> > I'm not too sure if SuperJPG is unique in this, but it can rotate JPGs losslessly :) > > What does it do if the dimensions are not multiples of 8 pixels? > Brian Pegasus Imaging Corporation The JPEG Wizard does this (amogst lots of other things). "Lossless Rotation of JPEG data is applied here. If the input file type is JPEG compressed data, the rotated image viewed on Open is losslessly rotated. There is no *generational loss* introduced in this operation." First though, Remember that at higher compression levels the colour information is coded in bloxks of 16x16 pixels not 8x9. The answer, well JpegWizard losslessly rotates jpegs - except that it crops the dimensions down to the next lowest multiple of 8 (or 16) pixels first starting at upper left = 0,0 Take a typical example, a 5-minute experiment if you like. Canvas size = 175 wide vs 123 pixels tall Version [3] at PS5 JPEG level 3 (4184 bytes) Version [6] at PS5 JPEG level 6 (6644 bytes) Open in jpeg wizard and resave (dumps some of the header crap PS insists on putting in. [3] 3823 bytes [6] 6245 bytes Now, using these two as source files for the test: > 90 degree CW [3] 112 x 175 (3648 bytes) [6] 120 x 175 (6207 bytes) > 180 degree CW [3] 112 x 160 (3462 bytes) [6] 120 x 168 (5885 bytes) > 270 degree CW [3] 123 x 160 (3677 bytes) [6] 123 x 168 (6225 bytes) > Horizontal mirror image [3] 123 x 160 (3643 bytes) [6] 123 x 168 (6076 bytes) > Vertical mirror image [3] 112 x 175 (3614 bytes) [6] 120 x 175 (6056 bytes) Fianlly, repeat the 180 degree rotation > (180 degree CW) * 2 [3] 112 x 160 (3461 bytes) [6] 120 x 168 (5893 bytes) Conclusions. Before the lossless transformations the the image is cropped from the right and/or bottom edge(s) down to the next lower multiple of 8 / 16 bytes. It all depends on the degree of compression applied.
