There are night lights which don't just emit light but project an entire image (example). If such a device is plugged in before Shabbat, would there be a problem of closing window shades or doing other things to darken a lit room which would make the image visible, having been previously invisible due to the amount of light in the room? Would this transgress the prohibition of writing which includes creating meaningful symbols?
I don't think there is any problem with doing this. You are not creating the image, you are just allowing it to be seen by removing something which prevented you from seeing it. There is a discussion regarding a form of invisible ink that by putting it next to the flame it can be seen, which according to the Pri Megadim is rabbinically prohibited. But there you are actually affecting the writing and revealing it, as opposed to this case where you are merely removing the light which was preventing you from seeing the image.