Lossless rotation by plug-in

[yannB74]

It would be much appreciated if the parameters appearing in the box for lossless rotation (by plugin) were given some explanations. Most of them are not obvious.
Since the plugin can be used for other transformations all these requested clarifications could be of help.
Additionally, if the Help section relative to the subject was updated with the requested clarifications it would be ideal.
Thanks,
YB

[Bhikkhu Pesala]

I don't use a camera, but I assume that anyone who does will have a pretty good idea what each item means. Which ones are unclear to you?

[JendaLinda]

I did my own research of the features, so I can provide some description.

The None transformation allows to do other operations than rotations / flips.

The Auto-rotate feature rotates the picture according to the orientation tag. This tag is usually set by phone cameras.

The file size may change during the transformation, because the data are rearranged. It's more complex to explain, but no image data are lost.

The Perfect transformation applies only to pictures, where dimensions are not multiples of 16. The remaining columns/rows cannot be rotated. If the picture dimensions are not multiples of 16 and the Perfect transformation is enabled, the plugin refuses to rotate/flip such picture.

The progressive JPG option allows progressive loading of the picture on a slow network connection. At first, a low resolution version of the picture is displayed, then the picture gets more and more detailed.

Optimize JPG file, this will rearrange the file structure and store it more efficiently. No data will be lost.

DPI is useful only for printing. It has no purpose in photos. DPI may be useful in scanned documents, where it stores the original scanner's DPI, so the document can be printed in the exact physical size as the scanned original.

JFIF marker is a style of file header. Most JPEGs are in JFIF format, so writing the JFIF marker should be enabled for general use.

The APP markers are metadata in the file. Most commonly it's the EXIF including information about the camera, date/time, rotation, thumbnail, GPS coordinates etc.

[yannB74]

Thank you JendaLinda, it is pretty useful.
YB

[j7n]

The Optimize checkbox selects between two variations of lossless compression: a fixed huffman table, and a table calculated for the current file. Counter-intuitively to the name, unchecking the optimization on already "optimal" files will output a bigger file that is no longer optimized. There is a third possible uncommon encoding, Arithmetic, that this plugin can't write. It is more efficient with storage space, but much slower to save and open. The plugin can be used to speed up loading of such files.

Progressive encoding is also slightly more efficient but slower than baseline.

The JFIF marker is one out of two structures that records the DPI value. The other is Exif. Without a JFIF marker, the entered DPI value may not be saved and be blank upon reloading. Absence of JFIF is required by some special software, and a very uncommon encoding in RGB color space (normal images are transformed into YCbCr space).

Among the other app markers is the color profile which may exist in photos to apply a correction to better match the image to the current display, and also an Adobe tag, which, if exists, will prevent the JFIF marker from being written. This is done by Irfan to be compatible with very uncommon RGB images written by some Adobe software.

[yannB74]

Thank you J7n for the added info but undoubtedly it needs some clarification in the text of the Help section to advise the user correctly.
It is easy to get lost and not doing the right thing.
Thks
YB

[Su2021]

For some reason, JPG Loseless Transform, changes the image section even when "Trabformation: None" is selected.
For example if you wish to only change the EXIF date, if will change most of the file content, including ofcourse the file size. A binary comparision will show that immediately.
This is ofcourse not needed, as changing only the date string with some hex editor can prove.

Even stranger, when selecting only "Apply original EXIF date/time to new file" (leaving the other options empty, tranformation None and "Keep all APP markers") will still change the file.
Anyone knows on any options that will let copy the EXIF date to the file system date without modifying the JPG files?

[JendaLinda]

The JPEG file is repacked. Depending on the "optimize" checkbox, the optimized or default Huffman tables are used to compress already quantized image data. This affects the file size, but doesn't affect image quality. Optimized Huffman tables make the file smaller, but there's no impact on compatibility or decoding speed.

Anyway, you can binary compare the pictures after decoding to pixels.