Implement p_width in canvas_item_add_polyline

[lyuma]

Co-authored-by: K. S. Ernest (iFire) Lee ernest.lee@chibifire.com

Antialiasing is not implemented. We found two techniques which could be implemented.

• The first technique described at https://github.com/tyt2y3/vaserenderer and http://artgrammer.blogspot.com/2011/05/drawing-nearly-perfect-2d-line-segments.html uses a polygon mesh for the edge pixels with a vertex color gradient to alpha.
• The second technique described at https://blog.mapbox.com/drawing-antialiased-lines-with-opengl-8766f34192dc does antialiasing in the fragment shader, but requires a normal vector to be passed to the vertex shader.

Line width is used in pull request #40513 - Below is a screenshot showing the technique in use.

Bugsquad edit: Fixes #40845.

[fire]

Someone is questioning the supported status.

It is in vulkan but won't be in webgpu.

https://www.reddit.com/r/rust_gamedev/comments/evlwzo/wgpurs_how_to_specify_line_width_when_drawing/

[lawnjelly]

I haven't looked at the renderer in 4.x yet, but this looks ok for something basic. The existing GLES2 and GLES3 renderers for TYPE_LINE do something like this (they use GL_LINES for an anti-aliased line around the edge).

What kind of results does this produce? Especially on sharp corners. It would be nice to see some pics with thicker lines. 😄 It might be easy to add a bevel but not sure if it is required.

The lack of anti-aliasing could also be a thing, which might be easier using a shader approach (as detailed in the linked article https://mattdesl.svbtle.com/drawing-lines-is-hard).

[erictuvesson]

fixes #40845

Edit: wanted to add a reference between the issue and PR, but doesn't seem to work when I am not the author.

[reduz]

This is kind of an issue I did not get into yet because Vulkan does not support antialiased lines, and I am not sure which approach is the best we can take to work around this problem. Geometry shaders would work but they are not supported on mobile.

[Chaosus]

Superseded by #43828, thanks for attempting to fix it anyway!