This semester, I’m taking a digital printmaking class, so I may have a couple of tutorials on here that are specifically related to topics surrounding screenprinting or preparing digital images for print. Here’s the first one…
When you are preparing an image for screenprinting, you have to separate out the various colors in order to make a stencil that you will ultimately use to print each color individually. There are lots of ways to separate out colors, but today I’m going to explain how to do it using the Threshold Layer Adjustment in Photoshop. This method is a bit different than separating by actual colors – instead, we’ll be using the luminosity values to determine the separations.
For this example, I’m going to start with a photographic image of a timepiece, and I’m going to prepare it to be printed in 4 colors.
First, open the image in Photoshop.
Looking at this image, I’ve decided ahead of time that I want to break it down into four colors – a light yellow, a light grey, a dark gold, and a dark grey (in order from lightest to darkest – which is the order that we’ll print it as well). Because the lightest color will actually just be filled in anywhere that we don’t have any of the other three colors, I’ll start by just making a new layer in Photoshop and filling it with a light yellow. Grab the light yellow color first by using the eyedropper tool on a spot with light yellow. Then make a new layer, go to Edit > Fill and use the foreground color, which should be your yellow. Name that layer light yellow and then hide it by clicking on the little eye next to the layer name.
For the next color, our light grey, we’re going to make a Threshold layer. Select the background layer and then go to Layer > New Adjustment Layer > Threshold.
Essentially, what threshold does is it takes a value of luminosity (brightness) and says “anything less than this value is going to be black, and anything greater than this value is going to be white.” Because the light grey layer will be below two other layers, we want the black area of this layer (which will eventually be the grey part) to be most of the image. The other two layers above will have progressively smaller areas of black (corresponding to their ultimate colors). Anywhere that is white will ultimately be transparent, showing the layer below (in this case, the light yellow layer). For this image, I’ll move my slider over to the right to around 180 or so. This is the part that is going to take some tweaking to find the right balance.
Right now this threshold layer exists as an adjustment on top of the layer below. What we need to do in order to print this properly for screenprinting is make this a pixel layer. First, duplicate the background layer (so we still have our original below). Then select the background layer as well as the threshold adjustment and then go to Layer > Merge Layers (or cmd-E on a Mac).
Next we want to get rid of the white part, so, using the magic wand tool, with contiguous unchecked in the top bar, click on the white area and push delete. If we want to now color this layer, go ahead and hide it for a moment by clicking on the eye next to the layer name, then using the eyedropper, grab a light grey from the background layer below. Then, using the magic wand (making sure contiguous is unchecked), select the black area and then go to Edit > Fill, and choose “foreground color.” You can now turn on the light yellow layer below to see how the composite looks so far. Make sure your new layer (which at this point should be named “light grey”) is above the light yellow layer.
Next you need to repeat all of the steps above (starting at “For the next color…”) to make another threshold layer. For this layer, choose a lower luminosity value and then color with the dark gold:
Finally, one more trip to the steps above to do the final dark grey layer and we’ll have a nice design separated out into four colors.
To screenprint, there are a few different paths to go down at this point (and I’m not going to get into too much detail for this part)—but essentially you’ll have to merge all the layers together so you have a flat, four-color image, then use the magic want to select each color from the flat image, duplicate that layer up to a new layer and convert to all black. Repeat for each color and then print each of those four print layers to be used to make your screen.