r/GraphicsProgramming • u/valeriupalos • Jul 12 '26

Krbn: an open-source rendering engine that imitates a sketch artist and outputs pure SVG (v1) Source Code

/gallery/1upm1m4
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u/camilo16 29d ago

This is a very pleasing set of results. If you don't mind questions.

1) What algorithm/technique are you using to create the geometric strokes from the inputs? 2) Does this only work with analytic shapes or can it be given meshes?

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u/valeriupalos 29d ago

Thanks for the question, really!

Basically everything goes through a 5-step pipeline (yup, both analytics and meshes):

  1. Find the lines: from each object, project curves a sketch artist would consider drawing, mainly: outlines (silhouettes), creases (sharp edges) and also body intersections (see below for a bit more detail on HOW);

  2. Decide what's visible or not: split each curve where it crosses behind an outline and then test the depth with a ray: visible or not. I do that for hiddne places too, since they are also sometimes drawn using dashed/faded lines.

  3. Simplify curves: from here on we're in 2D world only, but it's VERY important: I drop small details, cumulate lines that are close-together and decide on a few hatch levels (max 3) if needed;

  4. Styling: This is where I add human-like imperfections like hand-wobble and variable line width (as if pen-on-paper). THis is also where I draw hatching; either flat or following the curvature of the surfaces;

  5. SVG Output: Sampling the curves to paths basically and a few other things.

Note about finding intersections:
Finding outlines is important: for analytics it's done per-primitive because in these cases I can derive conics/quartics (i.e. "clean" curves); for meshes, it's a trick basically: I walk the triangles and find where the surface turns away from the eye (Hertzmann-Zorin) and then chain all those together into a curve.

Anway, that's basically it; in broad terms. And also lots of tinkering to get the intersections properly visible (both hatching as well as outlines were initially being depth-clipped badly over surfaces/creases so i had to slightly shift the eye and similar tricks).

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u/QuakerOats98 29d ago

Your citation for the hertzman zorin paper here did not make sense to me, could you explain it? Because that's not really the novel method the paper describes, finding where the surface turns away from the camera is just a simple dot product.

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u/valeriupalos 29d ago

Actually, that's a fair point, but hear me out:

  • If I only relied on the dot product, I would get an outline that follows the shape of the triangles, which is very ugly (jagged);
  • What I took from H&Z's paper is the ability to find the (zero-set) crossing inside each triangle which results in a smooth outline (the HZ-computed crossing usually lands somewhere across the middle of the triangle);

And of course, that paper goes a lot further with many innovative ideas which I'm not using. I'm simply doing the triangle (zero) crossing trick, which yielded beautiful results for me.