[pmwiki-users] [SPAM] [SPAM] [SPAM] Re: orientation flag and images

[Petko Yotov]

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On 08/08/2019 08:00, jdd at dodin.org wrote:
> Le 07/08/2019 à 23:28, Petko Yotov a écrit :
>> PmWiki doesn't do any image orientation/rotation at all. What you see 
>> is
>> done by the browser, only.
> 
> but same thing in Firefox and Chromium :-(, and why do the thumnail do
> not have the same orientation than the very same open image? Firefox
> do not see the thumb like an image?

The picture has an embedded thumbnail inside, and the thumbnail does not 
have the same orientation tag as the picture. Here is the list of your 
relevant orientation tags:

   $ exiv2 -P EIX web_P1020187* | grep Orient
   web_P1020187-2.jpg  Exif.Image.Orientation            top, left
   web_P1020187-2.jpg  Exif.Panasonic.CameraOrientation  Rotate CCW
   web_P1020187-2.jpg  Exif.Thumbnail.Orientation        left, bottom
   web_P1020187-2.jpg  Xmp.tiff.Orientation              top, left
   web_P1020187.jpg    Exif.Image.Orientation            left, bottom
   web_P1020187.jpg    Exif.Panasonic.CameraOrientation  Rotate CCW
   web_P1020187.jpg    Exif.Thumbnail.Orientation        left, bottom
   web_P1020187.jpg    Xmp.tiff.Orientation              left, bottom

You can see that web_P1020187-2.jpg has its first pixel at the top left, 
but its thumbnail has its first pixel bottom left. It is possible when 
the browser shows a tiny-size picture it takes the embedded thumbnail 
instead of scaling down the large picture.

After some research, it looks like modern Firefox and Chrome use the 
orientation tag only when the picture is opened directly. When the 
picture is embedded in a web page, they don't use that tag and display 
the physical pixels.

>> Browsers do not always behave identically, so I recommend using at 
>> least
>> `exiftran -a` on the uploaded (or soon to be uploaded) pictures.
> 
> well, I don't want any tool to do the rotation for me. I do it
> manually in digikam (but unedited images do not have the flag
> normalized, I guess, I also asked to digikam list)

I don't think you manually type the raw hex bytes to rewrite the files, 
you very likely use some software in your digital camera, or in your 
smartphone.

It is possible there is an omission/bug in the software and it only 
rotates the picture and not the thumbnail. It doesn't even rotate the 
picture but only changes the orientation tag.

At any rate, this is not something controlled or affected by PmWiki in 
any way.

>> Note that your desktop picture viewer (gPicView, gThumb, GIMP,
>> kolourpaint among others) and possibly your file manager may rotate 
>> the
>> pictures while viewing, so you wouldn't know if they are actually 
>> saved
>> upright or only have an orientation tag.
> 
> yes. Some app do, some others don't and even some looks at the ration
> L/l to try :-(

That's why when I have to deal with this I use a free/libre software 
tool to physically rotate the pixels of the image in the correct 
orientation before publishing it. GNU/Linux tools like exiftran, 
jpegtran and others can do lossless rotations (without reducing the 
picture quality by re-compression). I haven't noticed yet thumbnails 
rotated differently from the main picture, but I'll check this in the 
future, and probably remove the embedded thumbnails, with `exiftool 
-all= photo.jpg`.

> I use default Firefox setup (because I try to
> cope with my reader's install, not mine)

I agree, that's why I suggest fixing the pictures on your computer 
before uploading them.

Petko