PathCollection doesn't document cap and join styles

[kennykb]

I have an issue (attachment shows a reduced demonstration) where the miters of joined line segments in a PathCollection extend unreasonably far when the angle between them grows acute (use the trackball to rotate the figure in the demo to see the behaviour).

Changing the miter_limit parameter appears to have no effect.

I suspect that the cap and join parameters are necessary to confine the extension of the lines, but I've not found any documentation on what values they take. I haven't yet had time to decipher the complex shader code to figure it out.

I'd be happy to contribute a PR to elucidate this, but first I need to understand it myself!
miters.zip

[rougier]

Sorry for late reply. Can you post a screenshot to show the problem (just drag and drop images in the issue)

[kennykb]

Screenshot and script that generated it are in the attached ZIP

[rougier]

Easier for me if you post here.

[kennykb]

Script:

from glumpy import app, gl, glm, gloo, graphics, transforms
from glumpy.graphics.collections import PathCollection
from glumpy_cardboard_box import CardboardBox
import numpy as np

# Get the scale for our box and the size of an enclosing sphere. Give an
# initial value for the camera distance

h, w, d = 27.72, 20.78, 6.95
radius = 0.5 * np.linalg.norm([h, w, d]);
distance = 6.0 * radius

# Start up Glumpy

winWidth = 1920
winHeight = 1080
window = app.Window(width=winWidth, height=winHeight,
                    color=(0.0, 0.0, 0.25, 1.00))
@window.event
def on_init():
    gl.glEnable(gl.GL_BLEND)
    gl.glEnable(gl.GL_DEPTH_TEST)
    gl.glPolygonOffset(1, 1)
    gl.glEnable(gl.GL_LINE_SMOOTH)
    gl.glLineWidth(0.75)

@window.event
def on_draw(dt):
    window.clear()
    paths.draw()

vertices = np.array([[-1, -1, -1],
                     [-1, -1, 1],
                     [-1, 1, 1],
                     [-1, 1, -1],
                     [1, 1, -1],
                     [1, 1, 1],
                     [1, -1, 1],
                     [1, -1, -1],
                     [0, -1, -1]], dtype=np.float32)
scale = np.diag([w, -d, h])/2
vertices = vertices @ scale

transform = transforms.Trackball(transforms.Position())
viewport = transforms.Viewport()
paths = PathCollection("agg", transform=transform, viewport=viewport,
                       color='shared', linewidth='shared',
                       miter_limit='shared')

paths.append(vertices, color=[1, 1, 0, 1], linewidth=3, miter_limit=0.1)

window.attach(transform)
window.attach(viewport)

transform["distance"] = distance
transform["theta"] = 60.0
transform["phi"] = -30.0

app.run()

Using the trackball can result in images like:

[rougier]

Miter limit is used here and might be wrong. Is the path ok with round joins ? To ease debugging, best would be to have a two segment paths in ortho mode that show the problem.