We left off the process of preparing a transparent PNG file for DTG printing on a t-shirt in the previous post, having discussed why this is a useful approach, defined what kind of image works best for this process, and upscaling said theoretical image to a size that has sufficient resolution for printing at t-shirt size. In this post, we will open a potential PNG image in an image editor and remove parts of the image to take advantage of PNG transparency. After that, we will groom that PNG to make it print-ready.
Incidentally, you can find many t-shirts that embody this approach in both of my online stores - the Lazy River and Big Shadows. Check them out for tees, Hawaiian shirts, prints, cards and more.
Opening the File in an Image Editor
In order to work on the image, we need to open it in an image editor. Most people use Photoshop, and I have myself for quite a few years, but these days I'm cheap and I use GIMP, which is both open source and free. When I started using GIMP, about five years ago, Photoshop didn't have a lot of the features that it does now, namely all the AI-enabled stuff. Even so, at that time it did a few things that GIMP doesn't, such as curving text easily and allowing selection perimeters to be rounded; I missed those abilities when using GIMP, and still do, but at least I don't have another bloody monthly subscription. And GIMP really is a good, stable, ad-free and viable option for bread-and-butter editing like this as well as much more complicated projects. I highly recommend it.
I've chosen the following image, which has been upscaled using Topaz Gigapixel to 9,600 x 12,500, to work on for the purposes of demonstration:
This image ticks all the boxes for PNG printing, in this case, on a very dark tee. It has many areas made of solid background, so lots of unprinted shirt for breathability and flexibility. The darkest colours in it, apart from the background, are quite light and contrast nicely with the background. Finally, it's edges are pretty crisp and well-defined. That last one might seem questionable when looking at the image from a distance, given that much of the design's perimeter appears quite soft and fuzzy. If you look closely at the image's edges, though, you'll notice that the transitions from colour to black background are very granular and speckley/spattery rather than fading with a gradient. Later in this post, we will discuss the problems associated with images that fade smoothly into the background colour.
Select the Background Colour
Before you do anything else, make a duplicate layer of the image, leaving that bottom-most layer as a pristine and unaltered safety layer, and the top layer as your working layer. This is always a good idea when editing any image.
In order to remove/erase the background colour from our image, we first have to select it. In GIMP, you select a colour range using the Select by Color tool. I believe the PS version of this involves using the Color Range tool to create a mask.
When using the Select By Color tool, you have some options available. I like to keep Antialiasing switched on and Feathered Edges off or at least set to the bare minimum. The most important setting, though, is the Threshold level, which can range from 0 (no colours selected) to 255 (all colours selected). I've found that the best approach is to start small with a very low threshold and then apply larger levels until too big a range has been selected. Once you know that tipping point, back it off a bit. I'm going to set the threshold at 11 and make a selection by clicking on the darkest colour in the image, being the background colour:
In the above screen grab, you can see the perimeter of the selection on the image, represented by dashed and diagonal lines. In the actual program, those lines are slightly animated. In the upper right corner, a preview window indicates selected vs unselected areas in black and white, with white indicating selected areas. At this point, we want to Invert the selection, which reverses the selection and selects everything except the background colour range:
Now Copy, and then Paste as New Layer in Place. Also, Select None, as all those marching diagonal lines are distracting. You now have an image with three layers - the safety layer on the bottom, the working layer in the middle, and the pasted non-black layer on top. If you make the bottom two layers invisible, the result is this view:
The checkerboard is used to indicate that those areas are clear, invisible, and won't receive any ink when the file is printed. If you save the file as a PNG at this point, with the checkerboard showing and any background layers set to invisible, you could import that PNG file into the design interface of a printing platform such as Printful and call it done. Doing so would be premature, however, as such first-pass edits like this almost always need cleaning up, tweaking, and modifying before they are print-ready. We will get into that very shortly, but first we need to cover the DTG (Direct to Garment) printing process that Printful uses when printing on textiles.
How DTG Printing Works and the Quirk of Semi-Transparency
From the Brother website, "Direct to garment printing works differently from traditional screen printing, in that the technology gives you the ability to quickly print highly detailed images with multiple colors. A DTG printer’s print heads jet translucent CMYK ink and opaque white ink onto a garment’s surface. Applying heat to the ink allows it to dry and adhere to the fabric, keeping it in place." It's the use of an opaque white ink as the base of the print, upon which translucent colours are applied, which creates problems if parts of your design are not fully opaque nor fully transparent, but are semi-transparent. The printer needs to lay down that opaque white ink base wherever there is colour in the design, and that applies whether that colour is 100% opaque or only 50% opaque (or 20% or 75% or whatever). Semi-transparent colours printed on that white base look much too light, simply because the design doesn't have enough colour in that area and so not enough coloured ink is applied. Printful has a page dedicated to this which is worth having a look at.
I'm going to include an example to simulate how this looks. Here is the image I'm going to work with:
This image is a fairly strong candidate for the PNG treatment, having a lot of black and off-black that can be erased and some very strong mid-to-light tones with good contrasts against the background. Look closely at those black areas, though, and notice that most of them have some dark colours adjacent to them that fade into the black in a subtle way; it's those transitions we will deal with now. First, I'll erase the black background colour in the manner outlined above. I used a Select by Color tool threshold of 26, inverted the selection, copied and pasted, which produced this colour/non-black layer:
The image above on the left is the colour/non-black layer on an empty/transparent background. The middle image is that same layer on top of a solid black background. The last image is the original again, provided here so that it can be compared to the middle one - they look quite similar, nearly identical. Unfortunately, there are some semi-transparent areas that are going to create problems if they are not dealt with. We can identify these semi-transparent areas by placing the colour/non-black layer on a solid white background:
The close-ups above reveal some light grey areas of varying intensity - those are places where the layer is semi-transparent.
