you probably know what this is.
this is a luminosity mask, sometimes called luma matte. we use them daily when working in photoshop, compositing video layers or rendering 3d. we love them. we hate them.
matte can be pulled from image with few clicks or crafted by hand over a span of many hours.
it allows objects to be hidden or revealed, while itself remains invisible. even though it might be the most time consuming part of the whole process.
depending on their purpose, masks can be very rough and abstract or very detailed and descriptive. or both at the same time.
over the years i learned to appreciate them as works of art. i often find them more interesting than the final output of the project they are part of.