Support fractional HiDpi scaling with Qt backends #15656

timhoffm · 2019-11-10T16:04:20Z

PR Summary

We need to use the floating point versions devicePixelRatioF instead of devicePixelRatio to scale the canvas correctly when the system uses a fractional scaling.

Since we support older Qt versions, setting and getting devicePixelRatioF is wrapped in helper functions that fall back to the integer version or even no scaling.

Tested with a scaling factor of 1.2:

before:

after:

PR Checklist

Has Pytest style unit tests
Code is Flake 8 compliant
New features are documented, with examples if plot related
Documentation is sphinx and numpydoc compliant
Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way

timhoffm · 2019-11-10T21:38:00Z

There's an issue with

Traceback (most recent call last):
  File "/home/travis/build/matplotlib/matplotlib/lib/matplotlib/backends/backend_qt5cairo.py", line 25, in paintEvent
    surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, width, height)
TypeError: integer argument expected, got float
Fatal Python error: Aborted
Current thread 0x00007f065e8d2700 (most recent call first):
  File "/home/travis/build/matplotlib/matplotlib/lib/matplotlib/backends/backend_qt5.py", line 1044 in mainloop
  File "/home/travis/build/matplotlib/matplotlib/lib/matplotlib/backend_bases.py", line 3395 in show
  File "/home/travis/build/matplotlib/matplotlib/lib/matplotlib/cbook/deprecation.py", line 398 in wrapper
  File "/home/travis/build/matplotlib/matplotlib/lib/matplotlib/pyplot.py", line 266 in show
  File "<string>", line 58 in <module>

While I will try to fix this in isolation, we must check if other places also need an integer conversion. Also, should we cast to int or round to int?

anntzer · 2019-11-10T22:51:39Z

Probably do the same as what the qt version does?

tacaswell · 2019-11-11T03:00:46Z

lib/matplotlib/backends/qt_compat.py

+        obj.setDevicePixelRatioF(val)
+    if hasattr(obj, 'setDevicePixelRatio'):
+        # Not available on Qt4 or some older Qt5.
+        obj.setDevicePixelRatio(val)


do we want to cast to int here?

I would think so, given your other Qt PR, though that's for new Qt that wouldn't call this. Though I think it would come from getDevicePixelRatio, so we may expect int anyway?

tacaswell · 2019-11-11T03:03:10Z

lib/matplotlib/backends/backend_qt5cairo.py

@@ -19,8 +19,8 @@ def draw(self):
    def paintEvent(self, event):
        self._update_dpi()
        dpi_ratio = self._dpi_ratio
-        width = dpi_ratio * self.width()
-        height = dpi_ratio * self.height()
+        width = int(dpi_ratio * self.width())


We may want to use round here rather than int to move the jumps away from the integers.

However, this may have issues with not being in sync with what we do else were in the code (which is int).

tacaswell · 2020-06-11T22:55:08Z

It turns out Qt now has an enum to control how it rounds to pixels (https://doc.qt.io/qt-5/qt.html#HighDpiScaleFactorRoundingPolicy-enum).

You can test this behavior by setting the QT_SCALE_FACTOR=1.5 ENV in linux and running

import matplotlib.pyplot as plt
import numpy as np

th = np.linspace(0, 2*np.pi, 512)

fig, ax = plt.subplots()
ax.plot(th, np.sin(th))
ax.plot(th, np.cos(th))

ax.set_title(fig.canvas._dpi_ratio)

plt.show()

Without this patch it rounds down via truncating for non-integer ratios. I think what is going on is that we are getting the integer version of the dpi scaling and render the figure at (N, M) size. However, the widgets are seeing the fractional part and actually taking up (N1.2, M1.2) pixels on screen. When we paint the rendered figure Qt helpfully re-samples to the actual number of pixels and our anti-aliasing interacts really badly with that.

tacaswell

Take my review as only 1/2 as I did the rebase.

QuLogic

This test always fails for me when run with everything else, so I'm not 100% on this, but it does pass when run by itself.

lib/matplotlib/backends/backend_qt5.py

QuLogic · 2020-06-13T02:45:36Z

lib/matplotlib/backends/qt_compat.py

+        obj.setDevicePixelRatioF(val)
+    if hasattr(obj, 'setDevicePixelRatio'):
+        # Not available on Qt4 or some older Qt5.
+        obj.setDevicePixelRatio(val)


I would think so, given your other Qt PR, though that's for new Qt that wouldn't call this. Though I think it would come from getDevicePixelRatio, so we may expect int anyway?

timhoffm · 2020-06-13T10:48:46Z

I don‘t have the time to look into this in the next couple of days. Anybody can take over and push to this PR if necessary.

Author: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

QuLogic · 2020-06-15T22:00:04Z

lib/matplotlib/backends/qt_compat.py

+    if hasattr(obj, 'setDevicePixelRatioF'):
+        # Not available on Qt<5.6
+        obj.setDevicePixelRatioF(val)
+    if hasattr(obj, 'setDevicePixelRatio'):


This should be elif, no? Or you'll set pixel ratio two different ways.

I opened #17640 with this change.

jklymak · 2020-06-15T22:02:17Z

arghh.sorry @QuLogic

lumberbot-app · 2020-06-15T22:02:52Z

Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

Checkout backport branch and update it.

$ git checkout v3.2.x
$ git pull

Cherry pick the first parent branch of the this PR on top of the older branch:

$ git cherry-pick -m1 77f23da286def97e4f05f4c59f2d5c4f0bfe7af2

You will likely have some merge/cherry-pick conflict here, fix them and commit:

$ git commit -am 'Backport PR #15656: Support fractional HiDpi scaling with Qt backends'

Push to a named branch :

git push YOURFORK v3.2.x:auto-backport-of-pr-15656-on-v3.2.x

Create a PR against branch v3.2.x, I would have named this PR:

"Backport PR #15656 on branch v3.2.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to suggest an improvement.

