the main concept of the Umfeld graphic render is centered around the idea of shapes as used in Processing:

beginShape(TRIANGLES);
vertex(50,0);
vertex(100, 100);
vertex(0, 100);
endShape();

to make it more consistent implementing a renderer ( i.e derivatives of PGraphics ) all primitives ( e.g rect, ellipse, point, bezier etcetera ) are implement with begin-end-shapes. PGraphics in turn separates shapes into stroke and fill shapes, where stroke shapes are arranged as line strips and fill shapes as a list of triangles:

                    + -> emit_shape_stroke_line_strip -> triangulate, transform to world/screen space
begin-end-shape ->  |
                    + -> emit_shape_fill_triangles    -> buffer, transform to world/screen space

a derived renderer ( e.g PGraphicsOpenGLv33) can then receive, transform and draw line-strips ( emit_shape_stroke_line_strip ) and triangles as required. while this approach offers a consistent and concicse entry point for implementing other renderers ( e.g SDL_gpu, SDL_renderer, Vulkan or Metal ) it is probably not the most performant approach. there is room for numerous renderer-specific improvements here ( which may be applied over time ). however, for now this seems like a reasonable approach.

it also opens up interesting approaches that involve buffering of shapes ( AAMOF this one major optimization idea ).