The Stained Glass Tutorial
Ok 'tutorial' might be a strong word, but I've been doing stained glass style digital art for a few years now and I think I've finally gotten confident enough about what I'm doing to write up some tips if anyone wants to try their own hand at it. I've gotten a few asks about this that have sat unanswered because it would've been really long, so consider this the definitive answer to all of those.
A good amount of this info is stuff I've only started really paying attention to recently and isn't reflected in my past art as much, but hey. I'm learning too!
Here's a picture I took of a stained glass window (I believe it was originally from St. Wenceslas Cathedral but I am not 100% sure) side-by-side with my mockup of an unpainted version, with just the shapes of the glass panels. (I'll get to colors later.)
Note how many details are painted on rather than simply defined by the lead strips. Both hands are one piece. The hair and face are painted onto the same piece.
One of the biggest things I see people get wrong about replicating a stained glass style is in the shapes of individual glass pieces. The little bit of physical glass cutting I've done has given me a much better feel for what you can and can't make glass do.
Mistake #1: Using only straight lines or very geometric figures (like all triangles or something). I won't post examples of other people's art as 'bad' but I see this one a lot. There's nothing inherently wrong with this, there's definitely some stained glass windows with this look, but it sometimes feels less a stylistic choice as much as an artificial limitation people place on themselves.
So, for the record: glass can make curved shapes, you aren't limited to straight lines. However, there's a limit to how thin or concave these shapes can get. The picture probably shows it better than words, but if you're getting a very thin strip of glass between two larger chunks or a very thin, very curved piece, that's going to be a piece you want to break apart into two or more.
Mistake #2: Using glass pieces that are too small.
If you look at real stained glass windows, especially from the Gothic period onward, the individual glass pieces tend to be pretty large. This is because a lot of stained glass windows are painted, rather than simply using the thick lines of lead to differentiate every detail. The lead strips are used to separate colors, and only very sparingly to define details within a single color. If the pieces get too small it ends up resembling a mosaic rather than stained glass.
This is a counterexample of a window that uses a lot of tiny panels, but you have to be pretty intentional with this to keep it from looking very chaotic imo.
First of all: I just use pictures of stained glass sheets to get a really nice digital stained glass effect. It's that easy. I have a collection of like, over 1000 pictures of different stained glass colors/textures. It's pretty easy to find them but I could be convinced to make a public Google Drive folder with them if people are interested. This is stupidly time-consuming though, especially if you're also wanting to paint on details, and not necessary to get the 'look' of stained glass if you don't want to put up with this.
Medieval stained glass pages were pretty much exclusively uniformly colored, with a limited color palette. You see a lot of white, reds/red-browns, bright greens, cobalt blues, and peachy/pink as actual glass colors, occasionally rich purple and yellow/orange. Additional color variation was added with black or brown paint, and bright yellows were created using silver stain. These windows are pretty good examples of the kind of colors you see most often, so if you want to be super authentic to the Medieval stained glass look, this is gonna be the colors you're using.
(Also: in the first image, take note of what edges are defined by lead strips, vs which ones are painted!)
More modern Tiffany glass, on the other hand- or stained glass in the same style- uses a much wider variety of colors and textures. This sort of style, from my understanding, wasn't developed until around 1880-1900; this is when you start to see opalescent (multi-colored streaks), favrile (iridescent), and various types of mottled/ripple/textured glass, as well as a much wider range of colors. This is used in conjunction with painting for ecclesiastical scenes, but a lot of natural scenery uses simply the texture of the glass itself, as in the last photo.
I kind of completely neglected this when I started out and it's really helped me make sense of how to get that medieval stained glass look. There's only so much you can do with glass shapes, especially if you're trying to make human faces or text. Almost all of the details in these pieces are painted.
Apart from just shading/rendering, you can use painting to get more complicated shapes/silhouettes than just the glass would allow, especially for small spaces. Try to keep the actual shapes of the glass simple.
You can see this better in this image, in the trees and the castles, where the actual solder lines are kind of blob-like and most of the shape definition comes from paint. The sky and trees here also show a scratch-away technique of painting the entire surface of the glass black (or dark brown) and scratching out the desired colored patterns. This technique was used a lot for backgrounds, foliage, and decorative elements.
(^ My own example of this.)
An important thing to remember: Glass is transparent. The point is to let light through. Adding any paint over top of this- even a lighter color- is going to darken the appearance of the glass when you shine light through it. Your glass must be the lightest color you want visible, with any additional details painted over it preferably with Multiply (with mostly black or brown if you want real historical accuracy). You cannot use paint to make glass look brighter/lighter. If you wanted a face to have white in the eyes, the entire piece of glass it was painted on had to be white. This doesn't really matter that much if you're doing it digitally but I find keeping this in mind helps it look a bit more authentic.
Slapping this here again: in my traced version on the right, a lot of the colors look much lighter than the actual window. This is because I color-picked the lightest parts of those areas; anything darker would've had to be painted on.
^ Another of my own examples. Most of the details are 'painted' in.
Perhaps most importantly, if you are really dedicated to making authentic-looking stained glass art:
I have dozens upon dozens of references saved that I look at all the time for inspiration in decorative motifs, composition, etc. There's a pretty vast amount of stylistic difference in stained glass over the past thousand years and these people knew what they were doing way better than I do.