…t backends Merge pull request matplotlib#15656 from timhoffm/qt-fractional-hidpi Support fractional HiDpi scaling with Qt backends Conflicts: lib/matplotlib/backends/backend_qt5.py lib/matplotlib/backends/qt_compat.py - only backport relevant changes

anntzer · 2020-10-20T15:26:29Z

@timhoffm Actually, is the setDevicePixelRatioF shim ever used? AFAICT there's only setDevicePixelRatio, which directly takes a qreal (https://doc.qt.io/qt-5/qpixmap.html#setDevicePixelRatio), but no such thing as setDevicePixelRatioF. (On the getter side there's indeed both devicePixelRatio and devicePixelRatioF https://doc.qt.io/qt-5/qpaintdevice.html#devicePixelRatio.)

timhoffm · 2020-10-20T20:30:31Z

@anntzer You are probably right. I probably did the getter first and assumed that the setter follows the same pattern.

td;dr: Additional findings on the getter devicePixelRatio()

Digging a bit deeper: It seems that QPaintDevice initially only had int QPaintDevice::devicePixelRatio() but grew qreal QPaintDevice::devicePixelRatioF() in Qt 5.6. Note that QPaintDevice itself does not support setting the pixel ratio.

However, the derived classes QImage and QPixmap seemed to always have had floating point support via qreal devicePixelRatio() and void setDevicePixelRatio(qreal scaleFactor). (at least since Qt 5.5 - The Qt archive does not seem to have docs for older versions). I find that a little surprising in Qt - I would not have expected that they overload the API in the subclasses.

I was about to propose that we could replace _devicePixelRatioF(obj) by obj.devicePixelRatio(). That would work for QPixmap and QImage. But we also use _devicePixelRatioF in FigureCanvasQt, which inherits -> QWidget -> QPaintDevice but does not overload devicePixelRatio for qreal.

So indeed we could use devicePixelRatio() directly on all QPixmap, QImage and move the contents of _devicePixelRatioF to it's only needed usage in FigureCanvasQt._dpi_ratio.

timhoffm added the GUI: Qt label Nov 10, 2019

timhoffm added this to the v3.3.0 milestone Nov 10, 2019

timhoffm changed the title ~~Support fractional HiDpi scaling with Qt bakcends~~ Support fractional HiDpi scaling with Qt backends Nov 10, 2019

anntzer approved these changes Nov 10, 2019

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timhoffm added status: work in progress status: needs revision labels Nov 10, 2019

timhoffm force-pushed the qt-fractional-hidpi branch from 11a1a57 to ea8dfdb Compare November 10, 2019 21:37

timhoffm force-pushed the qt-fractional-hidpi branch from ea8dfdb to c205b60 Compare November 11, 2019 01:34

tacaswell reviewed Nov 11, 2019

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timhoffm mentioned this pull request Nov 18, 2019

UI scale level is different when running from interpreter than from script #15674

Closed

QuLogic modified the milestones: v3.3.0, v3.4.0 May 1, 2020

timhoffm mentioned this pull request May 18, 2020

Low quality window plots on hidpi display #17440

Closed

timhoffm mentioned this pull request Jun 4, 2020

FIX: support Qt 5.15 #17565

Merged

tacaswell force-pushed the qt-fractional-hidpi branch from c205b60 to eb0068c Compare June 11, 2020 22:34

tacaswell modified the milestones: v3.4.0, v3.2.2 Jun 11, 2020

tacaswell added Release critical For bugs that make the library unusable (segfaults, incorrect plots, etc) and major regressions. and removed status: needs revision status: work in progress labels Jun 11, 2020

tacaswell approved these changes Jun 11, 2020

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tacaswell force-pushed the qt-fractional-hidpi branch from eb0068c to c056bfa Compare June 12, 2020 01:12

QuLogic reviewed Jun 13, 2020

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Support fractional HiDpi scaling with Qt bakcends

17ff6ba

Author: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>

tacaswell force-pushed the qt-fractional-hidpi branch from c056bfa to 17ff6ba Compare June 15, 2020 19:43

QuLogic reviewed Jun 15, 2020

View reviewed changes

jklymak merged commit 77f23da into matplotlib:master Jun 15, 2020

lumberbot-app bot added the status: needs manual backport label Jun 15, 2020

QuLogic mentioned this pull request Jun 15, 2020

More qt fractional DPI fixes #17640

Merged

2 tasks

tacaswell removed the status: needs manual backport label Jun 16, 2020

timhoffm deleted the qt-fractional-hidpi branch June 16, 2020 11:39

anntzer mentioned this pull request Nov 3, 2020

Remove fallback to nonexistent setDevicePixelRatioF. #18877

Merged

7 tasks

j9ac9k mentioned this pull request Jan 23, 2021

have mkQApp configure QApplication such that it handles HIDPI displays on Windows pyqtgraph/pyqtgraph#1509

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Support fractional HiDpi scaling with Qt backends #15656

Support fractional HiDpi scaling with Qt backends #15656

timhoffm commented Nov 10, 2019

timhoffm commented Nov 10, 2019

anntzer commented Nov 10, 2019

tacaswell Nov 11, 2019

QuLogic Jun 13, 2020

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Support fractional HiDpi scaling with Qt backends #15656

Support fractional HiDpi scaling with Qt backends #15656

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timhoffm commented Nov 10, 2019

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