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While the approach described in my previous posting probably is a better approximation of the effect seen in the image posted by the OP, when I want text to be visible against a background that goes from very dark to very light tones, I usually prefer an approach based on the difference blend mode, as shown immediately below.  Even with text that is intentionally jammed together very tightly (for this demo), you can still make out individual characters, and I feel it retains detail in the image to a greater degree. 

However, there certainly are situations where the previous approach would work better visually.  The previous approach reminds me of a modern version of the old line printer ASCII art.

T