Alpha Channel or Alpha Channels?

Hi Folks,

How many times do I need to insert the Alpha Channel command in a single filter? (And where does it hook up?)

In my history, I have avoided Alpha Channels like the plague, but it looks like FF will make me HAVE to learn about them. All I know is that every pixel has information about transparency in it, and the alpha channel brings it out.

In theory I understand that you pull the alpha channel information fr om the original image, but on some filters, there are 3 or more things that add background opacity. I'm just confused about wh ere to insert it, then what it hooks up to. I guess it wouldn't hurt to understand the why of it, too, but that might be left for when one kind soul has time to put it in English for me.

I figure other new people will like to see this, both now and in the future.

Thanks,

Lime

Alpha is great for masking and blending. Here's an example:

1. Lay down Noise->Perlin Noise and Adjustments->Threshold.

2. Hook up the output of Perlin Noise to the Source of Threshold AND to the High of Threshold. You should see some of the Threshold preview goes black.

3. Now double click on the Low color for Threshold. Now let's change the alpha channel. Slowly drag the "A" all the way to the left, to 0. The black area fades away and a checkboard fades in. This is Filter Forge's way of telling you what the alpha value is. Imagine that the checkerboard is BEHIND the component. There's no checkerboard really there, it's just showing you that transparency is at play.

4. Lay down External->Image and Processing->Blend.

5. Set the Blend Opacity to 100%. The preview should go all white.

6. Drag the Threshold output to the Blend Foreground, and the Image output to the Blend Background. The areas of the Threshold that are opaque show up in the Blend preview, but the areas that are transparent (checkerboard) come fr om the Image. The Threshold is layered on "top" of the Image -- so the transparent pixels on top you see right through, like the holes in a mask.

7. To see real alpha blending, mess around with the Blend Opacity and particularly the Threshold Smooth values. Pixels that are semi-transparent are blended with the pixels underneath.

For example, a 50% alpha red pixel layered over a 100% alpha blue pixel means you want the resulting color to be half of the red, and the remainder (also 50%) to be blue. This is a perfect 50/50 split, so the result is 100% alpha magenta. If the blue pixel was also 50%, the result would still be magenta, but it would have an alpha of 50% and the checkerboard would show underneath. Mess around with a single Blend component to verify these results.

To see the alpha layer of some output more clearly (and also to manipulate it), use Channels->Get Alpha. It converts the alpha channel of some output to greyscale, wh ere white is 100% alpha (opaque) and black is 0% (fully transparent).

Hope this was helpful!

You can kind of put it where you need it. Remember though: it's still 'there' when you do processing on it, which costs processing horsepower. So it's better to add it at the end, if you can.

You might want to check this out, also:

http://www.filterforge.com/wiki/index...Techniques