Preconditions for lossless rotating

See also: How does rotating photos work?

To be able to perform perfect rotation, your photo width and height in pixels need to be of a standard size. Usually this is not a problem:

[…] most digital cameras produce images with dimensions that are exact multiples of the MCU size.

Ammara

When using the free version of JPEG Autorotate, your photos will not be rotated unless a perfect result is guaranteed.

With the Boosted and Pro licenses, you have two options:

• “Require perfect metadata and dimensions for rotation”
  • When you uncheck this option, also photos with non-standard sizes are rotated.
• “When images are non-standard dimensions, trim borders”
  • When you check this option, strips of the photo that cannot be losslessly rotated, will be removed from the borders of the photo.
  • Strictly speaking the rotation is no longer lossless if you check this. Often this is still what you want to do though, as the ugly strip of unrotated pixels is removed and the rest of the image still gets rotated without quality loss.

Photo sizes that can be rotated losslessly

Credit: Ammara / web archive link

Aspect Ratio 3:2		Aspect Ratio 4:3
MCU 8 x 8	MCU 16 x 16	MCU 8 x 8	MCU 16 x 16
24  x  16 48  x  32 72  x  48 96  x  64 120  x  80 144  x  96 168  x  112 192  x  128 216  x  144 240  x  160 264  x  176 288  x  192 312  x  208 336  x  224 360  x  240 384  x  256 408  x  272 432  x  288 456  x  304 480  x  320 504  x  336 528  x  352 552  x  368 576  x  384 600  x  400 624  x  416 648  x  432 672  x  448 696  x  464 720  x  480 744  x  496 768  x  512 792  x  528 816  x  544 840  x  560 864  x  576 888  x  592 912  x  608 936  x  624 960  x  640 984  x  656 1008  x  672 1032  x  688 1056  x  704 1080  x  720 1104  x  736 1128  x  752 1152  x  768 1176  x  784 1200  x  800 1224  x  816 1248  x  832 1272  x  848 1296  x  864 1320  x  880 1344  x  896 1368  x  912 1392  x  928 1416  x  944 1440  x  960 1464  x  976 1488  x  992 1512  x  1008 1536  x  1024 1560  x  1040 1584  x  1056 1608  x  1072 1632  x  1088 1656  x  1104 1680  x  1120 1704  x  1136 1728  x  1152 1752  x  1168 1776  x  1184 1800  x  1200 1824  x  1216 1848  x  1232 1872  x  1248 1896  x  1264 1920  x  1280 1944  x  1296 1968  x  1312 1992  x  1328 2016  x  1344 2040  x  1360 2064  x  1376 2088  x  1392 2112  x  1408 2136  x  1424 2160  x  1440 2184  x  1456 2208  x  1472 2232  x  1488 2256  x  1504 2280  x  1520 2304  x  1536 2328  x  1552 2352  x  1568 2376  x  1584 2400  x  1600	48  x  32 96  x  64 144  x  96 192  x  128 240  x  160 288  x  192 336  x  224 384  x  256 432  x  288 480  x  320 528  x  352 576  x  384 624  x  416 672  x  448 720  x  480 768  x  512 816  x  544 864  x  576 912  x  608 960  x  640 1008  x  672 1056  x  704 1104  x  736 1152  x  768 1200  x  800 1248  x  832 1296  x  864 1344  x  896 1392  x  928 1440  x  960 1488  x  992 1536  x  1024 1584  x  1056 1632  x  1088 1680  x  1120 1728  x  1152 1776  x  1184 1824  x  1216 1872  x  1248 1920  x  1280 1968  x  1312 2016  x  1344 2064  x  1376 2112  x  1408 2160  x  1440 2208  x  1472 2256  x  1504 2304  x  1536 2352  x  1568 2400  x  1600 2448  x  1632 2496  x  1664 2544  x  1696 2592  x  1728 2640  x  1760 2688  x  1792 2736  x  1824 2784  x  1856 2832  x  1888 2880  x  1920 2928  x  1952 2976  x  1984 3024  x  2016 3072  x  2048 3120  x  2080 3168  x  2112 3216  x  2144 3264  x  2176 3312  x  2208 3360  x  2240 3408  x  2272 3456  x  2304 3504  x  2336 3552  x  2368 3600  x  2400 3648  x  2432 3696  x  2464 3744  x  2496 3792  x  2528 3840  x  2560 3888  x  2592 3936  x  2624 3984  x  2656 4032  x  2688 4080  x  2720 4128  x  2752 4176  x  2784 4224  x  2816 4272  x  2848 4320  x  2880 4368  x  2912 4416  x  2944 4464  x  2976 4512  x  3008 4560  x  3040 4608  x  3072 4656  x  3104 4704  x  3136 4752  x  3168 4800  x  3200	32  x  24 64  x  48 96  x  72 128  x  96 160  x  120 192  x  144 224  x  168 256  x  192 288  x  216 320  x  240 352  x  264 384  x  288 416  x  312 448  x  336 480  x  360 512  x  384 544  x  408 576  x  432 608  x  456 640  x  480 672  x  504 704  x  528 736  x  552 768  x  576 800  x  600 832  x  624 864  x  648 896  x  672 928  x  696 960  x  720 992  x  744 1024  x  768 1056  x  792 1088  x  816 1120  x  840 1152  x  864 1184  x  888 1216  x  912 1248  x  936 1280  x  960 1312  x  984 1344  x  1008 1376  x  1032 1408  x  1056 1440  x  1080 1472  x  1104 1504  x  1128 1536  x  1152 1568  x  1176 1600  x  1200 1632  x  1224 1664  x  1248 1696  x  1272 1728  x  1296 1760  x  1320 1792  x  1344 1824  x  1368 1856  x  1392 1888  x  1416 1920  x  1440 1952  x  1464 1984  x  1488 2016  x  1512 2048  x  1536 2080  x  1560 2112  x  1584 2144  x  1608 2176  x  1632 2208  x  1656 2240  x  1680 2272  x  1704 2304  x  1728 2336  x  1752 2368  x  1776 2400  x  1800 2432  x  1824 2464  x  1848 2496  x  1872 2528  x  1896 2560  x  1920 2592  x  1944 2624  x  1968 2656  x  1992 2688  x  2016 2720  x  2040 2752  x  2064 2784  x  2088 2816  x  2112 2848  x  2136 2880  x  2160 2912  x  2184 2944  x  2208 2976  x  2232 3008  x  2256 3040  x  2280 3072  x  2304 3104  x  2328 3136  x  2352 3168  x  2376 3200  x  2400	64  x  48 128  x  96 192  x  144 256  x  192 320  x  240 384  x  288 448  x  336 512  x  384 576  x  432 640  x  480 704  x  528 768  x  576 832  x  624 896  x  672 960  x  720 1024  x  768 1088  x  816 1152  x  864 1216  x  912 1280  x  960 1344  x  1008 1408  x  1056 1472  x  1104 1536  x  1152 1600  x  1200 1664  x  1248 1728  x  1296 1792  x  1344 1856  x  1392 1920  x  1440 1984  x  1488 2048  x  1536 2112  x  1584 2176  x  1632 2240  x  1680 2304  x  1728 2368  x  1776 2432  x  1824 2496  x  1872 2560  x  1920 2624  x  1968 2688  x  2016 2752  x  2064 2816  x  2112 2880  x  2160 2944  x  2208 3008  x  2256 3072  x  2304 3136  x  2352 3200  x  2400 3264  x  2448 3328  x  2496 3392  x  2544 3456  x  2592 3520  x  2640 3584  x  2688 3648  x  2736 3712  x  2784 3776  x  2832 3840  x  2880 3904  x  2928 3968  x  2976 4032  x  3024 4096  x  3072 4160  x  3120 4224  x  3168 4288  x  3216 4352  x  3264 4416  x  3312 4480  x  3360 4544  x  3408 4608  x  3456 4672  x  3504 4736  x  3552 4800  x  3600 4864  x  3648 4928  x  3696 4992  x  3744 5056  x  3792 5120  x  3840 5184  x  3888 5248  x  3936 5312  x  3984 5376  x  4032 5440  x  4080 5504  x  4128 5568  x  4176 5632  x  4224 5696  x  4272 5760  x  4320 5824  x  4368 5888  x  4416 5952  x  4464 6016  x  4512 6080  x  4560 6144  x  4608 6208  x  4656 6272  x  4704 6336  x  4752 6400  x  4800

