Improving shading of procedurally generated pixel art sprites

Question

I've been working on a sprite generator and I'm having a hard time figuring out how to add shading automatically.

Here are some examples of the generated sprites

Currently what I'm doing is checking where a flat color meets a dark boundary, and then applying either a highlight or a shadow depending on the direction.

This is of course really boring. I've tried searching for other methods, but all I could find were AI examples. Does anyone know of any particularity simple yet clever ways of adding shading automatically?

I've thought of expanding the check up to 5 cells, and using a set of pre-determined patterns to generate the shading, but I'm just not too sure

Answer 1

Shading is about indicating the 3D shape of the object, so to shade an object well you have to generate its 3D shape. On your first pass, instead of making a flat colored shape, you could make a heightmap, then in your shading pass you can color and shade the heightmap based on adjacent pixels.

Answer 2

For those of you following or facing a similar problem, I believe I found a relatively easy solution. Considering that I'm travelling around 360 degrees to make a shape, I can use the angle to calculate the particular shading at that point (based on the direction). The same logic is commonly used to apply normal maps

While they don't look perfect, the code can probably be tweaked to produce better results.

Here is an example of the shaded sprites:

for (every vertex){

    var angle = angle(current, previous);

    var scalar = angle/TWO_PI

    fill(0, 0, scalar*100); //HSB

    vertex(current);

}