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Understanding how overprints and knockouts work

From: Illustrator Insider Training: Seeing Through Transparency

Video: Understanding how overprints and knockouts work

Okay, so we have started to get a little bit more of an appreciation of the fact that there is little bit more to transparency that meets the eye. You know, it's more than just blend modes or Opacity settings, but as we've seen already that inside the Transparency panel, we can use the Isolate Blending setting to make sure that elements within a group don't mix with elements that are outside of that group. Now there's another setting here called Knockout Group. However, before I can explain to you exactly what that setting does and how we might be able to use it in a real world situation, we first have to really get a good understanding of what a knockout is in general.

Understanding how overprints and knockouts work

Okay, so we have started to get a little bit more of an appreciation of the fact that there is little bit more to transparency that meets the eye. You know, it's more than just blend modes or Opacity settings, but as we've seen already that inside the Transparency panel, we can use the Isolate Blending setting to make sure that elements within a group don't mix with elements that are outside of that group. Now there's another setting here called Knockout Group. However, before I can explain to you exactly what that setting does and how we might be able to use it in a real world situation, we first have to really get a good understanding of what a knockout is in general.

So let's take a look now for a moment on this file right here. It's called flower.ai and it's very straightforward. I'm working right now in a CMYK document and I've simply taken two objects here, a blue flower, which looks just like that. I'm going to press Command+Z to move it back. And then a yellow circle that appears in the middle of it. So I have these two elements. Again, one is filled 100% yellow. One is filled 100% cyan. Now in order to get a better understanding of exactly what knockout is inside of Illustrator, I'm going to employ two other panels inside of Illustrator.

I'm going to go to the Window menu and I'm going to choose to open up my Attributes panel. We're going to see I have the ability to apply something called an overprint to my artwork. I'm also going to go back to the Window menu and I'm going to choose to open up my Separations Preview panel, which will allow me to view color separations on my screen right here inside of Illustrator. Now I'll do one other thing here inside of Illustrator as well. I'm going to go over to my View menu and turn on this setting here called Overprint Preview. This will allow me to preview overprints on my screen again here inside of Illustrator.

So let's see exactly what this Knockout is and how regular artwork gets printed inside of any document. Now I have two objects here inside of my document and the yellow object is sitting on top of the blue object that appears beneath it. Now let's for a moment understand exactly how printing works. I'm going to print out separations out of this, meaning I'm going to get a plate that is purely cyan and I'm going to have a plate that's purely yellow. This is now going to go through a printing press and first we're going to print the cyan color and then we're going to print the yellow color on top of it.

Now if I print the cyan, the blue part first, and then I put the yellow on top of that, I overlay the yellow on top of the blue, then the ink on the paper itself is going to mix in the area where the yellow and the blue overlap each other. When you mix yellow and blue, you get green. So that means that when I actually print this out on a printing press, this is going to look like a blue flower with a green circle in the middle, not a yellow one. So what normally happens with regular artwork inside of Illustrator is that when I create separations, artwork that appears on top of other artwork knocks out the artwork that appears beneath it.

In other words, Illustrator is automatically going to remove any blue that appears beneath the yellow circle. We can actually see this happening. I'm going to go here now to my Separations Preview panel and I'm going to turn off Cyan, turn off Magenta, and turn off Black. So all I'm viewing right now is the yellow plate itself. This is what's going to print out on the yellow plate. That actually won't print in the color yellow, but I'm right now previewing this using yellow so I can get in my head a better idea of what this is going to look like. The plate itself is actually completely solid black and then that printer just simply uses yellow ink for that plate.

However, this is the actual artwork that appears printed on the yellow plate itself. Now I'm going to turn on the Cyan plate. I'm going to turn off the Yellow plate, then you can see that on the cyan plate there is no ink that's going to print on the middle part here of this flower. That's because the yellow object that appeared on top of this knocked out or removed any artwork that appeared beneath it. This allows me to on a printing press get a blue flower with a yellow center, because now there's no mixing of the yellow and the blue inks on the paper itself.

