“ Sometimes you have to make a judgement about whether a dark area is image, or if it's background that has been darkened during processing. Generally, accidentally-enhanced background looks like a mid-grey or yellow/red tinted, soft-edged splodge, compared to darker-grey harder-edged areas that were originally ink. ”

“ Working from dark to light makes it less likely that you'll end up with grey background area that can't be distinguished from genuine grey image content on colour alone. In this case, I chose a point just above the centre of the roofline. I did not choose the darkest point in the background, because there is an area of the roof which is light enough that it would be removed by that. This means that there will some particularly dark areas of the background that we have to adjust manually later on. ”

“ You will see a histogram of the values in the image. Note that this is in "logarithmic" mode () —the right hand button, otherwise the white background will dominate the graph:
We can see a few things here:
There is a large spike at the right edge - this represents the many white pixels. Sometimes, if the colour erasure has left more light-coloured non-white pixels behind, you may see a small "hump" at the top end. There are no pixels darker than a certain value (the graph is empty below about the 1/5 mark) The brightness of the non-white pixels is fairly evenly spread. ”