Technical explanation

Ammara explains:

JPEG compresses images in blocks (tiles) that are usually 8 x 8 or 16 x 16 pixels, known technically as the “MCU” size (Minimum Coded Unit). If the image width or height are not exact multiples of the MCU size then “Partial MCUs” are added.

---

Consequently, when [editing] images it is a good idea to choose image sizes that result in whole MCU multiples (the table below can help to choose suitable sizes for various aspect-ratios and MCU’s). 

If either of the dimensions are not exact MCU multiples then special handling is required, and there are various choices:.

1) Trim the Image to the Nearest MCU Boundary.

Only complete MCU’s are rotated, and partial MCU’s are discarded (i.e. trimmed/cropped). This is technically not lossless, since a few pixels are discarded and hence the operation is not fully reversible, but it is often visually preferable to option 2, and it does retain the original quality in the remaining part of the image.

2) Don’t Trim – Leave Partial MCU’s “Unrotated”.

This results in an odd-looking strip of pixels on one or two sides of the image, however it is reversible (and therefore technically lossless) and retains the overall dimensions of the original image.

3) Fall-Back to “Lossy” Rotation.

If the width and height need to be retained, and the odd-looking “strip” produced by option 2 is not visually acceptible, then the only option is to rotate the uncompressed image, then re-compress to JPEG format, causing a (usually slight) loss of quality due to generational loss.

For further defails, see: Ammara / web archive link