Datashader min_alpha not in Holoviews

[Mossi8]

Hi, I am new both at Holoviews and Datashader. But I tried to get the same plot that I get using datashader using Holoviews but it is being impossible because the datashade function does not have this min_alpha parameter that is present if I only use datashader function tf.shade.

It is important, since as I zoom in I get only a few bins really intense which mask almost the rest of the image.

Thank you

Code in datashader (part of if):
background = "black"
export = partial(export_image, export_path="export", background=background)
cm = partial(colormap_select, reverse=(background=="black"))

def create_image(x_range, y_range, w=plot_width, h=plot_height):
cvs = ds.Canvas(plot_width=w, plot_height=h, x_range=x_range, y_range=y_range)
agg = cvs.points(data, 'X_coordinate', 'Y_coordinate', ds.count_cat("Gene"))
img = tf.shade(agg, how = "eq_hist", min_alpha = 255)
spread = tf.dynspread(img, threshold = 0.1, max_px = 1, how="over")
return spread

p = base_plot(background_fill_color=background)
export(create_image(*data_smFISH_coords),"5_smFISH")
InteractiveImage(p, create_image)

Code using Holoviews ():
import numpy as np
import holoviews as hv
import datashader as ds
from holoviews.operation.datashader import aggregate, datashade, dynspread, shade

hv.notebook_extension('bokeh')
dynspread.max_px=1
dynspread.threshold=0.5
shade.cmap="#30a2da" # to match HV Bokeh default
data_smFISH = data
data_smFISH["Gene"] = data_smFISH["Gene"].astype("str")
dic_data = {i : hv.Points(data_smFISH[data_smFISH["Gene"].apply(lambda x: x == i)]
[["X_coordinate", "Y_coordinate"]]) for i in data["Gene"].unique()}
Nd = hv.NdOverlay(dic_data, kdims=["Gene"])
agg = datashade(Nd, aggregator=ds.count_cat('Gene'))
dynspread(agg, how = "over")

[jlstevens]

Thanks for reporting this.

We should expose as much functionality from datashader as we can so I'm assigning this issue to the next milestone. I don't expect it will take too much effort to support. @philippjfr Can you confirm that there is no easy way to set min_alpha currently?

[Mossi8]

Yes, I have read through the datashader.py script and at some point it transforms the RGBA to RGB (as I understood), therefore, there isn't any alpha or min_alpha parameter to optimize. Which practically means that the plot is the same as the equivalent to using datashader with min_alpha = 0.

I have also tried different things, but nothing solved the problem.

Thank you for your time

[jbednar]

Right; this is an omission from holoviews.operation.datashader.shade(), which was probably written before we added min_alpha to datashader. Should be trivial; I'll make a PR.

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