In the examples above, the first image is again that colour/non-black layer, and the middle image simulates the base layer of opaque white ink that lies underneath it when the design is printed using DTG. The final image has the colour/non-black layer on top, the opaque white layer underneath that, with both stacked layers on top of a solid black background. As you can see, it's a disaster. This is probably a more extreme result than would occur when dealing with the actual physical mediums of shirt, printer and ink, but it's in the ballpark. When confronted with an area in the design that isn't 100% opaque, the computer is faced with missing information. And yet, the area must be filled with ink, and so the computer driving the printer makes up a solution that is, inevitably, lighter than you intended and looks weird. I haven't encountered a name for this effect in my research, so let's call it "White Fringing." Colour Fringing is a term used in printing to describe colour bleed and aberration, so it seems appropriate.
How to Fix White Fringing
There are at least three ways to deal with this problem: Erasing, Painting, and Stacking.
Erasing: As the name suggests, you can simply erase those semi-transparent areas. You can use the Eraser tool or create a selection of those areas using the Free Select tool and then delete its contents. When using the Eraser tool, make sure it is at full strength and with a hard edge. When using the Free Select tool, make sure it has no feather, or at most 2.
Painting: But what if those semi-transparent areas portray some detail that you don't want to lose? In that case, you can add colour to those semi-transparent areas by using the Paintbrush tool or, again, create a selection of those areas using the Free Select tool and then fill its contents with the Bucket Fill tool. As with Erasing, make sure the edges are hard and not feathered and the application of colour is at 100% opacity.
Stacking: The approach I like the most is Stacking, in which you create duplicate layers of the colour/non-black layer and make a stack of them on top of a solid white background. Each additional layer adds opacity to the semi-transparent areas, gradually strengthening them and filling them in. The following cropped screengrab shows a stack of 5 duplicated layers on a solid white background:
As you can see, the semi-transparent areas are already much more solid. It can take quite a few duplications to achieve full opacity, though, sometimes as many as 20 or more. What I tend to do is make a stack of four and then merge those duplicated layers together into one merged layer. Then I duplicate that layer for an effective 8 layer stack (2 layers made of 4 layers each). Repeat as needed, noting the changes in the image as you add duplicate layers or chunks of layers. You will know you are finished when adding one more duplicate layer results in no change in the image. Below are examples of stacking, with the stacks on a white background (left) and on a black background (middle), and the original image last (right) to serve as a basis for comparison:
It took about 30 stacked layers to achieve opacity in this case.
Another Problem with Edges, the Reflectivity of Materials Problem
There is one more problem concerning dark edges of a print (when printing on a black or very dark shirt) that needs discussion, and again this is a problem that doesn't have a name. Let's call it the "Reflectivity of Materials Problem." Raw and unprinted shirt material is not very reflective of light, while printed ink on top of a shirt is slightly reflective and thus a little lighter in tone. We can think of this as differences in sheen, with raw shirt being flat and printed areas being satin. That creates a problem of gradation when print colours right next to raw black shirt are off-black or very dark. On your computer monitor, the jump from full black to off-black or very dark charcoal (e.g.) is very small. On a shirt, that same jump from black shirt to off-black is disproportionately large. This problem also applies to white shirts and off-white printed ink, but less so than for black shirts.
When thinking about this problem, it's useful to think of a 100% black design printed on a black shirt. The print doesn't just disappear on the shirt, and the inked parts will look very dark grey in comparison to the raw shirt. That has big ramifications if your design's edges are trying to blend into the background of the black shirt. I'm going to try and illustrate this, but it's tricky to do on-screen because it's a materials problem. Probably the easiest and clearest way to demonstrate this is with a simple greyscale gradient with discrete steps down to black, like this one:
Let's say that your design's perimeter uses a similar gradient, a darkening down to black, either in discrete steps like this or more smoothly. Using a Select by Color tool threshold of 5 and clicking on the full-black border results in a selection that looks like this:
As you can see, the black border and last two steps of the gradient were selected. Now let's Invert that, Copy, and Paste New Layer in Place. We will also place that layer over a solid black background:
At this point you might be thinking that the transition from the darkest tone in the pasted layer to the black background looks pretty effective, relatively smooth and not jarring. If you squint your eyes a bit to simulate distance (an old scenic painting trick), that transition looks even better. Unfortunately, that transition does not translate faithfully to actual ink on an actual black shirt. Because of the Reflectivity of Materials Problem, the printed area will be a bit lighter in the context of the black shirt material. It will look more like this:
Now the transition from the darkest tone to the black background is bigger, much less subtle, and jarring. Printful has a page dedicated to this problem, "Why does my design look different from what I intended?" It's a big enough problem that Printful has gone so far as to include a print preview button on their design interface that emulates this issue, I would imagine in response to incessant complaints about it. I think they have gone overboard with the extent of the problem as portrayed in their print preview, making it look worse than it actually is. Here is that same gradient layer from above put on a black t-shirt, with the preview turned off (left) and on (right):
The easiest way to evade this problem when creating PNGs for printing on a black or very dark shirt is to simply avoid using off-black and very dark colours at your design's perimeter; transitions from printed area to raw shirt material should consist of medium-dark to very light colours and tones. That's not to say there can be no black tones or very dark colours in the design at all; they're fine as long as they are surrounded by printed ink. One of the great things about DTG printing is that, because it's basically an inkjet printer, it can capture vivid colours and tremendous subtlety of gradation, up to and including photographic quality. Just make sure that you manage that subtlety where raw shirt and printed areas transition.
Stay Tuned for Part Three!
This installment has turned out to be quite long, so I'm going to wrap this up for now. Part three, coming soon, will finish up this subject by covering Proofing and Cleaning, Text, Using the Design Interface, and some other tidbits.