conversion between alpha channel and grayscale on ipad

[sobriquet]

There are online image editors and many use flash, so they don't
work on the ipad. There are also html5 versions and they do work,
like Polarr (v2.polarr.co), which works for me in the Dolphin
browser.

Does anyone know of any online html5 image editor (or ios app)
that allows one to convert between black and white images
and an alpha channel with transparency to mask things out
(so black is transparent, gray is partially transparent and
white is opaque)?

I can use the magic wand in the outdated PSTouch app and
selectively cut out a color, and even introduce some gradual
transparency by feathering the selection before cutting it out,
but it's hard to obtain a kind of mask that is equivalent to
a grayscale image (where shades of gray represent grades of
transparency).

For instance, say I want to mix these images:
https://dumpyourphoto.com/photo/QwXmtl4W3C
https://dumpyourphoto.com/photo/ubpBNSunWl

I might convert the first image to grayscale as a basis for
a mask:
https://dumpyourphoto.com/photo/jGZapwAdDi

When this is applied as a layer mask to the first image to
preserve certain areas I get an image with graduated
transparency:
https://dumpyourphoto.com/photo/IDzlJyp8Iw

It can be put on top of the second image to obtain a mixed
result:
https://dumpyourphoto.com/photo/puSrvniSE6

[sobriquet]

It seems the MediBang Paint app does this. It can convert an
image to a selection based on brightness value (as well as
converting to grayscale).
Pretty extensive feature set in the app, though drawing
with the WICS2 stylus doesn't seem quite as smooth as
in the Procreate app.