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PDF backend can now do alpha on hatches. #17049

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PDF backend can now do alpha on hatches. #17049

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6 changes: 0 additions & 6 deletions doc/users/next_whats_new/2020-03-24-svg-hatch-alpha.rst
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6 changes: 6 additions & 0 deletions doc/users/next_whats_new/hatch-alpha.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
The SVG and PDF backends can now render hatches with transparency
-----------------------------------------------------------------

The SVG and PDF backends now respect the hatch stroke alpha. Useful applications
are, among others, semi-transparent hatches as a subtle way to differentiate
columns in bar plots.
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This could be a nice demo in the gallery, or perhaps another subplot in https://matplotlib.org/3.2.0/gallery/lines_bars_and_markers/filled_step.html

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Cool idea. As a side note, because hatch color/alpha is linked to the Patch's edge's, a simple plot with hatch alpha will usually not look so good because the translucent edge will half-overlap with the fill. One can plot edge-less with lw=0, but to have an opaque outline one must replot a full opacity, edge-only plot on top of the first.
It's a bit hackish. An independent hatch style API (#7059) would go a long way to simplifying this.

25 changes: 19 additions & 6 deletions lib/matplotlib/backends/backend_pdf.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1288,14 +1288,18 @@ def _write_soft_mask_groups(self):
self.output(*content)
self.endStream()

def hatchPattern(self, hatch_style):
def hatchPattern(self, hatch_style, forced_alpha):
# The colors may come in as numpy arrays, which aren't hashable
if hatch_style is not None:
edge, face, hatch = hatch_style
if edge is not None:
edge = tuple(edge)
if forced_alpha: # reset alpha if forced
edge = edge[:3] + (1.0,)
if face is not None:
face = tuple(face)
if forced_alpha: # reset alpha if forced
face = face[:3] + (1.0,)
hatch_style = (edge, face, hatch)

pattern = self.hatchPatterns.get(hatch_style, None)
Expand All @@ -1310,10 +1314,14 @@ def writeHatches(self):
hatchDict = dict()
sidelen = 72.0
for hatch_style, name in self.hatchPatterns.items():
stroke_rgb, fill_rgb, path = hatch_style
ob = self.reserveObject('hatch pattern')
hatchDict[name] = ob
res = {'Procsets':
[Name(x) for x in "PDF Text ImageB ImageC ImageI".split()]}
if stroke_rgb[3] != 1.0:
res['ExtGState'] = self._extGStateObject

self.beginStream(
ob.id, None,
{'Type': Name('Pattern'),
Expand All @@ -1324,7 +1332,9 @@ def writeHatches(self):
# Change origin to match Agg at top-left.
'Matrix': [1, 0, 0, 1, 0, self.height * 72]})

stroke_rgb, fill_rgb, path = hatch_style
if stroke_rgb[3] != 1.0:
gstate = self.alphaState((stroke_rgb[3], fill_rgb[3]))
self.output(gstate, Op.setgstate)
self.output(stroke_rgb[0], stroke_rgb[1], stroke_rgb[2],
Op.setrgb_stroke)
if fill_rgb is not None:
Expand Down Expand Up @@ -2245,15 +2255,15 @@ def alpha_cmd(self, alpha, forced, effective_alphas):
name = self.file.alphaState(effective_alphas)
return [name, Op.setgstate]

def hatch_cmd(self, hatch, hatch_color):
def hatch_cmd(self, hatch, hatch_color, forced_alpha):
if not hatch:
if self._fillcolor is not None:
return self.fillcolor_cmd(self._fillcolor)
else:
return [Name('DeviceRGB'), Op.setcolorspace_nonstroke]
else:
hatch_style = (hatch_color, self._fillcolor, hatch)
name = self.file.hatchPattern(hatch_style)
name = self.file.hatchPattern(hatch_style, forced_alpha)
return [Name('Pattern'), Op.setcolorspace_nonstroke,
name, Op.setcolor_nonstroke]

Expand Down Expand Up @@ -2311,13 +2321,15 @@ def clip_cmd(self, cliprect, clippath):
(('_cliprect', '_clippath'), clip_cmd),
(('_alpha', '_forced_alpha', '_effective_alphas'), alpha_cmd),
(('_capstyle',), capstyle_cmd),
# If you change the next line also fix the check in delta
(('_fillcolor',), fillcolor_cmd),
(('_joinstyle',), joinstyle_cmd),
(('_linewidth',), linewidth_cmd),
(('_dashes',), dash_cmd),
(('_rgb',), rgb_cmd),
# must come after fillcolor and rgb
(('_hatch', '_hatch_color'), hatch_cmd),
# If you change the next line also fix the check in delta
(('_hatch', '_hatch_color', '_forced_alpha'), hatch_cmd),
)

def delta(self, other):
Expand Down Expand Up @@ -2346,7 +2358,8 @@ def delta(self, other):
break

# Need to update hatching if we also updated fillcolor
if params == ('_hatch', '_hatch_color') and fill_performed:
if (params == ('_hatch', '_hatch_color', '_forced_alpha')
and fill_performed):
different = True

if different:
Expand Down
11 changes: 8 additions & 3 deletions lib/matplotlib/backends/backend_svg.py
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Original file line number Diff line number Diff line change
Expand Up @@ -350,12 +350,17 @@ def _get_hatch(self, gc, rgbFace):
"""
Create a new hatch pattern
"""
forced_alpha = gc.get_forced_alpha()
if rgbFace is not None:
rgbFace = tuple(rgbFace)
if forced_alpha: # reset alpha if forced
rgbFace = rgbFace[:3] + (1.0,)
edge = gc.get_hatch_color()
if edge is not None:
edge = tuple(edge)
dictkey = (gc.get_hatch(), rgbFace, edge)
if forced_alpha: # reset alpha if forced
edge = edge[:3] + (1.0,)
dictkey = (gc.get_hatch(), rgbFace, edge, gc.get_forced_alpha())
oid = self._hatchd.get(dictkey)
if oid is None:
oid = self._make_id('h', dictkey)
Expand Down Expand Up @@ -398,8 +403,8 @@ def _write_hatches(self):
'stroke-linecap': 'butt',
'stroke-linejoin': 'miter'
}
if stroke[3] < 1:
hatch_style['stroke-opacity'] = str(stroke[3])
if stroke[3] != 1.0:
hatch_style['stroke-opacity'] = short_float_fmt(stroke[3])
writer.element(
'path',
d=path_data,
Expand Down
1 change: 1 addition & 0 deletions lib/matplotlib/collections.py
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Original file line number Diff line number Diff line change
Expand Up @@ -299,6 +299,7 @@ def draw(self, renderer):
gc = renderer.new_gc()
self._set_gc_clip(gc)
gc.set_snap(self.get_snap())
gc.set_alpha(self._alpha)

if self._hatch:
gc.set_hatch(self._hatch)
Expand Down
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58 changes: 40 additions & 18 deletions lib/matplotlib/tests/baseline_images/test_artist/clip_path_clipping.svg
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