[gthumb-list] The Exif Orientation tag, again

[Stefano Pettini <stefano pettini gmail com>]

Dear all,

I suppose you already discussed before this topic, but I think that
the Orientation tag processing in gthumb should be slightly cleaned
up.

This is how I see the tag: it's merely an hint to the viewing software
on how to display the picture. It has nothing to do with the X and Y
resolution of the picture, the X and Y Exif dimensions, or any other
kind of metadata that describes properties with a kind of horizontal
or vertical concept. I see it as an extra hint to take into account
one second before showing up a picture. All the image data and
metadata should always be valid, with or without the Orientation tag.
With this I mean that every Exif tag should always match how the JPEG
is physically encoded, not how it's meant to be displayed. Exif
dimension and X/Y resolution and so on should always match real
parameters.

If we agree, let's go on :-)

This is for example how Nikon cameras work. I have untouched images,
captured keeping the camera in vertical position, with:

General, Dimension: 2896x1944.
Exif, PixelXDimension: 2896.
Exif, PixelYDimension: 1944.
Orientation: left, bottom (= vertical)

Looking at the code and behavior of the latest gthumb, we reset the
Exif orientation tag during lossless rotations. I think it's wrong and
this has the effect of not applying some transformations or applying
them twice.

If I can suggest a general policy to handle the Orientation tag:

1) When importing pictures, if a "reset orientation" option is
selected, images should be rotated or mirrored to reset the
orientation, updating X/Y Exif fields.
2) When performing lossless rotation or mirror operations:
- (option 1) The orientation tag is left unchanged and the picture is
physically rotated, updating the X/Y Exif fields.
- (option 2) Only the orientation tag is updated to reflect the
transformation, nothing else is changed.
3) When cropping the image, or performing operations that affect image
dimensions, the orientation should be reset and the physical image and
X/Y Exif fields updated.
4) When performing operations that don't affect image dimensions, like
filters, the orientation tag and other Exif X/Y fields should be left
unchanged.
5) Provide an explicit option to remove the orientation flag, that
behaves like point 1.

Or (simplified policy):

1) When importing pictures, if a "reset orientation" option is
selected, images should be rotated or mirrored to reset the
orientation, updating X/Y Exif fields.
2) When performing lossless rotation or mirror operations, the
orientation tag is left unchanged and the picture is physically
rotated, updating the X/Y Exif fields.
3) The orientation is reset, removing the tag and update X/Y Exif
fields, when opening the Edit window or just before any operation that
changes the image.

What do you think?

I know it's a topic you already discuss, I still have some pictures
rotated by gthumb 2.10.x with invalid exit information inside :-)

Thanks.

Regards,
Stefano