A little bit off topic but: GL_LINES doesn't have a performant analog on lots of other platforms, even Unity. Drawing a line properly requires turning the two endpoint vertices into a quad and optionally adding endcaps which are at least triangular but can be polygons. From my understanding, that requires a geometry shader since we're adding virtual/implicit vertices. Does anyone know if mesh shaders could accomplish the same thing?
Also I wish that GL_LINES was open-sourced for other platforms. Maybe it is in the OpenGL spec and I just haven't looked. I've attempted some other techniques like having the fragment shader draw a border around each triangle, but they all have their drawbacks.
To draw lines instead of a geometry shader you can use instancing, since you know how many vertices you need to represent a line segment's bounding box. Have one vertex buffer that just contains N vertices (the actual attribute data doesn't matter, but you can shove UVs or index values in there) and bind it alongside a buffer containing your actual line information (start, end, color, etc). The driver+GPU will replicate the 'line vertex buffer' vertices for every instance in the 'line instance buffer' that you bound.
This works for most other regular shapes too, like a relatively tight bounding box for circles if you're drawing a bunch of them.
In my experience, drawing quads with GL_POINTS in OpenGL was way faster than drawing quads with instancing in DirectX. That was noticable with the DirectX vs. OpenGL backends for WebGL, where switching between the two resulted in widely different performance.
drawing using GL_LINES is old school fixed function pipeline and it's how modern graphics hardware works. If you want a single line, draw a small rectangle between V1 and V2 using geometry. The thickness is the distance between P1 and P2 / P3 and P4 of the rectangle. A line has no thickness as it's 1 dimensional.
Draw in screen space based on projected points in world space.
set gl_Color to your desired color vec and bam, line.
I'm not sure exactly what you mean, but you can both output line primitives directly from the mesh shader or output mitered/capped extruded lines via triangles.
As far as other platforms, there's VK_EXT_line_rasterization which is a port of opengl line drawing functionality to vulkan.
hundredrabbits' game Verreciel uses a reimplementation of webgl-lines, to pretty good effect, if I may say so:
https://github.com/mattdesl/webgl-lines
https://hundredrabbits.itch.io/verreciel
PS— I still play Retro, and dream of resuscitating it :)