setDownsampling & PlotCurveItem

[noxfir...@gmail.com]

Hello everyone,

I am currently struggling to use pyqtgraph for realtime ploting of 64 curves à >1k samples with a samplingrate of >1kHz. I found the "Multiple plots" example which shows the use of PlotCurveItem to increase the performance which really improves the update time. But as I draw every curve in a seperate plotwidget refresh rate is not as high as in the example. So I wanted to try downsampling. But if I call setDownsampling for a plot the following exception is generated:

Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\pyqtgraph\graphicsItems\PlotItem\PlotItem.py", line 952, in updateDownsampling
    c.setDownsampling(ds, auto, method)
AttributeError: 'PlotCurveItem' object has no attribute 'setDownsampling'

Interestingly the program continues to run. To reproduce this behaviour just add p.setDownsampling(ds=True) to the Multiple plots" example e.g. in line after

p.setLabel('bottom', 'Index', units='B') .

best regards,

Nox

[noxfir...@gmail.com]

PS: Exception is thrown as well if example is left unchanged and downsampling is setup via GUI context menu.

[vas...@gmail.com]

You need to set Downsampling of PlotItem, not of PlotCurveItem.

# example

import pyqtgraph
graphics_layout_widget = pyqtgraph.GraphicsLayoutWidget()
plot_item = graphics_layout_widget.addPlot()

# this works well

plot_item.setDownsampling(auto=False, ds=2, mode='mean')
plot_curve_item = plot_item.plot()
# this produce an exception
# plot_curve_item.setDownsampling(auto=False, ds=2, mode='mean')

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[noxfir...@gmail.com]

Hello,

Thank you for this very interesting hint and example code. Sadly using plot() instead of PlotCurveItem comes with a decreased performance. I measure ~50ms (plot) vs ~40ms (PlotCurveItem) per frame for Multiple plot example which is the reason why I started using PlotCurveItem in the first place. Of course the downsampling helps to catch up/surpass this effect but only for higher down sample rates. Additionally the issue remains that using the GUI context menu to set downsampling does raise an exception for the unmodified multiple plots example.

best regards

Am Dienstag, 30. Mai 2017 03:17:19 UTC+2 schrieb Vasilije Mehandzic:

You need to set Downsampling of PlotItem, not of PlotCurveItem.

# example

import pyqtgraph
graphics_layout_widget = pyqtgraph.GraphicsLayoutWidget()
plot_item = graphics_layout_widget.addPlot()

# this works well

plot_item.setDownsampling(auto=False, ds=2, mode='mean')
plot_curve_item = plot_item.plot()
# this produce an exception
# plot_curve_item.setDownsampling(auto=False, ds=2, mode='mean')

On Thu, May 25, 2017 at 8:17 PM, <noxfir...@gmail.com> wrote:

Hello everyone,

I am currently struggling to use pyqtgraph for realtime ploting of 64 curves à >1k samples with a samplingrate of >1kHz. I found the "Multiple plots" example which shows the use of PlotCurveItem to increase the performance which really improves the update time. But as I draw every curve in a seperate plotwidget refresh rate is not as high as in the example. So I wanted to try downsampling. But if I call setDownsampling for a plot the following exception is generated:

Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\pyqtgraph\graphicsItems\PlotItem\PlotItem.py", line 952, in updateDownsampling
    c.setDownsampling(ds, auto, method)
AttributeError: 'PlotCurveItem' object has no attribute 'setDownsampling'

Interestingly the program continues to run. To reproduce this behaviour just add p.setDownsampling(ds=True) to the Multiple plots" example e.g. in line after

p.setLabel('bottom', 'Index', units='B') .

best regards,

Nox

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