A line drawing is an image made up of single or multiple lines. It doesn't matter if the lines are straight or curved, thick or thin. There is no shading or color variation in these photos. The lines in a line drawing can be black and white or several options. It still usually uses a single color, which could be black but isn't required or a mixture of colors.
Every color scheme in a vector line design or raster image is worthy of generating vector line art. Every shade has a line drawn through it by the creators. The more color variants in an image, the more lines will be formed.