange...
Thank's for taking a look at my file.
You know if there is any way to file a bug report regarding the stacking order?
This forum post stands as a bugreport. I'll try to fix it for the next release.
So, just found another bug...
I changed the bump height on the base material to 0,05 and the alpha disappeared.
Changed it back to 0.1 that I had before and still the alpha is gone. Had to press re-do a couple of times to get it back.
This seems like a pretty buggy software.
That's strange, I'll try to reproduce it.
Also note, that instead of material stacking, you can use the layer shader, which gives you more control and kinda the recommended way with Arnold.
Your original scene should work now with this wip build, please give it a go if you have some time:
https://safeshare.solidangle.com/index.php/s/ljxlVTXMM3eaGLz
password: c4dtoawip
I'm also working on some other issues regarding the native decal workflow reported earlier.