Glitch in transparent areas of background when using Background Image

[sethcshort]

Hi,

I've used background image a few times successfully before, but for some reason this time I'm getting some strange glitch. It seems like data from some other image is being inserted behind the background image (which is a PNG with transparency) and strange pixels can be seen through the transparent areas of the image.

Attached are samples. First is taken from the background PNG image I'm using. The squares are transparent, I put a black background behind so we can see there's nothing visible inside the squares.
Second is a screenshot out of KeyShot. You can see strange stray pixels in some kind of pattern visible behind the transparent squares.
Third is a sample from a render out of KeyShot. You can see that the glitch is visible in the render as well.

I'm using Windows 7, KeyShot 6.2.105

Thanks,
Seth

[sethcshort]

Working on another file, I'm getting a similar glitch. This time the erroneous pixels extend out past the edge of my BG. See attached.

While the glitch seems to have the same kinds / radius of curves, it seems to be a different color. It kind of resembles some of the other graphics I've used as backgrounds... but I've dug through my files and I can't find a direct correspondence. There might be some scaling going on... perhaps it's the edge of a previous background I used?

This file is not based in any way on the previous file. It's from a different source and has never had a background applied previous to this (the other one hadn't either).

Thanks,
Seth

[guest84672]

What are you trying to achieve?

[sethcshort]

Ideally, I'd like to render the 3D scene with shadows and/or reflections that take into account the transparency of the background, so when I compose it with the background image in Photoshop, I don't have shadows overlapping the transparent areas. This seems to be currently happening when I render with the background visible, and I've used it previously to good effect.

Currently, due to the glitch, I'm rendering the 3D objects with a black background, which results in a general shadow that ignores the background transparency. For my current purposes, this is fine. But it would be nice to give it that little extra realistic detail.

Seth

[Rex]

Hey Seth,

Perhaps what you are looking for could be better achieved by adding a plane to the scene and applying a diffuse and opacity map?

[LayC42]

Or you can use a flat white material to all your meshes an render it out against a black background without shadows. So you'll get a useful alpha map of your renders to cut unwanted areas in Photoshop.

[sethcshort]

Thanks for the tips. As I mentioned, the process I'm using now is working fine, but if it becomes otherwise, I'll give those a try.

I posted this because I noticed the graphical glitches, and that they affected the render, so I thought I'd point it out since it seems to be indicative of a bug.

Seth