Short description

Intent of https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/graphicsItems/ImageItem.py#L419-L440 is to combine levels and lut such that apply levels is a no-op in the subsequent call to makeARGB()

Code to reproduce

import pyqtgraph as pg
import numpy as np
pg.setConfigOptions(imageAxisOrder='row-major')

app = pg.mkQApp()
imv = pg.ImageView()
imv.show()
data = np.random.randint(0, 256, size=(4096, 4096), dtype=np.uint8)
imv.setImage(data)
app.exec_()

Expected behavior

apply levels should take very little time in makeARGB() profiling

Real behavior

both apply levels and apply lut take significant time to execute

> Entering functions.makeARGB
  check inputs: 0.0219 ms
  apply levels: 374.5632 ms
  apply lut: 230.2356 ms
  allocate: 0.0088 ms
  reorder channels: 84.9686 ms
  alpha channel: 11.7705 ms
< Exiting functions.makeARGB, total time: 703.3772 ms

Tested environment(s)

• PyQtGraph version: 0.11.1.dev0
• Qt Python binding: PySide2 5.15.2 Qt 5.15.2
• Python version: 3.7.9
• NumPy version: 1.19.4
• Operating system: Windows 10 x64
• Installation method: pip install -e .

Additional context

As far can I understand, the intended behavior relied on code that was changed in #793.

For uint8 data,

• a lut of size 256 is created
  https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/graphicsItems/ImageItem.py#L424
• levels is automatically calculated as [0, 255]
  https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/functions.py#L1121-L1122
• and value of scale before Fixes incorrect default value for scale parameter in makeARGB. #793 would be set to lut.shape[0] - 1 == 255

The following lines rely on levels[1] == scale in order to bypass apply levels
https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/functions.py#L1182-L1183

So for instance, if #793 were reverted (just for the purpose of demonstration of this issue), we would get the following:
(i.e. apply levels is practically a no-op)

> Entering functions.makeARGB
  check inputs: 0.0324 ms
  apply levels: 0.0243 ms
  apply lut: 215.7016 ms
  allocate: 0.0088 ms
  reorder channels: 69.7846 ms
  alpha channel: 10.7770 ms
< Exiting functions.makeARGB, total time: 298.7649 ms

If, on the other hand, the lines performing the combination of lut and levels were commented out altogether, https://github.com/pyqtgraph/pyqtgraph/blob/master/pyqtgraph/graphicsItems/ImageItem.py#L422-L440
we would get an even better timing:
i.e. both lut=None and levels=None passed into makeARGB(), making both apply levels and apply lut no-ops

> Entering functions.makeARGB
  check inputs: 0.3414 ms
  apply levels: 0.0269 ms
  apply lut: 0.0055 ms
  allocate: 0.0036 ms
  reorder channels: 84.2574 ms
  alpha channel: 12.9225 ms
< Exiting functions.makeARGB, total time: 97.5745 ms