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Fix the misplacement of rasterized object in DrawingArea in the vectorized backends #30171

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Fix the misplacement of rasterized object in DrawingArea in the vectorized backends #30171

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12 changes: 12 additions & 0 deletions lib/matplotlib/backends/backend_mixed.py
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Original file line number Diff line number Diff line change
Expand Up @@ -86,6 +86,18 @@ def start_rasterizing(self):
self._raster_renderer)
self._bbox_inches_restore = r

def get_image_magnification(self):
"""
Return the magnification factor for rasterized content.

When using `.MixedModeRenderer`, the figure DPI is set to the vector
renderer's DPI (typically 72 for PDF/SVG/PS) but rasterized content
is drawn at ``self.dpi`` (the user's requested DPI). This factor
converts from vector-space display coordinates to raster-space pixel
coordinates.
"""
Comment on lines +93 to +98
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Copilot AI Apr 8, 2026

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MixedModeRenderer.get_image_magnification currently returns self.dpi / self._figdpi unconditionally. When MixedModeRenderer is in raster mode (self._renderer is the Agg raster renderer), callers typically expect magnification to match the active raster renderer (usually 1.0) to avoid unintended extra resampling/memory use for images drawn while rasterizing. Consider returning 1.0 (or delegating to the active renderer) when rasterizing, and only returning dpi/_figdpi when in vector mode.

Suggested change
When using `.MixedModeRenderer`, the figure DPI is set to the vector
renderer's DPI (typically 72 for PDF/SVG/PS) but rasterized content
is drawn at ``self.dpi`` (the user's requested DPI). This factor
converts from vector-space display coordinates to raster-space pixel
coordinates.
"""
In vector mode, `.MixedModeRenderer` converts from the figure DPI
(typically 72 for PDF/SVG/PS) to ``self.dpi`` (the user's requested
DPI). When rasterizing, defer to the active raster renderer so image
drawing uses that renderer's native magnification.
"""
if self._raster_renderer is not None and self._renderer is self._raster_renderer:
return self._renderer.get_image_magnification()

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return self.dpi / self._figdpi

def stop_rasterizing(self):
"""
Exit "raster" mode. All of the drawing that was done since
Expand Down
48 changes: 43 additions & 5 deletions lib/matplotlib/offsetbox.py
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Expand Up @@ -37,6 +37,7 @@
from matplotlib.patches import (
FancyBboxPatch, FancyArrowPatch, bbox_artist as mbbox_artist)
from matplotlib.transforms import Bbox, BboxBase, TransformedBbox
from matplotlib.backends.backend_mixed import MixedModeRenderer
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Importing MixedModeRenderer at module import time pulls in backend code (backend_mixed -> backend_agg) from a core artist module, which can increase import time and risks circular-import issues. Prefer avoiding the hard import (e.g., use a local import inside the rasterization branch, or switch to a capability/attribute check rather than isinstance(renderer, MixedModeRenderer)).

Suggested change
from matplotlib.backends.backend_mixed import MixedModeRenderer

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DEBUG = False
Expand Down Expand Up @@ -689,9 +690,6 @@ def draw(self, renderer):
self.dpi_transform.clear()
self.dpi_transform.scale(dpi_cor)

# At this point the DrawingArea has a transform
# to the display space so the path created is
# good for clipping children
tpath = mtransforms.TransformedPath(
mpath.Path([[0, 0], [0, self.height],
[self.width, self.height],
Expand All @@ -700,7 +698,28 @@ def draw(self, renderer):
for c in self._children:
if self._clip_children and not (c.clipbox or c._clippath):
c.set_clip_path(tpath)
c.draw(renderer)
if c.get_rasterized() and isinstance(renderer, MixedModeRenderer):
# When using MixedModeRenderer (PDF/SVG/PS), the figure DPI is
# set to the vector renderer's DPI (typically 72 pt/inch), so
# dpi_cor ≈ 1.0. But rasterized children are drawn into a
# RendererAgg buffer at renderer.dpi (the user's requested DPI).
# The child's transform is dpi_transform + offset_transform,
# where offset_transform is a translation in vector-DPI display
# units. Both must be scaled to raster-DPI pixel coordinates.
mag = renderer.get_image_magnification()
raster_dpi_cor = dpi_cor * mag
off_mat = self.offset_transform.get_matrix().copy()
scaled_off = off_mat.copy()
scaled_off[:2, 2] *= mag # scale tx and ty to raster DPI
self.dpi_transform.clear()
self.dpi_transform.scale(raster_dpi_cor)
self.offset_transform.set_matrix(scaled_off)
Comment on lines +701 to +716
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The scaling branch should be skipped when the renderer is already rasterizing (e.g., this DrawingArea is inside a rasterized parent). In that case points_to_pixels is already in raster-DPI units, so multiplying by mag again will over-scale (dpi factors can effectively be applied twice). Consider guarding with and not renderer._rasterizing (and/or renderer._raster_depth == 0) so the correction only applies when entering rasterization from vector mode.

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c.draw(renderer)
self.dpi_transform.clear()
self.dpi_transform.scale(dpi_cor)
self.offset_transform.set_matrix(off_mat)
Comment on lines +717 to +720
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This code mutates dpi_transform and offset_transform and then restores them, but restoration isn’t protected against exceptions. If c.draw(renderer) raises, the transforms remain scaled and can corrupt subsequent draws/layout. Wrap the temporary scaling/restoration in a try/finally to guarantee state restoration.

