View Full Version : Question: Transparency in DFX+ (compared to Commotion)?

07 July 2003, 12:48 AM
Hello, everyone,

I, too, am one of those Lightwave users that will get their hands on DFX+ very soon. At the moment I am experimenting with the DF4 demo. I have some experience with Commotion 4.

Here's what I was trying to do with the demo version:
I have a render of an object against a colored backdrop. As I have learned so far, there always has to be a backdrop colour in LW. I have picked green to make the problem more obvious:

The image has the following alpha channel included:

Continued in the next post...

07 July 2003, 12:49 AM
In Commotion I can choose "Premultipy..." and pick a colour, the backdrop colour.

Commotion then uses the alpha channel to calculate the backdrop away. Here's the result:

Continued in the next post...

07 July 2003, 12:49 AM
I wanted to do the same in DF.

There's only a "Post-Multiply" button:

This simply multiplies the RGB channels with the Alpha values. But that, of course, doesn't get rid of the green, as you can see in the result:

What do I have to do in DF4/DFX+ to get the same results as with Commotion?


07 July 2003, 01:10 AM
Ok, i think i see what is going on here... and it should be possible to do via a channel boolean if i understand correctly.

I need to sit down and figure out the exact operation, but you should be able to multiply your image by a colored background and the alpha in your existing image to produce the same result. (Or negatives thereof)

Ermm.. I need to know what is exactly that you are doing between the green BG and the brown BG. I can't quite figure out the math given just the images.

Is Commotion giving you a brown BG after you premultiply by the green color (aka a spill suppression?) or is it a color correct node you are using, etc?

07 July 2003, 02:05 AM
Ok, i'm stumped. Maybe it's too late in the day, or something. I'd send the info you've given here, and a copy of your original images to tech at eyeonline, and see if they can find you a solution. I know Stuart will be able to find it for you, but he's off at Siggy right now, and won't be back till Monday.

I'll keep at it in the meantime, to see if I can figure it out.

07 July 2003, 03:24 AM
Ok, I have it after having a long and involved IM session with one of the developers. (Thanx Daniel!)

This is an un-merge function. It can be done with a custom tool.

I couldn't figure why I was getting errors... Since your images alpha and color don't quite line up (Due to jpg compression i assume) and it being late here, I was going nuts. Daniel finally pointed out that the blurred areas don't match.

I have included a flow with a few examples, which you should be able to work with after rendering quite easily.

07 July 2003, 05:29 AM
Why not just render to a no color (black) bg?

I know both Maya and XSI also have an option to pre-mult or not. Does Lightwave?

07 July 2003, 08:06 AM
thank you sooo much for your efforts.

I'm sorry I couldn't answer your questions, but I just had to get some sleep after posting (it was 2 o'clock in the morning over here). And I am sorry that I didn't mention, that the images were just parts I cut out from bigger ones, which is why they didn't quite match. The brown background was just the brown background of the Flow/Composite, no special operations involved.

I am so pleased now. First of all with what efforts you went through to solve my problem. Secondly that it all happened so fast. That's great.

Most important in the long run, I think: I learned an important lesson about the art of compositing, about writing your own tools with DF (DFX+ does have that custom tool, too, doesnt it?) and about how good the support is.

I am very confident now, that buying DFX+ was a good decision (I ordered the LW upgrade mainly because of DFX+). Although this might not have been my only question ;-)

Once again:
Thanks a lot.


07 July 2003, 08:08 AM

Black isn't no color as I had to find out. It will produce the same results as a green backdrop only that the fringe is black instead of green.

Lightwave doesn't offer premultiplying.


07 July 2003, 03:05 AM
Originally posted by marcboy
Black isn't no color as I had to find out. It will produce the same results as a green backdrop only that the fringe is black instead of green. Hmmm... are you sure? From my albeit very limited experience with the DF demo, it would appear that it automatically pre-multiplies the alpha if the background is black. So turn post-multiply off (assuming you have it on), and there'll be no black fringing. This is all assuming you're using an image with an embedded alpha, not a separate file.

It does, however seem a shame that there's a bit of a rigmaroll to get it to pre-multiply any other bg colour...


07 July 2003, 03:15 AM
Actually, Jason's unmerge custom tool works perfectly well - not much of rigmaroll after all ;)

07 July 2003, 05:55 AM
marcboy, a couple of points:

The Custom tool isn't part of the base DFX+. As an "advanced" tool, it comes with Module 1. Luckily, Module 1 is apparently included in the LW8 bundle :-) You could perform the same operation with 3 Channel Booleans instead; it might even be faster. See attached flow.

Second, rendering against black generally IS desirable.. you can normally take a render over black with combined alpha and comp it straight on, nothing extra required. If you've rendered against black and you're still getting black fringing when you comp it on, there may be some problem with mismatched alpha (e.g. the alpha fringe extends out further than the colour edge), or the merge process is wrong for the images (e.g. using Subtractive merging with premultiplied images).

It could just be the mismatched positioning and low JPEG quality, but I had some trouble trying to get the example pics you posted to give good results - the blurred sphere seems somewhat less blurred in the alpha channel than in the colour channels, which defeats the unmerge process.

As for Lightwave and premultiplied images, if Lightwave will produce an image with antialiased colour edges on a black background and antialiased alpha, that should effectively be a "premultiplied" image. The colour channel values should be no greater than the corresponding alpha value (and transparent areas are therefore black), which is what you would get if you multiplied a non-antialiased image by its alpha.

07 July 2003, 08:13 AM
Daniel, thanks for the flows and for becoming a CG Talk member to help me.

I remember that a while ago I rendered an image in LW with a black background and saved it as 32bit PNG. As far as I remember it did show a black fringe, though. That was when I decided to render the scenes of my film in one go as opposed to foreground and background layers.

But I will experiment and try things out again tonight.


07 July 2003, 10:50 PM
OK, you're right, I can indeed use my renders in DF right away if they are rendered on black. That's great.

Sorry for all the trouble my question caused. But I for one learned a lot in the process.


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