VirtualDub: Using INTERNAL logo filter

[terrypin]

Any VirtualDub users around please? I posted this first in the VirtualDub forum but so far no response.
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I can use Donald Graft's excellent plug-in logo filter to add a transparent text logo. For example a BMP consisting of black text on white appears on my AVI as opaque black text, because I make white the transparent colour in the filter definition dialog.

But so far I'm darned if I can see how to accomplish the same thing with the internal logo filter. (Why do I want to? Curiosity & challenge.)

Do I have to be a graphics expert to know how to achieve this? Terms like the following, which appear in the dialog, are beyond me:

"Enable per-pixel alpha blending using alpha channel"

"Non-premultiplied alpha is better -the alpha channel is created
separately."

"Use gray channel of secondary image for alpha channel"

Could someone step me through how to do this please, or point me to some documentation in terms I have some chance of understanding? Better still, a worked example would set me straight.

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Terry, West Sussex, UK

[jagabo]

The alpha channel is a transparency map. It indicates how transparent each pixel of the logo image is. Black inicates the pixel is transparent, white indicates the pixel is opaque, and grays indicate different levels of transparency in between.

Given a logo and a grayscale alpha channel (not premultiplied):

+


A more likely scenario:

+

[terrypin]

Got it, thank you, that succinct illustration was exactly what I needed!

In practice, for my example of an opaque white logo, making it white on black, say WhiteTextLogo.bmp, and using that identical file for the secondary image, gives me the required result.

I'll try that 'premultiplied' option next, and hope the penny will drop easily there too.

Much appreciate your prompt reply.

--
Terry, West Sussex, UK