View Full Version : Alpha Issues

09-17-2013, 04:54 AM

Im having some issues with alpha image transparency.

The alpha image works well to cut out the image, however the alpha information cuts out everything behind it.

Anyone have any ideas why this is?...Ive spent hours playing with all kinds of settings and cant get it to work correctly.



Ryan Roye
09-17-2013, 05:10 AM
In object properties, look for this, which will allow you to determine whether an object influences the alpha channel or not. You usually want background objects to be "constant black", and the foreground objects be left at their default of "Use surface settings" (so, the trees in your pic should have a white alpha channel to go with them). Note that you don't need a green screen or any special colored background to cut out imagery in the case of 3d production, though there are a bunch of little things you'll discover (like unpremultiply alpha being used to prevent color bleeding).


If you want even more control over how alpha is embedded into your images, there are some additional options in the surface editor within the advanced tab that are worth experimenting with.

09-17-2013, 05:29 AM
Thanks Chazriker,

The green is just an really obvious box in the background showing that the trees still cut through it...(picked a crap colour lol!! no greenscreen techniques...that made me look like a noob!!)

"Raytrace Occlusion" in the render globals fixes the colour image - fills in the gaps with shadows, but the alpha still the same.

I will have a play with those settings and see what happens....Thanks!

09-17-2013, 05:53 AM
Me again,

I tried to play with the object alpha channel settings, but it didnt fix my problem.

Just to point out...the leaves have a colour jpg and an alpha mask jpg in the trans. (standard setup)

Here's a better example of the problem...I have 5 trees stacked in z space which would completely block out anything behind it which it does with the colour, however the alpha information from the front tree cut through all behind it.


09-17-2013, 07:12 AM
The problem you see is the effect of ray recursion limit is set too low or has reach it's max value. Ususally when you do leaves as single polygons with alpha textures, you use clipmap instead of transparency to avoid this problem. Clipmap is found under object properties and not in the surface editor.

09-17-2013, 12:19 PM
Thanks Ztreem...I just worked that out...I separated the different leave groups into there own layers and one for the trunk, and used the respective clip map texture for the leave objects via the uv map... and it works spot on! no need for raytrace occlusion etc, as well it renders quicker i noticed.

Thanks! for replying as it confirms it for me that this is the way to go. :)


EDIT: I tested the Ray Recursion limit...it was set to 6, when I set it to max 64 it fixed it...It worked as low as 15. Thanks Again!!

09-17-2013, 02:25 PM
Late to the party but I was going to suggest Ray Limit right off. Trees, especially, burn up a lot of recursions. Every leaf layer all the way through the tree to the background eats a ray. I've run into this many times.

Another issue is that LW doesn't (at least used to) handle JPEGs well as a general rule. I've used JPEGs as transparency or clip maps on leaves and, turning on some specularity, the whole poly would show as a ghost. Using TIFFs or TGAs cured the problem.

09-17-2013, 07:47 PM
Thanks spherical...I would never have worked that out, as I had no idea what ray recursion did or how it impacts on the render if at all (still dont really know...RTFM time, now that I know what to look for)....I have been doing wrong for such a long time :(

I'm using jpg's as I thought that the taga, tiff and png alpha channel was causing the issue...I prefer to work with png's, lw seems to like taga or tiff formats a lot better, but they dont preview as icons as you search through them in windows.

Thanks to all for your help...much appreciated!!! :)

Clip Maps are a little tricky, as they don't save with the object...a clip map node (something like that) would be nice as part of the surface settings to be able to save it with the object...The work around is to setup the clip maps on the object and save that scene, then use the "Load from scene" option to import the object with its clip maps.

09-17-2013, 09:54 PM
Ray Limit prevents unnecessary rays from being computed. If it doesn't make any difference in the final render, you may as well truncate them and save time; sometimes a lot of time. IIRC, you can obtain a function for Windows that does generate icons for other image file types.