Blend modes; Multiply to transparency?

[Mar]

Hello everyone!

My scenario is this: I have a high contrast black-and-white fishnet pattern, this one in fact, and want to remove the white background. As you can see there is some small white specks amidst the black threads which makes the magic wand produce less than desirable results.

Now, what I tried was to simply add a new transparent layer beneath the B&W original and setting the original's blend mode to Multiply. In my mind, this would superimpose the black threads over the transparent blackground, alas I was proven wrong. The annoying thing is I can fill this new layer with red and have the threads come through beautifully with no aliasing and no white specks. Indeed, the process seems very simple if only there was some way of preserving the transparency through the process. Am I doing something wrong and is there a solution better than the not-so-magic wand? I've heard there is an [outdated] plugin that does approximately what I'm after but I'd rather keep to "vanilla Paint.net" if possible.

[Sarkut]

Mar,

To eliminate the white from the mesh image:

Use Grim Color Reaper plugin, set to white.

Save As... PNG.

Grim Color Reaper

HOW TO INSTALL PLUGINS

Edited April 6, 2010 by Sarkut

[Mar]

It performs beautifully, thank you

(On the same note, would anyone kindly explain why Multiply didn't act as I had hoped?)

[Sarkut]

This is my best guess.

It seems that what is being multiplied with Multiply

blend mode are the RGB numbers..

The Alpha channel is separate from the calculation.

The Color Picker tool shows Alpha as

R255, G255, B255 (white), so it multiplies as white.

White is 100%, or one.

1 X 1 = 1

White X Alpha = White

[Mar]

If I fill the background with #000000, Alpha: 0.0, I should expect to get a completely black result as the net is #000 * #000 and the spaces are #FFF * #000.

The result is actually the same as the original: a black net with white, non transparent holes. It seems there is either an invisible white bottom layer that gets added at the very end of the process, or that #000000:0.0 somehow translates to #FFFFFF:1.0...

[Sarkut]

Good point.

I'm sure someone knows the answer.

In the meantime I will regard it as

simply being the will of Vishnu.

Edited May 5, 2010 by Sarkut

[Simon Brown]

It's blending the colour you're filling with the background, not replacing it. To get it to replace, set the blending mode (not blend mode) to overwrite.

[Sarkut]

I'm sticking with my Vishnu hypothesis.

[Mar]

It's blending the colour you're filling with the background, not replacing it. To get it to replace, set the blending mode (not blend mode) to overwrite.

I think you misunderstand what I'm trying to do (or maybe I'm the one misunderstanding what you're trying to say )

To elaborate, I'm not tampering with the original layer in any way other than setting it to multiply. Doing this will let the layer below show through in the white areas, but not if the layer below is transparent. Sarkut reasoned this was because the Multiply layer blending mode disregards the alpha channel, thus blending white with white resulting in white. While this holds true for a completely transparent background with a colour value of #FFFFFF, it shouldn't work that way with a completely transparant #000000.

[Sarkut]

Okay. I think the way it works is that the degree of Opacity determines

the degree that the blend mode is in force at a given pixel coordinate.

Where the Opacity is zero on either layer,

the multiply blend mode is rendered null.

Wherever zero opacity is present on either layer

Normal blend mode is in effect.

The amount of transparency apparently multiplies also.

If each layer is at 50% transparency, when merged

in multiply mode they will have 25% transparency.

When both start with 80%, they have 64% when merged

with multiply blend mode.