upgrade to a geometry or compute shader for line drawing, using only a single line points array for input.

[trusktr]

To fix #5, we need to take advantage of WebGL 2 geometry shading (generate the vertices in the shader) or WebGPU compute shading (similar, but different), so that the only thing we need to pass in is the user line points.

Ultimately the user will provide only a single array of line points, then shading handles the rest, which includes generating points or pixels around the user points to draw the region of the line, including joint miters, rounded corners, end caps, etc.

This will be better for both drawing fidelity and performance because,

• only a single array of user line points needs to be uploaded, and all line drawing options are provided as uniforms
• geometry is no longer computed on the CPU
• more possibilities in line drawing options are easily possible without much performance loss, whereas adding things like better miters, line caps, etc, will more drastically decrease performance if calculated on the CPU

closes #5

[LeXXik]

I don't want to be that guy, but there are no geometry shaders in WebGL.

[trusktr]

@LeXXik I meant "generating vertices/geometry with the vertex shader" rather than the traditional approach where all points are passed in as attributes up front.

Example:

https://webgl2fundamentals.org/webgl/lessons/webgl-drawing-without-data.html

I wasn't aware I was referring to a native non-ES OpenGL feature! :)

For thick line drawing, we'd send in only the points along the line (the user's line path), and then we send in a number indicating the number of points we will "generate" (the number of times the vertex shader should be invoked where we calculate the line points including end caps, etc, all in the shader).

Thus, we'd be "generating geometry with the vertex shader", thus I was calling it "geometry shader".

But I'm not sure if there will be a performance gain if we calculate the line points every frame. I suppose it could be a win for animated lines, otherwise if a line is not animated we might want to calculate on the CPU once up front.

[LeXXik]

Ah, I see where I got confused. Thank you for the explainer!

For a static line (where points don't move/change), the CPU side will be cheaper, I suppose, as the calculations would be done only once and stored in the vertex buffer with a simple shader. As opposed to being calculated on GPU every frame.

[trusktr]

A use case I'm imagining is for realtime plotting systems that have plot lines with lots of points, and having to upload data to the GPU each frame. In that case I wonder if uploading the only the path each frame would be faster than calculating/uploading all line vertices each frame.