If you work in Photoshop or something similar you know what an advantage it is to keep different elements of your art on different layers. This allows you to add, subtract, resize, or move elements in your scene around, or remove the background altogether.
Since I've been doing more digital art I'm also doing more art where the subject can separated from the background scene and saved as a PNG file with transparency. This is useful for putting the image on products, particularly things like t-shirts and coffee mugs where you don't want a square photo-like picture on the product.
One of the things to keep in mind when creating images for shirts, and some other products, is there is no such thing as white ink. So when the printing company puts an image that contains white or light colors onto a dark shirt they have to put a white undercoating down first, then spray the darker colors over it where the image is not going to be white. They put the white coating down based on the pixels in your image so if your image doesn't have clean edges you may get a white outline or stray white spots outside the main image. So it's important to find any stray pixels or jagged edges and clean them up.
These stray pixels can occur without you realizing it, if you accidentally bump your brush against your drawing tablet when using a light color or lower opacity. Or perhaps you erase part of the image. It might look like you got it all but even 1 tiny mostly transparent pixel will cause a white undercoat to be laid down there on your shirt or other product. Finding that tiny pixel can be hard just by eye-balling the image. So here's a quick little trick to bring those little guys out of hiding.
Here's my original image, one of a silly chicken in a Santa hat I created for Christmas stockings and shirts. It looks pretty clean outside the edges, right?
Well, looks are deceiving. I know it's not that clean in reality because I created this from another image, moving the chicken's head a bit and erasing the old part of the head and neck. I'm sure there are pixels that I missed that need cleaning up. But I can't see them. So what we do to bring them out is right-click on the layer in the Layers panel and select Blending Options. In the panel that pops up go down to the bottom and select Stroke. Play with the size until you can see spots appearing outside the edges of your image, but don't make it so big it covers small pixels near the edge. Pick a color that's different from your image edges so you can see it better. Here's what I get with mine.
As suspected, there are stray pixels around the hen's neck where I didn't quite erase it all when adjusting the bird's head position. I need to clean those up so they don't produce spots when printed on dark fabric. To do that I just use the eraser tool and dab those spots. The nice thing about the Stroke blending option is that, as you erase the pixels, the Stroke outline will disappear too, letting you know you got it all. (Don't use the Stroke under the Edit menu for this...you won't the same results.) Here's the chicken with edges cleaned up.
When you get your image all cleaned up right-click on the layer in your Layers panel and select Clear Layer Style. Now it's back to how it appeared originally but you know you've cleaned up any stray pixels so you'll have a nice clean print on your shirts and products.
And here's my chicken on a "Merry Chickmas from your local chicken farmer" shirt.
