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Add print_rgba to backend_cairo. #10828

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Merged
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Mar 19, 2018
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Add print_rgba to backend_cairo. #10828

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33 changes: 31 additions & 2 deletions lib/matplotlib/backends/backend_cairo.py
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Original file line number Diff line number Diff line change
Expand Up @@ -47,6 +47,26 @@
from matplotlib.font_manager import ttfFontProperty


def _premultiplied_argb32_to_unmultiplied_rgba8888(buf):
"""
Convert a premultiplied ARGB32 buffer to an unmultiplied RGBA8888 buffer.

Cairo uses the former format, Matplotlib the latter.
"""
rgba = np.take( # .take() ensures C-contiguity of the result.
buf,
[2, 1, 0, 3] if sys.byteorder == "little" else [1, 2, 3, 0], axis=2)
rgb = rgba[..., :-1]
alpha = rgba[..., -1]
# Un-premultiply alpha. The formula is the same as in cairo-png.c.
mask = alpha != 0
for channel in np.rollaxis(rgb, -1):
channel[mask] = (
(channel[mask].astype(int) * 255 + alpha[mask] // 2)
// alpha[mask])
return rgba


class ArrayWrapper:
"""Thin wrapper around numpy ndarray to expose the interface
expected by cairocffi. Basically replicates the
Expand Down Expand Up @@ -436,15 +456,24 @@ class FigureCanvasCairo(FigureCanvasBase):
supports_blit = False

def print_png(self, fobj, *args, **kwargs):
self._get_printed_image_surface().write_to_png(fobj)

def print_rgba(self, fobj, *args, **kwargs):
width, height = self.get_width_height()
buf = self._get_printed_image_surface().get_data()
fobj.write(_premultiplied_argb32_to_unmultiplied_rgba8888(
np.asarray(buf).reshape((width, height, 4))))

print_raw = print_rgba

def _get_printed_image_surface(self):
width, height = self.get_width_height()
renderer = RendererCairo(self.figure.dpi)
renderer.set_width_height(width, height)
surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
renderer.set_ctx_from_surface(surface)

self.figure.draw(renderer)
surface.write_to_png(fobj)
return surface

def print_pdf(self, fobj, *args, **kwargs):
return self._save(fobj, 'pdf', *args, **kwargs)
Expand Down