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Simplify webagg blitting. #23182

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merged 1 commit into from
Jun 3, 2022
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Simplify webagg blitting. #23182

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48 changes: 13 additions & 35 deletions lib/matplotlib/backends/backend_webagg_core.py
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Original file line number Diff line number Diff line change
Expand Up @@ -171,6 +171,9 @@ def __init__(self, *args, **kwargs):
# sent to the clients will be a full frame.
self._force_full = True

# The last buffer, for diff mode.
self._last_buff = np.empty((0, 0))

# Store the current image mode so that at any point, clients can
# request the information. This should be changed by calling
# self.set_image_mode(mode) so that the notification can be given
Expand Down Expand Up @@ -227,17 +230,18 @@ def get_diff_image(self):
if self._png_is_old:
renderer = self.get_renderer()

pixels = np.asarray(renderer.buffer_rgba())
# The buffer is created as type uint32 so that entire
# pixels can be compared in one numpy call, rather than
# needing to compare each plane separately.
buff = (np.frombuffer(renderer.buffer_rgba(), dtype=np.uint32)
.reshape((renderer.height, renderer.width)))

# If any pixels have transparency, we need to force a full
# draw as we cannot overlay new on top of old.
pixels = buff.view(dtype=np.uint8).reshape(buff.shape + (4,))

if self._force_full or np.any(pixels[:, :, 3] != 255):
buff = pixels.view(np.uint32).squeeze(2)

if (self._force_full
# If the buffer has changed size we need to do a full draw.
or buff.shape != self._last_buff.shape
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may be nice to add a comment as well about this line. It took me a while to hunt down why it was ok to compare the initial np.empty((0,0)) with the buffer and this line is the escape

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Suggested change
or buff.shape != self._last_buff.shape
# If the buffer has changed size we need to do a full draw
or buff.shape != self._last_buff.shape

# If any pixels have transparency, we need to force a full
# draw as we cannot overlay new on top of old.
or (pixels[:, :, 3] != 255).any()):
self.set_image_mode('full')
output = buff
else:
Expand All @@ -246,7 +250,7 @@ def get_diff_image(self):
output = np.where(diff, buff, 0)

# Store the current buffer so we can compute the next diff.
np.copyto(self._last_buff, buff)
self._last_buff = buff.copy()
self._force_full = False
self._png_is_old = False

Expand All @@ -255,32 +259,6 @@ def get_diff_image(self):
Image.fromarray(data).save(png, format="png")
return png.getvalue()

@_api.delete_parameter("3.6", "cleared", alternative="renderer.clear()")
def get_renderer(self, cleared=None):
# Mirrors super.get_renderer, but caches the old one so that we can do
# things such as produce a diff image in get_diff_image.
w, h = self.figure.bbox.size.astype(int)
key = w, h, self.figure.dpi
try:
self._lastKey, self._renderer
except AttributeError:
need_new_renderer = True
else:
need_new_renderer = (self._lastKey != key)

if need_new_renderer:
self._renderer = backend_agg.RendererAgg(
w, h, self.figure.dpi)
self._lastKey = key
self._last_buff = np.copy(np.frombuffer(
self._renderer.buffer_rgba(), dtype=np.uint32
).reshape((self._renderer.height, self._renderer.width)))

elif cleared:
self._renderer.clear()

return self._renderer

def handle_event(self, event):
e_type = event['type']
handler = getattr(self, 'handle_{0}'.format(e_type),
Expand Down