Hello Patrick, fantastic tip, thank you!
I went to work with your suggestion and came up with the following (in case
it's useful for anyone). I admit it's kind of a workaround, but for now it
does what I want :)
import numpy as np
import pyqtgraph as pg
def interp0(x, xp, yp):
"""Zeroth-order hold interpolation with same (base) signature as
numpy.interp."""
def f(x0):
if x0 <= xp[0]: return yp[0]
if x0 >= xp[-1]: return yp[-1]
k = 0
while x0 > xp[k]: k += 1
return yp[k - 1]
return np.asarray([f(x) for x in x])
class DiscreteColorMap(pg.ColorMap):
def map(self, data, mode = pg.ColorMap.BYTE):
data = interp0(data, self.pos, self.pos)
return super().map(data, mode)
I'm taking my first steps with Python, though, so suggestions for
improvements welcome!
Kind regards
Edward
On Tuesday, April 11, 2023 at 9:27:09 AM UTC Patrick wrote:
> Hi,
>
> I think the colormaps are always interpolated. If you run the pyqtgraph
> Color Maps example you'll see even the discrete maps imported from
> matplotlib get interpolated.
> It might be possible to define your own colormap with two stops at the
> same location. It might break, but it might also work...
> I've hard coded in my own colormaps before using something like:
>
> #...
> self.cbar = pg.HistogramLUTItem(self.overview_composite_image)
> self.cbar.gradient.restoreState({"mode": "rgb",
> "ticks": [(0.00, (0, 0, 0)),
> (0.25, (0, 0, 128)),
> (0.50, (144, 0 , 0)),
> (0.85, (224, 224, 0)),
> (1.00, (255, 255, 255))]})
> #...
>
> So you could try putting two stops for each colour something like:
>
> self.cbar.gradient.restoreState({"mode": "rgb",
> "ticks": [(0.00, (0, 0, 0)),
> (0.25, (0, 0, 0)),
> (0.25, (0, 0, 128)),
> (0.50, (0, 0, 128)),
> (0.50, (144, 0 , 0)),
> (0.85, (144, 0, 0)),
> (0.85, (255, 255, 255)),
> (1.00, (255, 255, 255))]})
> #...
>
> It's possible you may need to make the next range start at 0.25000001 etc
> if things break because of divide-by-zero errors.
>
> The "proper" way to do it would be to write a ColorMap subclass
> DiscreteColorMap or similar that handles the functionality.
>
> Patrick
> On Monday, 10 April 2023 at 7:36:56 pm UTC+9:30 [email protected]
> wrote:
>
>> Hello,
>>
>> Is it possible to apply a colour map to an image without interpolation
>> between the colours? Sort of a discrete colour map?
>>
>>
>> Kind regards
>> Edward
>>
>
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