I am mapping a PNG image (with transparency) to the color attribute of a lambert material. Then I apply this lambert to a polygon plane. The idea is that we would only see the part of the image that is visible thanks to the alpha embedded on the PNG, which works as expected.
The problem is that for some reason the bounding box of the plane this material being applied to is casting some shadows in areas that should be transparent. Something like my alpha is not completely transparent and areas that are supposed to be transparent are still casting some shadows.
I make sure that my alpha is 100 % transparent when I export the PNG out of Photoshop. I also observe the problem when using TIFF or PSD files with transparency.
I’m using Maya 2009 on a Mac and rendering with Mental Ray (but I also observe the same artifact when rendering with Maya Software)
I've been struggling with this for ages and have tried a number of bizarre workarounds that I've seen elsewhere on the web such as nesting two surface shaders, then using a color blend etc... The solution is actually frustratingly simple:
On the (lambert) material attributes, click on the "Raytrace Options" tab and set "Shadow Attenuation" to 0. For some reason it's preset to 0.5 and this causes the unwanted subtle shadow.