How to make dark gray background on TIF image transparent or white in QGIS [duplicate]

Question

see also: Imported TIF has dark gray background instead of white

That question addresses why the TIF imported into QGIS has a dark gray background when the color set is set to 4096 when exported from PDFCreator. That file's gdalinfo shows NBITS=4

Here, in this question, I would like to know if there is a way to render the dark gray transparent, much in the way that entering a "0" in properties->transparency->no data value->additional no data value will make the black lines transparent.

Alternatively, is there a way to have QGIS render the dark gray as white?

Answer 1

I looked at the histogram, and there is a huge peak at 15 for all bands. Two other tiny peaks are likely the colors of small bits of color. Setting the additional no data value to 15 got rid of all the gray, and left black lines.

However, it also turned full black the bits of color (themselves 'grayed' out just like the background was)

I could not figure out how to leave the color bits with some color. If anyone can tell me how to un-gray the color'd bits, let me know.

I tried playing around with the histogram min/max (I'm not sure if that is a display choice, or actually modifies the underlying image). I tried playing around with adding a custom transparency band (a single line, with entries in the three bands)((a '15' in bands duplicated the effect of the additional no data value of 15). I tried playing the drop down box, none/blue/etc, just at the top of the custom area, but couldn't figure out what it was doing.