The idea is similar to photo mosaic in that the pixels of a reference image are replaced with something. Here, each pixel is replaced with an ASCII symbol that best represents the intensity, for example white space or a full-stop have maximum whiteness, so depending on size of the set of symbols used, the maximum intensity range(white) in greyscale can be replaced by white space and lowest inensity(black) be replaced with '@' or '#' .
And the result is a text instead of an image. The result is saved in asci.txt with variants.