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What is the feather when make a new channel?

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Old 09-07-2008, 08:28 PM
kevs kevs is offline
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Re: What is the feather when make a new channel?

Thanks Tommy that was excellent.
My comparisons are:
1) selection with lasso 0 feathering.
2) using a channel for a mask.

With the lasso at 0, I see a hard edge (viewed in quickmask)
With the channel I usually see soft creamy edges.

Why? (or did you answer that.. maybe you did)
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Old 09-07-2008, 09:10 PM
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Verywierd Verywierd is offline
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Re: What is the feather when make a new channel?

When you make a selection using any tool, you are drawing a line between two areas. Feathering can make the line fuzzy to some extent, but it is still a line.

On the other hand, when you use a channel, you are getting a selection based on the actual lighting/visual conditions. So, if the photograph was of two sheets of straight edged plastic placed side by side, you would probably get a sharp line or selection, apparently unfeathered. However, in most cases edges are curved or softened by various factors such as lens focus, parallax effect, mist, dust, light falloff and shadow etc. So, the "edge" can actually be a gradual and uneven fading. In an 8bit image, this can be up to 256 shades of grey and of each colour channel, spreading out to either side of what your eyes perceive as the "border" of the selection.

In other words, when you use a selection tool, feathering is limited to only the same number of pixels to either side of the selection line, no matter what the shape, colour or lighting condtions.

When you use a channel, the effective "feathering" can extend much further and in a more gradual and subtle manner, depending on the shape of the objects involved and the lighting conditions. So a channel selection may or may not "feather", depending on what was actually in the scene at the time, and therefore looks more natural than an arbitrary selection line.

Note that the above is a very unscientific explanation and is likely to make a scientist puke.
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Old 09-08-2008, 04:10 PM
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TommyO TommyO is offline
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Re: What is the feather when make a new channel?

I think your terms are causing some confusion. But, let's see if this makes sense.

I prefer not to use the word "channel" when referring to a selection. The channel is more than that. However, if you really mean the whole channel, then yes all edges will be soft. This is by design, as the channel is a gray-scale representation and contains no hard edges normally (unless representing vector shapes).

The lasso with no feather always produces hard edges.

Other selection tools may or may not produce hard edges, based on their design. Some simply cannot select individual pixels when their tool is dragged on a pixel boundary. So, they partially select adjacent pixels, resulting in a soft edge. If the tool is set to a larger diameter, it may increase the partial selection effect.

Again, reference the black and white image. In it, the tool was set to 1 pixel. Even with that small a setting, the partial selection has the effect of creating a soft edge.
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Old 09-09-2008, 07:27 AM
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Sveltepig Sveltepig is offline
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Re: What is the feather when make a new channel?

Hi Kev
I understand what you are asking. All of these answers are responding correctly, but I think maybe your difficulty understanding the point they are all trying to make is that you are not grasping what a "feather" does to a selection. If you have already figured this all out and moved on, then excuse my butting in, but if not, perhaps my explanation will help.

I think the most basic way to understand a mask/alpha channel/selection is to see solid black as hiding all pixels and solid white as showing all pixels. If you draw a line (with a marquee or lasso tool it will create a sharp delineation between solid black and solid white and give a sharp edge to your selection. If you feather that selection the edge will seem less sharp because what feathering does is anti-alias the line between black and white, or in other words, blur that line a little. If you blur black and white together, you get grey, which does not hide all pixels (like solid black) or show all pixels (like solid white). Instead, it shows some of the pixels, or gives the pixels along the edge of the line a transparency, which has the effect of making the edge less crisp and appear softer.

If you use a copy of the red, green or blue channel to create a selection, the selection seems soft because there were shades of gray in the original channel, and not just solid black ares next to solid white areas.
All the grey areas will give degrees of transparency to your selection, which will result in a soft, seemingly feathered selection.
The shade of grey will determine how transparent that area of your selection appears. Light gray (closer to white) will show more, dark grey (closer to black) will hide more. 50% grey (128r, 128g, 128b) will hide/show 50% of the pixels in that area.

The main difference between this and a manually feathered selection is that a feather setting will be uniform around the edge of the selected pixels, whereas the selection created by a channel selection will have different shades of gray in different areas, and for that reason will not be uniform.

The best way to experiment with this information is to create your own alpha channels from scratch and then paint onto them with black and white brushes, at different opacities and with soft and hard-edged brushes. Then use the alpha channel as a selection to see the results when applied to pixels.

I often use this method to explain to people when teaching masking, and it seems to be easy to understand, but naturally it helps to see it in action while listening to the theory. I hope this has helped.

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Old 09-09-2008, 05:01 PM
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Markzebra Markzebra is offline
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Re: What is the feather when make a new channel?

Most of this advice is confusing - THEY ARE THE SAME. "The channel is more than that" - rubbish, its exactly the same as that.

Back to my original comment "Selections and channels are interchangeable" - they are precisely the same - no difference. Each channel has a selection equivalent. Each selection has its channel equivalent.

Let me try and be more specific - With both selections and channels each pixel has 256 possible values… in channels this is 256 possible levels of grey, and in a selection its 256 possible levels of transparency. Get it?
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Old 09-09-2008, 08:59 PM
kevs kevs is offline
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Re: What is the feather when make a new channel?

Very Weird, Karin, Tommy , M
great, info and insight.'
Thanks for helping me on this new journey into compositing. I'm on PHotoshop for for eight years , and I find this more complicated than anything else. So this all helps a lot. all those answers were terrific.

1) I have the Katrin Eesemann and Kelby Channel book, any other books or DVD you recommend I get.
2) Once you master a lot of this stuff and get good, are any of these plug ins like Fluid Mask etc, even better than what you are doing manually? thanks
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