Saving RGB images without background

[tricky]

I've added a message to an old thread that deals with this, but for clarity (sort of!) I thought I might add a new one here.

Is it possible to save an RGB image that 'floats' off its background? In other words I don't want antialiassed edges to be blended with the background, be that a solid colour or an image.

I am pretty sure this has something to do with it being premultipled or straight.

We have usually saved as TGA files and opened these up in Photoshop and chosen 'remove black matte' to get rid of any edge pixels. However, I am using an old copy (version 6.5!) of After Effects, and am rendering out a bunch of animated trees as a seperate pass to comp over the top of a background. If I save as TGAs, I get a white edge around all the leaves/branches. Presumably this is because when I rendered the sequence, I had a white background.

Is there a way of saving an image file that doesn't multiply the edges against the background colour, which is supported by older software like After Effects 6.5?

[rivoli]

TGAs can be stored either premultipled or not, you can choose in the file properties when saving. alternatively, if ae 6.5 supports it, you can still save the output as premultiplied, and tell ae that you have multiplied it over white. as long as it's a solid colour it should work.

[tricky]

So, saving as a TGA, I should untick the premultiplied option if I want it to sit nicely on a background, right?

[rivoli]

yes, you'll get an awful beauty pass, but it works alright.

[tricky]

How do you mean?

[instinct]

You can set the matte color and interpretion on import in AE

[rivoli]

it's only the way a straight rgb pass looks, as it doesn't get actually blended with the background. but it works as expected once it's masked with the alpha.

[instinct]

See here:

[img]
http://www.geniusdv.com/weblog/archi...ects_pixel.jpg
[/img]

If you set it to premultiplied and set the color to white that should do the trick

Regards,
Thorsten

[jop]

I use exr with straight alpha, it works like a charm, plus you cant set 16bit or even 32bit color depth.
But I don't know how AE 6.5 can handle it...

Anyway, no matter the background you have, straight alpha will give you perfect results, because objects edges pixels won't be antialiased against the background, but will remain "plain". It's the alpha wich will then "cut" the edges and blend border pixels with the background you want as a post process. So you won't have fringes anymore, even with thin objects.
Premultiplied alpha works in most cases, but it may introduce white fringes on thin objects.