《The lossless translation》 -----本字幕由雷天音 整理制作----- Now we've been fairly liberally switching back and forth between selection
【----QQ 568031962----】 outlines and alpha channels and from alpha channels to selection outlines and
【----ACELY STUDIO----】 that might raise a red flag for you, because after all many times in Photoshop
when you convert between different things, for example, if you take a TIFF
image and you save it out as a JPEG image or if you take an RGB image and
convert it over to CMYK something gets lost in the translation and you don't
wan to be converting back and forth too many times, because every time you run
a conversion you subtly harm the image. So for example if you were to take a
photograph and convert it from RGB to CMYK and then back to RGB and then back
to CMYK, over and over and over again you would ultimately destroy the quality
of that photograph. I mention this because I want you to know that does not
hold true when converting between selection outlines and alpha channels. Going
from a selection outline to an alpha channel or from an alpha channel to a
selection outline is a completely lossless translation. So feel free to do it
as many times as you like, and that's the big moral of this exercise by the
way, if you believe me, then go ahead and skip on to the next exercise, if you
want to see some proof stick with me. I'm going to go up to the File menu, and
I'm going to choose the New command, and I'm going to create an image here that
measures 600 x 600 pixels, no special reason for 600 x 600, just enough room to
play on the screen and I'm going to stick with the Grayscale image, that's what
it's set to for me. It may be set to something else for you by default, but a
Grayscale image will be good enough for what I'm trying to show you. I'm going
to go ahead and set the bit damp from eight bit to 16 bits, so that we have as
many luminous levels as we might reasonably want inside of this image and then
I'm going to go ahead and click OK in order to create that new document. Now
I'm going to grab my Elliptical Marquee tool here and bear in mind, because
we're working inside of a 16 bit Grayscale image, we can take advantage of as
many as 32,000 different luminance levels. I'm going to go ahead and make a big
circle and then add some some subcircles to it here and if you're not sure what
I'm up to, don't worry about it. I'm not really interested in you following
along at this point, and I am going to show you how these tools work, how the
selection tools work beginning in the next chapter, so you'll see all of the
different tricks of the trade that are available to you. All right, having
created the strange blob, I'm going to go up to the Select menu, I'm going to
choose the Modify command, and I'm going to choose Feather and the Feather
command allows me to blur the edges of my selection outline and I'm going to
blur these edges to the tune of about 18 pixels, I figure, so we're blurring
approximately 18 pixels in and 18 pixels out. Not exactly, because it's a
Gaussian algorithm as it turns out, but approximately. Now I'm going to click
OK in order to accept that modification. Now you would hardly know that the
selection outline is blurred at this point. Certainly the corners are smoothed
off, but the marching ants aren't capable of representing a blur, because the
marching ants just trace the threshold between the most selected pixels and the
least selected pixels and that's it. However the blur has occurred even though
the marching ants aren't showing them to you, the blurred edges are there. Now
I'm going to go back to my Rectangular Marquee tool and I'm going to cut a few
slivers inside of this selection so that I have some nice sharp edges mixed in
with my soft edges here. All right, so I've created a selection outline. Now it
may not be, you know, anything I'm going to win an award for, it's not a
selection outline I'd use inside of an actual photographic image, but it will
serve to illustrate how selection outlines and alpha channels work here, so I'm
going to go ahead and fill this selection with black by pressing Alt+
Backspace, or Option+Delete, icuase currently black is my foreground color
inside of this image. Now I'm going to go ahead and save this out as an alpha
channel, so I'm going to go to the Channels palette and notice that I've got a
Gray channel go in right now, because this is a Grayscale image. I'm tgoing to
go down to this little guy here, at the bottom of the palette and I'm going to
Alt, or Option click on it so that I can name the channel as I make it and I'll
call this one Blob or something to that effect and I'll click ok and there's my
Blob channel right there, then save the selection as an alpha channel, I can
now deselect the image, I don't need the selection anymore, because I've saved
it. If I ever want to come back to that selection I can just load it from the
alpha channel and because it's a lossless translation, I'm not worried about
keeping that selection outline around for the moment, and as you can see, it's
just an inverted version, the, the Blob channel is actually an inverted version
of the Gray channel, they're just opposites of each other right now. Now I'm
going to go to the Layers palette, I'm going to make a new layer, I'm actually
going to press Ctrl+Shift+N, or Cmd+Shift+N on the Mac, to bring up the
new layer dialog box and I'll call this one Test layer and I'll click ok, in
order to make that layer, there it is. Let's go ahead and fill the entire layer
with white, assuming that the background color is set to white, as it is by
default, you can press Ctrl+Backspace, or Cmd+Delete on the Mac and I have a
white layer. Now, what I want to do is rebuild the background item, so I want
to rebuild that black blob there with the sort of rectangles cut through it. So
go back to the Channels palette and I'll load up my alpha channel as the
selection outlined by Ctrl clicking on it or Cmd clicking on it, on the Mac. So
there's my selection outline and if what I'm telling you is to be believed it's
exactly the same selection outline we had before. I'll go back to Layers
palette, make sure that Test layer is active, which it is and I'll press Alt+
Backspace, or Option+Delete in order to fill it with black. Now let's press
Ctrl+D, Cmd+D on the Mac in order to deselect that image. So we've got test
layer and we've got that background. Background's the original selection, Test
layer is the selection that went through a complete conversion right, it went
to an alpha channel and then back to a selection outline. So let's see if it's
identical. If I turn it off and then turn it back on, we're not seeing any kind
of shift on screen, and we are viewing the image at the 100% view size, so we
are seeing all the pixels, so that leaves me to believe that the two are
identical, cause I'm not seeing any kind of shift, but let's make sure. There's
a Blend mode inside the Layers palette here that allows you to compare two
layers to each other and is called the Difference mode. I'm going to go ahead
and choose, I'll go ahead and click on the word Normal there and I'll choose
Difference and what we should get if there's no difference, we should get
absolute black, which is what we've got every single pixel now on screen is
black. If there were any differences, they would show up as either dark gray
pixels, or light pixels, or anything but black pixels and right now, every
single one of these pixels is black. Hence, these two images are absolutely
pixel for pixel identical to each other and of course hence converting from a
-----本字幕由雷天音 整理制作----- -----更多Lynda字幕请访问我的人人----- -----联系邮箱：acely@126.com----- selection outline to an alpha channel, back to a selection outline is a
completely lossless conversion. You can do it as many times as you like.