Question About Lossless Transformations

[esaleris]

Nabocorp -

Thank you for a wonderful product. I really appreciate it and was happy to pay for the full version today!

In any case, I do have a question. I was doing lossless rotation on my pictures, when I noticed that rotating a picture 90 degrees four times (which returns it to the original orientation) resulted in an image that was not the same size as the original (in this case, a difference of 300 bytes).

I remembered reading something about EXIF thumbnails so I made sure to remove all of them before making this test. Theoretically, shouldn't the sizes match up? In addition, when I manually change an EXIF tag, and then do "Auto-Transform" four times, the files are different also... but with different file sizes... Shouldn't these methods be exactly the same?

Thanks for taking the time to answer, in advance...

[nabocorp]

Hello,

I already noticed this but can't tell exactly why... Probably in the internal JPEG structures describing the image. I tried to do the same thing with ACDSee and gets the same result: file sizes are different. If you really want to make sure the transformations are lossless you can open both files in Paint Shop Pro and perform image arithmetics: substract one image to the other (check all channels, divisor = 1, bias = 0, and uncheck clip color values). You should get a 100% black images saying that all pixels are the same. Of course if you open an image in Paint Shop Pro and manually rotate it by 90° (and save it each time to JPEG and close it and reopen it) and finally do the substract stuff you'll see that the difference image is not black at all!


Regards,
nabocorp

[esaleris]

Hey nabocorp and all -

Thanks for addressing my question so quickly. Here are some comments after experimenting with it.

Topic 1:

I went ahead and did the test in Fireworks (don't have PSP) and tested the implementation of the "lossless rotation" by turning picture about 5 times full circle (360). The images were identical, whatever the file size may have been.

In addition, I noticed something: pictures will always have the same byte size when rotated. Say you rotated 1.jpg 4 x 90 degrees. It is the exact same as 1.jpg rotated 80 x 90 degrees. I did an "fc.exe" compare and no file differences were detected.

Topic 2:

Cam2pc uses the jpegtran code for the lossless transformations, right? Were there any modifications made? I ask because Windows XP Image and Fax Viewer also implements jpegtran, but its rotated images have a slightly different size from when it is done with cam2pc. I'm not sure if all softwares that use jpegtran code come up with the same rotated image sizes.

Topic 3:

I manually edited the EXIF data for one of the pictures and did "Auto-transformation" a couple of times. EXIF Orientation 8; then Auto-transform. The sizes were a bit different again. How is "Auto-transformation" different from the default lossless?

Topic 4:

Could we request that lossless transformations be performed with the timestamp kept? When I do a lossless transformation, the timestamp changes. I usually just go to the Image menu and change it back, but by default, I think it would make sense to keep the date, no?

Thanks for your time! Later!