Although most colour separations are built in a vector program, such as Adobe Illustrator or Corel, I regularly am tasked with colour separation because the image is complex or there is a placed or imported JPG image along with the vector artwork. In this article I describe anti-aliasing, which Photoshop uses to soften the edges of images when it converts them from vectors to rasters.
Screen printing | by Scott Fresener | Stitch & Print 1-2018
Anti-aliasing is the enemy
The anti-aliasing problem is one that a normal vector artist would not identify, but that is an issue for colour separators and a “disease” in the graphics world. Photoshop uses the dreaded anti-aliasing function to soften the edges of images when it converts them from vectors to rasters.
Why anti-aliasing? Honestly, I think it is a leftover from the era of slower computers and lower resolution graphics. It is Photoshop’s way of helping to soften jagged edges and smooth them, which was a huge deal years ago with lower resolution graphics. Now, with fast computers and graphics with minimum 300 dpi, it is almost a non-issue to my mind.
When you build or create an image in a vector program, the entire image is made up of the “math” between point A and point B. There is no image resolution at this point, unless you place or import a raster image (you know…, the one you borrowed from Google Image), which however is not the subject of this article.
When you save a vector image as a PDF or an EPS in Corel or Adobe Illustrator, the vector data is still math. But when you open the image in Adobe Photoshop, the math is converted to pixels and one of the questions you get is whether you want anti-aliasing turned on.
The default setting in the window in Figure 1 is 72 dpi and anti-aliasing is checked. You have to change the resolution to 300 dpi and uncheck anti-aliasing. Figure 2 shows a close-up of the image in Photoshop with anti-aliasing checked. Figure 3 shows the same image with antialiasing off and at 300 dpi.
Why turn it off?
OK, so why the fuss? Wouldn’t the image be better with smooth rather than jagged edges from converting it into pixels? It all sounds good on paper, and that’s the point: If you are printing to paper, DTG, decals or wide format posters, or using sublimation, it will slightly soften and blend the edges, especially on large format posters. It is only when you are trying to colour separate an image that it makes any difference, and this is where the softness really gets in the way of a clean set of separations.
Notice in Figure 2 how a variety of lighter colours are now visible around the edges. When you colour separate this in Photoshop, these “colours” end up on separations as glows or halos around things. They need to be erased and that takes time. Figure 3 shows the same image without anti-aliasing. Notice how clean and sharp it is. It is indeed a little jagged, but we are zoomed in very close and if you are a “vector snob” you need to get over it! These jagged edges will never show up on a shirt at 300 dpi.
Grey is the real problem colour to separate in an image, because if anti-aliasing is on, a lot of the smaller “softening” pixels are grey. I spend more time cleaning up grey separations than any others.
And when it comes to colour separations, it’s all about time – time your customer never pays for. In Photoshop, typical separations are done using the Colour Range tool to “pull” a colour and turn it into a “channel”. This is called a “channel” separation and it is how most high-end separations are achieved. The image on the left in Figure 4 shows a channel separation of the grey from the file
with anti-aliasing turned on. The right-hand image is a grey channel separation with anti-aliasing turned off. There is a HUGE difference. If I were doing this job, I would have to erase all of the glows and light edges from the image on the left.
Just two simple mouse clicks – turn off anti-aliasing and change 72 dpi to 300 dpi (a topic for another article), make life easier on the colour separator. I often get Photoshop files to separate where the customer/artist has tried to do me a favour by importing their file into Photoshop from AI or Corel. In 95% of these cases, they have left anti-aliasing on and the resolution at 72 dpi. I always have to go back and ask if a vector file of the job is available, because I want to control how it is imported into Photoshop.