"[T]he person endowed with hyperphantasia can watch a movie and later watch it in their memory and the two are indistinguishable...."

"The polar opposite is hypophantasia—the much reduced ability to form mental images. These extremes of the 'mind’s eye,' as researchers refer to them, are not so much a disorder, as they are opposite ends on a continuum. Most people—including myself—fit somewhere near the middle of the continuum. But even if you fit somewhere close to hypophantasia, you can with practice improve your mental imagery. Finally sharpening that initial image will get you started on fashioning others and will provide insight into your own personal 'mind’s eye.'"

From "The Complete Guide to Memory: The Science of Strengthening Your Mind" by Richard Restak.

Here's the memory exercise (which is easy for those of us with hyperphantasia and hard if we've got hypophantasia):

On a table or desk, arrange ten items in any configuration.... For this I recommend you include several familiar or even prized objects.... Study the items carefully for three minutes. Now close your eyes and practice picturing them in their specific arrangement. Can you do that? If not, start with five objects. The key is to see them as unique, easily distinguishable from one another, and yet placed in a specific arrangement. When you can do that, study them in depth, one at a time. In this exercise I selected a pen, which I helped design with the Italian luxury company Montegrappa.... Now the pen stands out in enhanced clarity compared to the other objects.... When you perform this exercise, you will notice that you use a kind of high-power mental lens to visualize the details by zooming in and out.... The clearer you can see the object in your mind’s eye, the easier it is to remember.... [C]larity and detail in mental imagery is directly related to the quality of your memory....

Here's the Wikipedia article on hyperphantasia. It sounds enjoyable and useful, but I'm not going to feel bad about not having it, because I see that it is "correlated to several mood disorders, particularly anxiety, major depressive disorder, and bipolar disorder, and having hyperphantasia may exacerbate symptoms of such disorders by subserving ruminating thoughts as well as acting as an 'emotional amplifier.' For example, vivid ‘flash forwards’ to suicidal acts may increase occurrences of suicide." It may also aggravate PTSD, schizophrenia, and Parkinson's disease.