Replacing whiteness with transparency

“I want to replace the whiteness in pixels with transparency, basically for lighter pixels to be made proportionally as transparent.”

OK, no problem! Open up Photoshop.

Start with your flattened image

1. Place embedded > import image

Add Saturation/Hue or Threshold adjustment layer

2. Layer > New Adjustment Layer > Hue/Saturation > turn down Saturation until image has no color

OR

3. Layer > New Adjustment Layer > Threshold > will result in 100% black or 100% white results

Add Invert adjustment layer

4. Layer > New Adjustment Layer > Invert

Add Curves adjustment layer

5. Layer > New Adjustment Layer > Curves > tweak Curves until everything you’d like to be solid/opaque is white, and everything you want fully transparent is black

Select image then copy the result

6. CTRL + A (select entire image) and CTRL + SHIFT + C (copy the combined grayscale result)

Create a layer mask on original layer

7. Layer > Layer Mask > Reveal All

Enter direct edit mode

8. ALT + Click in the layer mask icon

Paste the black and white image

9. CTRL + SHIFT + V ("paste in place")

Exit layer mask direct edit

10. Click original layer

Finish!

11. Hide all adjustment layers (unless you used Threshold layer, keep Threshold layer visible)

(I originally learned the trick from this video by Uni Photoshop Skills! Check them out, the channel has a lot of useful photoshop tutorials.)