This is how it looks without the shaders. Obviously the lighting was tuned for how the shader works, so the character is too dark without it, but it should give you an idea.
The cel shading or ramp shading is essentially free. It's just a modification to the standard material. You can easily convert the standard material to a shader material with one button in Godot, then edit it as you like. In this case I just altered the light() function to sample from a ramp texture (sort of like a palette map) rather than doing the normal dot product of the light angle. This essentially give you steps, so you get large areas of a single color rather than a smooth gradient. So this part is free.
The second part is the outline. I am using a 3x3 sobel filter. Not the one from the Godot docs, cause that one looks too dated, but I found some other open source code that fit my needs better. This has a thinner line and also better anti-aliasing. The result of the sobel is a black image with a white outline. Then I change the color to a brownish red, and multiply that by the screen buffer. This allow the color to better match the environment. Additionally, on the full screen ColorRect (that the shader is attached to) I set the alpha to about 60% so it is not as noticeable. This gets a subtle effect but it actually does quite a lot without being in your face. This is not free, but also not that expensive either. It takes 9 texture samples and some simple math. My other shaders (like Godot Super Scaling) were also doing 9 samples (sometimes more) and the cost is low. About 1 ms on a good computer. For this, it should be even faster as I'm not doing any heavy math, mostly addition and subtraction, which are cheap. So my guess it would be even less than 1 ms.
As far as I can tell, I'm happy with the look of the character, so I would consider that done. The background was setup really quickly this morning just for the render, so it needs a lot of work. I want it to look more like a painting, so I will explore NPR (non-photorealistic rendering) to make it look more like visual novel or anime backdrops. This shouldn't be too hard, but I haven't wrote one before, so it might take be a couple days. I also have to remodel parts of the background because the walls are too thin and the shadows don't work. But this is fairly easy.
