View Full Version : Composite Question

10-25-2009, 10:06 PM
I have a very complex scene and have found I need to reduce objects/polys. So I've rendered images of some middleground objects and am placing the images on polys. Since the work is for stills, this works ok. However, I have a faint remnant of the alpha that I can't get rid of.

I can't do this as post in Photoshop because in the scenes I'm doing there are a lot of interlacing objects.

I constructed a simple scene to demonstrate my problem. Attached is a screen shot. Simply, how do I place a 32 bit image in a scene and acheive true 100% transparency where it's needed.

I know this must be a simple question for most of you, but I'm a simple minded person. I don't get it. Any help is much appreciated.

10-26-2009, 06:51 AM
Why not use a clip map, it's in your object properties.

10-26-2009, 07:48 AM
Thanks for the reply, Crashburn.

Isn't a clip map 0 or 100% only? At least that's what I get. And, as I attempted (poorly) to show, my objects have varying degrees of transparency. I've also tried rendering out a separate Alpha and plugging that into transparency...same result as posted.

Surely this is possible to do. If not, why is 0% alpha not translatable to 100% transparent? Lightwave reminds me often that I'm not too bright.

More help is appreciated.

10-26-2009, 07:56 AM
Check out the Render Globals>Output tab. Play with the alpha formata and go with the one that works. Premultiply Alpha ought to work for your purposes.

10-26-2009, 08:28 AM
Thanks, Robert. Didn't work. Same result. Tried every combination with the two renders involved. I've used this method before and never noticed the alpha shadow before. Not till I came to this busy "Jackson Pollock" sort of busy background. It made me wonder if it's always been there.

Again thanks. Other ideas, please. I know the situation must have something to do with my ignorance.

10-26-2009, 08:59 AM
what are your render settings, have you got render transparency enabled etc and tried changing those?

10-26-2009, 09:27 AM
Thanks for the reply CrashNBurn. Attached are my render settings. No GI used, and, as shown above, image plane is set to not be seen by GI anyway. Though I need ray trace shadows for my render, off or on makes no difference here.

I take it from the replies so far, I should not be having this problem. Any othe guesses as to what stupid thing I could be doing?

10-26-2009, 09:38 AM
Have you tried enabling Raytrace Occlusion, seem to remember that helps with transparencies. Could be wrong, but worth a try.

10-26-2009, 10:02 AM
With all lights excluding the image plane and no ambient light, I assumed ray traced "anything" would have no effect. But I tried...No dice.

To make the green "football" image I arranged the simple model in Layout against a black background and saved as a png32. Did I need to do something different?

I appreciate the suggestion. Any other ideas?

10-26-2009, 12:34 PM
May be you have an alpha in the rest of the rectangle too. In that case, use a gradient and key out that part. E.g., see attached a simple example that compares invert with gradient.

10-26-2009, 02:35 PM
Thanks for the reply Sekhar, and the work you did. Your scene rendered fine. Then I added the backgrd I have been using in the above example and there the alpha remnant is back. I believe I understood the logic of the gradient, but couldn't do any better than the attached. Also tried to shut off lights, GI to the plane.

By the way, the backgrd, though a bit fussy is nothing special. (See second attached.) I think the alpha remnant is only obvious because of the fussy backgrd.

I have a feeling that there is something simple I'm doing or not doing. Somebody is gonna say, "I can't believe he didn't...."

Other ideas.

10-26-2009, 03:01 PM
If what you're showing are the Gradient and Invert in my example, do NOT use the Invert - just pass the alpha through the Gradient. I gave both to show you the difference. What the Gradient basically does is take the input alpha and anything below a certain value outputs as 100% and the rest as 0%, essentially keying out the semitransparent background.

10-26-2009, 03:42 PM
I understand. I was trying different things (Note the Keys are also reversed). Sorry to be confusing. Attached is an image from your scene with keys adjusted to show no alpha (no invert)...nice. Second attached is same settings with my backgrd made visible at render...Alpha is visible and I can't key it out. Third attached is your scene with my background attached.

Thanks for helping...anything else. As I say, I figure I'm doing something stupid.

10-26-2009, 04:50 PM
I'm not sure I'm following all of this correctly but perhaps there is an issue with Alpha from your image map?
Try saving out a Black and white version of the image map and apply that as your Transparency Map. Gradients in imported transparency maps work fine.
Can't help you with nodes. I'm still using layers and this technique always works for me.

Good Luck.

10-26-2009, 04:59 PM
Yeah, posting the scene helps. Looks like there's a problem in your BrainShell node setup - disconnect the HoneyComb(2) output and it renders fine.

10-26-2009, 05:56 PM
Thank you very much. Embarrassing that you had to isolate the problem in my own work, but I'll take it. I fixed the node problem. Some how the reduced opacity of the honeycomb bump and its appearance was influenced by the overlying transparency...

Problem fixed...Thank you, thank you.