Whoever made this feature: fuck you and your shit. You are retarded and should be fired from your job!

Yeah, I just finished rotating some fucking pictures which I spent hours taking. And no there is no backups because I did assume this program was done by someone who had brains which actually worked. Fucking retarded dipshit moron monkey coder. FUCK

best thing about it: I was suppose to make panoramic images from them, but now the loss in quality makes all of the images totally worthless

In one sense, the test with Photoshop or similar is excellent. It provides clear feedback regarding whether any _noticeable_ difference is caused by rotation or other re-compression of an image.

In another sense, it still doesn’t really answer the question. After all, JPEG works based on the fact that tiny changes are not noticed by humans.

I wrote a simple program to load two different image files and compare them pixel by pixel, taking into account rotation for one of the image in order to make sure the correct pixels were compared. I then took a number of different-sized pictures, including sizes that are compatible with lossless rotation of JPEGs, copied them, rotated the copies, and then ran them through my program.

In fact, it turns out that the pixels are _not_ preserved exactly. In most cases, the RGB values wound up only off by 1 or 2, often only for one channel (e.g. the red channel might have dropped from 254 to 253, while the other channels remained unchanged). These are changes that are likely imperceptible, whether viewing the image itself or a Photoshop manipulation intended to highlight changes. But they are there.

So, if you want true lossless rotation, don’t use Windows. Use one of the many standalone tools available that do in fact rotate JPEG images without changing the actual data in the image.

Should help to replace the default right click rotate in File Explorer with your favorite application. If anybody tried it with the listed by raymond applications, please comment if it worked OK

I rotated/saved/rotated/etc a fairly large JPEG 12 times in Windows 7 Photo Viewer, ending up back at the original orientation. Then I loaded that, along with the original image file, into Photoshop and pasted the adjusted image as a layer on top of the original photo.

Using the ‘Difference’ overlay mode and then flattening the layers gave a pure black result. I checked this using Image>Adjustments>Equalize and there is definitely only one brightness level in the image (black, zero). If there was as much as a pixel different between the two source layers, that would have been visible.

Andre

Lossless rotation only works on JPEGS with dimensions divisible by 8. AFAIK WLPG (is that WPV?) rotates losslessly if possible, if not then it’ll do it’s best.

I’d imagine other software works similarly.