Image Capture makes TIFFs 85% smaller when rotation is applied (?!)

[jdmc]

When I use Image Capture to scan a 4x6 print photo on my flatbed scanner at 600 dpi, with no rotation applied to the bounding box, the resulting TIFF file is generally around 11 MB in size, and the same scan saved as JPEG is about 1.2 MB in size. However, I've discovered that if I use any nonzero amount of rotation on the bounding box, the resulting TIFF is about 1.8 MB in size, while the same scan saved as a JPEG is (again) 1.2 MB in size. In other words, I'm consistently finding that applying rotation shrinks the file size of the resulting TIFF by a whopping 85% — nearly as big a difference as saving in JPEG. Other than the tilt, however slight, the two TIFFs are visually indistinguishable, and both have exactly the same dimensions in pixels. If I open each TIFF in GraphicConverter 12 and re-save each as a TIFF with ZIP compression, both of the resulting files are about the same size as the original zero-rotation TIFF (i.e., the shrinkage of the rotated-capture TIFF goes away).

Here's a small screenshot to illustrate the results of my test:

Again, I have found this result to be consistent across dozens of scans. What on earth is going on here, to make the very-slightly-rotated-capture TIFF so dramatically smaller than the orthogonal-capture TIFF?

 

[jdmc]

The only remotely plausible explanation I've been able to come up with so far is that Image Capture would actually be using JPEG compression inside TIFF files that are captured with a rotated bounding box, but… why would it do that?

 

[Slartibart]

LZW compression is available in TIFF, maybe your initial capture is uncompressed TIFF? You can e.g. check (besides numerous other things) the image (format) details using macOS’s sips.

 

[jdmc]

LZW compression is available in TIFF

Yes, as is ZIP and RLE compression, but none of those comes close to an 85% compression ratio.

maybe your initial capture is uncompressed TIFF?

I'm not capturing an orthogonal scan and then rotating it; I'm capturing an image directly from the scanner with a rotated bounding box, rather than a non-rotated bounding box. All else is the same.

 

[jdmc]

UPDATE: I found a question-and-answer on Stack Exchange/Ask Different — Why does Image Capture scan tiff files with different compression formats? — that seems to confirm that my suspicion is correct: Image Capture does indeed use JPEG compression inside a TIFF captured with a rotated bounding box, but LZW compression if captured with an orthogonal bounding box. I find this utterly bizarre, but there it is.