Suggested change
c.draw(renderer)
self.dpi_transform.clear()
self.dpi_transform.scale(dpi_cor)
self.offset_transform.set_matrix(off_mat)
try:
c.draw(renderer)
finally:
self.dpi_transform.clear()
self.dpi_transform.scale(dpi_cor)
self.offset_transform.set_matrix(off_mat)

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else:
c.draw(renderer)

_bbox_artist(self, renderer, fill=False, props=dict(pad=0.))
self.stale = False
Expand Down Expand Up @@ -899,7 +918,26 @@ def get_bbox(self, renderer):
def draw(self, renderer):
# docstring inherited
for c in self._children:
c.draw(renderer)
if c.get_rasterized() and isinstance(renderer, MixedModeRenderer):
# offset_transform and ref_offset_transform store display-unit
# translations computed at vector DPI (typically 72 pt/inch for
# PDF/SVG/PS). When rasterizing, the RendererAgg buffer uses
# renderer.dpi (user DPI). Scale the translation components so
# the child draws at the correct position in the raster buffer.
mag = renderer.get_image_magnification()
off_mat = self.offset_transform.get_matrix().copy()
ref_mat = self.ref_offset_transform.get_matrix().copy()
scaled_off = off_mat.copy()
scaled_off[:2, 2] *= mag # scale tx and ty only
scaled_ref = ref_mat.copy()
scaled_ref[:2, 2] *= mag
self.offset_transform.set_matrix(scaled_off)
self.ref_offset_transform.set_matrix(scaled_ref)
c.draw(renderer)
self.offset_transform.set_matrix(off_mat)
self.ref_offset_transform.set_matrix(ref_mat)
Comment on lines +936 to +938
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Same as in DrawingArea.draw: temporary mutation of offset_transform/ref_offset_transform should be protected with try/finally so state is restored even if c.draw(renderer) errors.

Suggested change
c.draw(renderer)
self.offset_transform.set_matrix(off_mat)
self.ref_offset_transform.set_matrix(ref_mat)
try:
c.draw(renderer)
finally:
self.offset_transform.set_matrix(off_mat)
self.ref_offset_transform.set_matrix(ref_mat)

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else:
c.draw(renderer)
_bbox_artist(self, renderer, fill=False, props=dict(pad=0.))
self.stale = False

Expand Down
51 changes: 49 additions & 2 deletions lib/matplotlib/tests/test_offsetbox.py
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Expand Up @@ -12,8 +12,9 @@
from matplotlib.backend_bases import MouseButton, MouseEvent

from matplotlib.offsetbox import (
AnchoredOffsetbox, AnnotationBbox, AnchoredText, DrawingArea, HPacker,
OffsetBox, OffsetImage, PaddedBox, TextArea, VPacker, _get_packed_offsets)
AnchoredOffsetbox, AnnotationBbox, AnchoredText, AuxTransformBox,
DrawingArea, HPacker, OffsetBox, OffsetImage, PaddedBox, TextArea, VPacker,
_get_packed_offsets)


@image_comparison(['offsetbox_clipping'], remove_text=True)
Expand Down Expand Up @@ -507,3 +508,49 @@ def test_anchored_offsetbox_tuple_and_float_borderpad():
# in the y-direction.
assert pos_tuple_asym.x0 > pos_float.x0
assert pos_tuple_asym.y0 < pos_float.y0


@check_figures_equal(extensions=["pdf", "svg", "png"], tol=5)
def test_rasterized_artist_in_drawing_area(fig_test, fig_ref):
"""Rasterized artists in DrawingArea must be correctly positioned in
vector backends. Regression test for
https://github.com/matplotlib/matplotlib/issues/28549
"""
from matplotlib.collections import PatchCollection
from matplotlib.patches import Rectangle

def make_plot(fig, rasterized):
ax = fig.subplots()
box = DrawingArea(10, 10)
rects = PatchCollection(
[Rectangle((0, i), width=10, height=1) for i in range(10)]
)
rects.set_rasterized(rasterized)
box.add_artist(rects)
ax.add_artist(AnchoredOffsetbox(child=box, loc="center", pad=0))

make_plot(fig_test, rasterized=True)
make_plot(fig_ref, rasterized=False)


@check_figures_equal(extensions=["pdf", "svg", "png"], tol=8)
def test_rasterized_artist_in_aux_transform_box(fig_test, fig_ref):
"""Rasterized artists in AuxTransformBox must be correctly positioned in
vector backends. Regression test for
https://github.com/matplotlib/matplotlib/issues/28549
"""
from matplotlib.patches import Ellipse

def make_plot(fig, rasterized):
ax = fig.subplots()
ax.set_xlim(0, 10)
ax.set_ylim(0, 10)
# Use ax.transData as the typical real-world use of AuxTransformBox
box = AuxTransformBox(ax.transData)
el = Ellipse((5, 5), width=4, height=2)
el.set_rasterized(rasterized)
box.add_artist(el)
ax.add_artist(AnchoredOffsetbox(child=box, loc="center", pad=0))

make_plot(fig_test, rasterized=True)
make_plot(fig_ref, rasterized=False)
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