Let me turn on the CMYK plates here, the complete composite here, and let's go for a moment now. I'm going to select just this yellow object right here and in my Attributes panel I'm going to set that object to Overprint. When I do so, I'm overriding Illustrator's default behavior, the default behavior of knocking out objects that appear beneath other artwork. By choosing Overprint Fill I'm instructing Illustrator to actually keep the blue background beneath the yellow one. And because I have Overprint Preview turned on right now, I'm now seeing a simulation of what this is going to look like when printed on a printing press.

Because now the blue flower does not have that middle area knocked out. Again, to show you what that looks like on the plates itself, I'm going to turn off Cyan, Magenta, and Black. The Yellow plate looks exactly the same. Let me deselect it so it's easier to see. But if I now turn on the Cyan plate and turn off the Yellow plate, you can see that now there's no white circle. There's no knockout that's created in this plate. So it's just important to understand here the terminology. When you have regular artwork that is stacked on top of each other, those elements on the top of the stacking order by default will always knockout the objects that appear beneath it, meaning it'll remove the artwork that appears directly beneath it.

However, I can choose to override that behavior by telling objects to overprint. When I tell an object to overprint, that means that it should not remove the fill beneath it and that way I will combine these two elements on press. This is kind of a poor man's way of actually achieving more colors than you're actually willing to pay for on press. Just to give you an example, if these were two Pantone colors, just say blue and yellow here in this case, and I only wanted to pay for printing two colors on a press, I could simulate actually three colors by using these two colors, but then specifying that wherever these colors overlap, they also create the illusion of a third color, because I'm actually mixing those two colors together.

In fact, if I turn this on, it looks like that yellow circle is transparent. It's not. It's just set to overprint so that those two colors actually mix on press. But again, the important thing here really that I just kind of want to drive home is this terminology. We know something called overprinting, but we also know something called knocking out. And again knockout means the artwork that appears underneath is actually removed. Keep that in mind, because in the next movie we're going to learn exactly what the Knockout Group setting does in the Transparency panel.

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This video is part of

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  1. 7m 4s
    1. Welcome
      1m 6s
    2. The history of vector transparency
      4m 2s
    3. Getting the most out of this training
      1m 30s
    4. Using the exercise files
      26s
  2. 6m 21s
    1. Transparency living in a world of PostScript
      2m 56s
    2. Transparency...it's everywhere
      2m 13s
    3. Transparency across Adobe applications
      1m 12s
  3. 42m 20s
    1. Deconstructing the Transparency panel
      7m 48s
    2. Adding transparency to gradients
      4m 59s
    3. Using the Isolate Blending setting
      5m 20s
    4. Understanding how overprints and knockouts work
      6m 26s
    5. Using the Knockout Group setting
      6m 47s
    6. Using the Knockout Group setting without a group
      6m 2s
    7. Understanding the Opacity & Mask Define Knockout Shape setting
      4m 58s
  4. 36m 26s
    1. What is an opacity mask?
      3m 37s
    2. Learning from channels in Photoshop
      7m 20s
    3. Creating an opacity mask
      6m 44s
    4. Editing an opacity mask
      5m 31s
    5. Using a gradient as an opacity mask
      4m 44s
    6. Using image pixels as an opacity mask
      4m 4s
    7. Using a complex appearance as an opacity mask
      4m 26s
  5. 53m 30s
    1. Understanding transparency flattening
      5m 58s
    2. Learning the two rules of flattening
      8m 1s
    3. Understanding the concept of complex regions
      7m 47s
    4. Exploring the Transparency Flattener options
      11m 44s
    5. The relationship between flattening and stacking order
      8m 22s
    6. Using the Flattener Preview panel
      8m 3s
    7. Creating and sharing flattener presets
      3m 35s
  6. 24m 37s
    1. Working with PostScript (EPS) files
      7m 22s
    2. Placing Illustrator files into InDesign layouts
      3m 59s
    3. Copying graphics from Illustrator
      2m 41s
    4. Saving PDF files
      4m 41s
    5. Using the PDF/X standards
      4m 36s
    6. Printing files from Illustrator
      1m 18s
  7. 34s
    1. Next steps
      34s

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