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There is a pretty easy image which I want to behave monochrome:

enter image description here

You can see on the left, and also on top there is one thin line which seems semi-transparent.

I dropped the colors by selecting Image > Mode > Grayscale, then selecting Image > Mode > Duotone (with only black as tint).

(I'm not sure this is the right way to my goal)

So, there are those semi-transparent sides, but okay, I don't care, because if I drop a background behind it they are gone:

enter image description here

However, how can I limit the colors to be really duo? I.e. only full black, and full white (and maybe 100% transparent as third)?

Back to my image: I want to reduce the size: Image > Image size (with Automatic resample):

Now even with the background:

enter image description here

And also without the background:

enter image description here

I got a massive undeniable semi-transparency at the sides. Now it's not hidden by adding a solid background underneath.

So: my goal is to have an image with only Black and White (no intermittent shades), with optionally a 100% transparency. This way I will be able to resize the image without those annoying transparency effects on the sides (I guess/hope).

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Black and white is traditionally not a duotone. It's one color - black. White is really never seen as a color. On a press white doesn't exist, except in very special circumstances. If you want white pixels for screen display as well as transparency, then you sort of have to use Greyscale mode.

Duotone is not necessary and is of no benefit here. Duotones restrict the number of colors, but do not restrict tints (%) of those colors. Greyscale is the best you can do.

If you want only black and white pixels in a raster image then "Bitmap" is the proper mode in Photoshop... convert to greyscale, (cuz ya have to) then to Bitmap. However, this will not allow transparency. In Bitmap mode, white is seen as being transparent. So, again, Greyscale is the best you can do if you need black, white, and transparent pixels.

When resizing, to prevent the introduction of grey pixels caused by anti-aliasing, you will want to use the Nearest Neighbor interpolation method.

This prevents the introduction of partial opacity pixels (% of colors) which are used to "smooth" edges (anti-alias) and ensures all pixels are solid. In your use description, it would keep all pixels either black, white, or transparent - no greys.

enter image description here

With transform handles active, the interpolation method is in the Control Bar across the top of the screen...

enter image description here

Comparison....

enter image description here

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  • Amazingly well described. Thank you!
    – Daniel
    Oct 4 '21 at 5:31

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