How DaVinci Resolve handles alpha channels

Hi,

I am new to DaVinci Resolve and I am using it to put together a new demo reel. However when I bring in my images, some are not behaving properly when I layer them up in the edit page. In this case, it is just a Solid Colour generator for my background with the CG render placed on top of that. But I am getting a bad looking darkened edge around my image. This was using the default Straight Alpha but even when I right click on the image on the timeline, the go to Clip Attributes and change this to Premultiplied, it does not look right. I downloaded Nuke just to compare the process and have included the comparison below.

In the cropped image if you look closely, even the tips of the hair look more like they should in Nuke compared to DaVinci Resolve.

It would be awesome if anybody can offer some insight or advice on this please?
Thank you,
Paul.

Hi again. Any takers on this?

I am new on the forum so not too sure of what the response time usually is for posts but hopefully somebody on here has an answer for this.

Thanks!

[Peter Chamberlain]

Post a link to a full res source frame so someone has a chance to examine

Alpha handling in Resolve is wonky, there’s bazillion threads about it with ample examples, and example frames.

Try Fusion Standalone, I bet it works fine there.

Post a link to a full res source frame so someone has a chance to examine

Here's a sample frame. I don't have permission to post the full frame and the full res 4K frame was too big to attach also.

But this will show the exact same issue, but is just HD instead of UHD.

Alpha handling in Resolve is wonky, there’s bazillion threads about it with ample examples, and example frames.

Try Fusion Standalone, I bet it works fine there.

When you say standalone, do you mean it is a fully separate program to Davinci Resolve and not the built in Fusion?

Yes.

https://www.blackmagicdesign.com/products/fusion

Yes.

https://www.blackmagicdesign.com/products/fusion

Note: You can use the standalone Fusion if you pay for the Studio license of Resolve...

I should add that I brought your sample PNG into LumaFusion on my iPad just for fun and I see the same dark fringe on it that you show in your first Resolve image (straight alpha). Not sure what this data point means but thought I'd share.

Sample image is perfectly fine. Are you in a color managed project or doing any kind of manual color management? That can mess with colors in semitransparent regions.

Steve, thanks for your input, this is the kind of help I am looking for so thanks for doing that.

Yeah, I don't think I will be going to spend 360 Euro just because the alpha is behaving weird. It is a strange one, but maybe I can just use Nuke as a workaround for that section. It may be something I decide to purchase later on down the line, but just not now.

I would LOVE to know the reason behind why is it happening though.

Thanks for checking it Hendrik. As for a colour managed project, are you referring to the image sample I posted here or the settings I am using in Davinci Resolve?

Thanks for checking it Hendrik. As for a colour managed project, are you referring to the image sample I posted here or the settings I am using in Davinci Resolve?

Settings in Resolve. The interplay between straight color values, premulted color values and alpha is delicate if any kind of color transformations are happening. First off, alpha is not color, it is non-color data representing coverage. Secondly, color values in RGB channels, once premulted, have a direct association with alpha due to the meaning of premultiplication operation. If premult, unpremult or color ops are done in wrong order, this association is bent or broken, causing regular blend operations like merge over to overshoot (light fringe) or undershoot (dark fringe) because the way background is scaled back using alpha does not match how the foreground is scaled and their combined result will not amount to what it should.