This short post is about the taming of the pyqtgraph widget HistogramLUTItem and preventing a linked ImageItem to update manually set levels in the histogram.

Problem

I love the histogram widget that shows the distribution of the grey-levels of an image and can be easily linked to an ImageItem

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import pyqtgraph as pg
...
win = pg.GraphicsLayoutWidget()
view = win.addViewBox(lockAspect=True)
self.pg_img = pg.ImageItem()
view.addItem(self.pg_img)

self.histogram = pg.HistogramLUTItem(self.pg_img, fillHistogram=True)
win.addItem(self.histogram)
...

Now, say your camera is pushing new images every 1s to ImageItem and you want to keep the max,min levels fixed such that you can do alignment or alike. I initially thought I could prevent these updates in the parent ViewBox of the ImageItem with methods such as disableAutoRange() or inside of the histogram itself with disableAutoHistogramRange() - I was mistaken unfortunately.

Solution

Whenever the histogram levels are updated there is a sigLevelsChanged signal emitted. This is the culprit for resetting our manually set levels. My solution allows the user to toggle between auto-update and allowing to adjust the histogram levels through the widget sliders. Let’s check in which mode we are when setting the image data

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if self.bln_fix_hg_levels:
	self.pg_img.setImage(img, levels=self.hg_levels)
else:
	self.pg_img.setImage(img)

and elsewhere maybe we have a checkbox which clicked event leads to a toggled flag and get’s us to update the self.hg_levels manually

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check = QCheckBox("Fix hg levels")
check.clicked.connect(self.upon_click)
...
def upon_click(self):
	self.bln_fix_hg_levels = self.sender().isChecked()
	if self.bln_fix_hg_levels:
		self.update_hg_levels()
		self.histogram.sigLevelsChanged.connect(self.update_hg_levels)
	else:
		self.histogram.sigLevelsChanged.disconnect(self.update_hg_levels)

in the update_hg_levels method we finally grab our levels

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def update_hg_levels(self):
	self.hg_levels = self.histogram.getLevels()

It works pretty well - happy me. As always it is good to look into the documentation ;).