Sunday, December 01, 2019

A little something called Super-resolution [part deux]

After looking at handheld super-resolution image making a friend suggested that image stitching, rather than image stacking, could lead to useful improvements in image file size.  So, naturally, I had to take a closer look.

Before we look at the results, let's compare the two approaches.

Image Stacking for Super-Resolution -
  • Shooting handheld
  • Fire-off a half dozen images of the scene using a high-speed multiple exposure mode
  • In processing -
    • Align the images
    • Stack the images as layers in PS or Gimp
    • 2X bi-cubic 600DPI (minimum) up-rez every image
    • Set the opacity of each layer
    • Flatten the image
    • Apply an Unsharp Mask of 2 pixels
Pros -
  • Image noise is reduced significantly
  • Light/Dark transition zones are smooth and "creamy"
  • 2x up-rez gives a somewhat useful, if not exactly brilliant, increase in viewable detail
  • If all else fails, at least there is an image to begin with, up-rez'd or not.  

Cons -

Image Stitching for Super-Resolution -
  • Set exposure to something at accurately expresses highlight and shadow detail - use this combination of aperture, shutterspeed, and ISO for taking all the "section"
  • Shoot small-ish "sections" of the scene where images sequentially overlap eachother, making sure you've covered the entire scene (see: Breznier Method)
  • In processing -
Pros -

Cons - 
  • Images need to be planned with final images visible only after processing - which means it really helps to "pre-visualize" a scene
  • Depending on the software and accuracy of shooting image "sections" there may be distortions (example of failing to accurately rotate the lens around the nodal point)
  • Limited to static subjects
  • Slow setup time - suggest manual exposure metering to help the "sections" stitch correctly and to keep the overall final scene exposure even and correct

Comparison of Resultant Images -

[If you click on the image it'll take you to the Flickr hosting site. Once there, look at the file at full resolution. In many cases the differences between lenses is small and likely can't be seen until you take a squint at the comparison at 100 percent.]

Super-Resolution ~ comparing stitched and "cubic uprez"

The obvious conclusion is this - even though the stitched image is 1400 pixels shorter in the long dimension than the stacked up-rez'd image, the stitched image clearly resolves small details better than the stacked image.

My friend is, of course, correct.  Check out the section titled "4-Way Focusing Rail..."  The image stitch approach can be very nice indeed, but only if you plan ahead.

No comments: