better draw with clipper

[chibai]

https://github.com/mozman/ezdxf/files/6605558/test_hatch.zip

The issue dxf file is test_hatch.zip from issue 454.
The mainly problem is that the holes is not fully inside external paths.
I found that clipper library can fix this problem.
Below is the very draft code

from typing import List, Tuple
import pyclipper
import sys
import matplotlib.pyplot as plt
import ezdxf
from ezdxf import bbox
from ezdxf.math.bbox import BoundingBox
from ezdxf.lldxf import const
from ezdxf.entities import Hatch
from ezdxf.path import (
    Path,
    from_hatch_boundary_path,
    fast_bbox_detection,
    winding_deconstruction,
    )

from ezdxf import recover
from ezdxf.addons.drawing import RenderContext, Frontend
from ezdxf.addons.drawing.matplotlib import MatplotlibBackend
import matplotlib.pyplot as plt
from matplotlib.axes import Axes
from matplotlib.lines import Line2D
from matplotlib.patches import Arc, PathPatch
from matplotlib.collections import PatchCollection
from matplotlib.path import Path

# Safe loading procedure (requires ezdxf v0.14):
try:
    doc, auditor = recover.readfile('test_hatch.dxf')
except IOError:
    print(f'Not a DXF file or a generic I/O error.')
    sys.exit(1)
except ezdxf.DXFStructureError:
    print(f'Invalid or corrupted DXF file.')
    sys.exit(2)

# The auditor.errors attribute stores severe errors,
# which may raise exceptions when rendering.
if not auditor.has_errors:
    fig = plt.figure()
    ax = fig.add_axes([0, 0, 1, 1])
    ctx = RenderContext(doc)
    out = MatplotlibBackend(ax)
    Frontend(ctx, out).draw_layout(doc.modelspace(), finalize=True)
    fig.savefig('test_hatch.png', dpi=300)

#############clipper util################
def polygon_union(polygons:List[List[Tuple[float, float]]]) -> List[List[Tuple[float, float]]]:
    pc = pyclipper.Pyclipper()
    pc.AddPaths(polygons, pyclipper.PT_SUBJECT, True)
    union_polygons = pc.Execute(pyclipper.CT_UNION, pyclipper.PFT_NONZERO,pyclipper.PFT_NONZERO)
    pc.Clear()
    return union_polygons

def polygon_difference(sub_polygons:List[List[Tuple[float, float]]], clip_polygons:List[List[Tuple[float, float]]]) -> List[List[Tuple[float, float]]]:
    if not clip_polygons:
        return sub_polygons
    pc = pyclipper.Pyclipper()
    pc.AddPaths(sub_polygons, pyclipper.PT_SUBJECT, True)
    pc.AddPaths(clip_polygons, pyclipper.PT_CLIP, True)
    difference_polygons = pc.Execute(pyclipper.CT_DIFFERENCE, pyclipper.PFT_NONZERO,pyclipper.PFT_NONZERO)
    pc.Clear()
    return difference_polygons


##################get hatch path###############
def get_hatch_polygons(hatch_entity: Hatch):
    external_polygons = list()
    hole_polygons: List[List[int], int] = list()
    boudPaths = hatch_entity.paths.rendering_paths(hatch_style=hatch_entity.dxf.hatch_style)
    loops: List[Path] = list()
    for boundary in boudPaths:
        path = from_hatch_boundary_path(boundary=boundary, ocs=hatch_entity.ocs(), elevation=hatch_entity.dxf.elevation.z)
        for sub_path in path.sub_paths():
            sub_path.close()
            loops.append(sub_path)
    # remove text boxes in external paths
    external_paths, holes = winding_deconstruction(polygons=fast_bbox_detection(paths=loops))
    external_paths = [p for p in external_paths if not (isinstance(p.user_data, const.BoundaryPathState) and p.user_data.textbox)]
    for external_path in external_paths:
        try:
            external_path = external_path.counter_clockwise()
        except ValueError:   # cannot detect path orientation
            continue
        external_polygons.append([(v.x, v.y) for v in external_path.flattening(50)])
    for hole_path in holes:
        try:
            hole_path = hole_path.clockwise()
        except ValueError:
            continue
        hole_polygons.append([(v.x, v.y) for v in hole_path.flattening(50)])
    ###################clipper the external path with holes#########################
    total_external_polygons = polygon_union(polygons=external_polygons)
    total_hole_polygons = polygon_union(polygons=hole_polygons)
    hatch_polygons = polygon_difference(sub_polygons=total_external_polygons, clip_polygons=total_hole_polygons)
    return hatch_polygons

def polygons2path_patch_data(polygons):
    codes = []
    vertices = []
    for polygon in polygons:
        if len(polygon) > 2:
            codes.append(Path.MOVETO)
            vertices.append(polygon[0])
            for pt in polygon[1:]:
                codes.append(Path.LINETO)
                vertices.append(pt)
    return vertices, codes
                

    
################draw with pyclipper##################
fig, ax = plt.subplots()
ext: BoundingBox = bbox.extents(entities=doc.modelspace(), flatten=0)
visible_layers: List[str] = [layer.dxf.name for layer in doc.layers if layer.is_on() and not layer.is_frozen()]
for e in doc.modelspace():
    entity_type = e.dxftype()
    entity_layer = e.dxf.layer
    if e.dxf.invisible == 0 and entity_layer in visible_layers and entity_type == 'HATCH':
        hatch_polygons = get_hatch_polygons(e)
        v, c = polygons2path_patch_data(hatch_polygons)
        patch = PathPatch(Path(v, c), color = "#808080", fill=True)
        ax.add_patch(patch)
ax.set_xlim(ext.extmin[0], ext.extmax[0])
ax.set_ylim(ext.extmin[1], ext.extmax[1])
w_size = h_size = 2048
fig.set_size_inches(w_size / 100, h_size / 100)
plt.subplots_adjust(top=1, bottom=0, right=1, left=0)
plt.axis('off')
plt.savefig("clipper_draw.png", format='png', dpi=100)
fig.clear()
plt.close()

[chibai]

Here is ezdxf drawing

[chibai]

here is clipper drawing, which is same as BricsCAD

[chibai]

here is autocad showing

[mozman]

Thank you for you contribution, but... 😉

This solution solves this special problem, but does not work for all combinations of boundary paths. This is the included example DXF hatch_boundaries.dxf which should look like this:

The Matplotlib rendering looks like this:

Not perfect but usable

This is the pyclipper result:

ahh ... nothing

So there is much work and time to invest to get this solution working for general cases, which I will not do because I am happy with the current state.

Perfect rendering of DXF files was never a goal of this project!

To emphasize it again: ezdxf and the included drawing add-on is and will never be a substitute for a real DXF viewer or even a CAD application, there will always be minor or major errors in the rendering.

[chibai]

Got it~