Wrong EXIF Orientation converting from AVIF to JPG

[joojoooo]

Converting a JPG image to AVIF using avifenc and then converting the AVIF back to JPG using avifdec will write the wrong Orientation in the EXIF data.

Version

avifenc -V
Version: 1.0.4 (dav1d [dec]:1.4.1, libgav1 [dec]:0.18.0, aom [enc/dec]:v3.8.2, rav1e [enc]:0.7.1 (UNKNOWN), svt [enc]:v1.7.0)
libyuv : available (1883)

JPG to AVIF

avifenc -q 70 input.jpg output.avif
Directly copied JPEG pixel data (no YUV conversion): input.jpg
Successfully loaded: input.jpg
AVIF to be written: (Lossy)
 * Resolution     : 4000x2252
 * Bit Depth      : 8
 * Format         : YUV420
 * Chroma Sam. Pos: 0
 * Alpha          : Absent
 * Range          : Full
 * Color Primaries: 2
 * Transfer Char. : 2
 * Matrix Coeffs. : 6
 * ICC Profile    : Present (632 bytes)
 * XMP Metadata   : Absent
 * Exif Metadata  : Present (33975 bytes)
 * Transformations:
    * irot (Rotation)      : 3
 * Progressive    : Unavailable
Encoding with AV1 codec 'aom' speed [6], color quality [70 (Medium)], alpha quality [100 (Lossless)], tileRowsLog2 [0], tileColsLog2 [0], 1 worker thread(s), please wait...
Encoded successfully.
 * Color AV1 total size: 1367241 bytes
 * Alpha AV1 total size: 0 bytes
Wrote AVIF: output.avif

* irot (Rotation) : 3 is the angle and means (90 * ANGLE) degree rotation anti-clockwise, which is correct, 90*3=270° anti-clockwise which equals the original JPG Orientation of 90° clockwise (CW) :

$ exiftool -a -u -g1 input.jpg | grep Orientation
Orientation                     : Rotate 90 CW
$ exiftool -Orientation# input.jpg 
Orientation                     : 6

AVIF to JPG

avifdec output.avif upsidedown.jpg
Decoding with AV1 codec 'dav1d' (1 worker thread), please wait...
Image decoded: output.avif
Image details:
 * Resolution     : 4000x2252
 * Bit Depth      : 8
 * Format         : YUV420
 * Chroma Sam. Pos: 0
 * Alpha          : Absent
 * Range          : Full
 * Color Primaries: 2
 * Transfer Char. : 2
 * Matrix Coeffs. : 6
 * ICC Profile    : Present (632 bytes)
 * XMP Metadata   : Absent
 * Exif Metadata  : Present (33975 bytes)
 * Transformations:
    * irot (Rotation)      : 3
 * Progressive    : Unavailable
Wrote JPEG: upsidedown.jpg

Issue

6 and 8 EXIF Orientations are swapped in avifImageGetExifOrientationFromIrotImir()

$ exiftool -a -u -g1 upsidedown.jpg | grep Orientation
Orientation                     : Rotate 270 CW
$ exiftool -Orientation# upsidedown.jpg 
Orientation                     : 8

Resulting in an upsidedown JPG

A temporary "solution" is manually changing Rotation before using avifdec:

$ exiftool -QuickTime:Rotation=90 output.avif

This will create an avif file with a wrong rotation but will make avifdec wrongly output a correct orientation of 6 (Rotate 90 CW) JPG

[y-guyon]

Thank you for your interest in libavif and for your input contribution.

I think I handled Exif orientation based on web resources such as https://www.daveperrett.com/articles/2012/07/28/exif-orientation-handling-is-a-ghetto/#eh-exif-orientation.
Now that I look at the spec at https://archive.org/details/exif-specs-3.0-dc-008-translation-2023-e/page/44/mode/2up, it seems the values of Orientation tag do not match.
Maybe one means "what was the camera orientation when the picture was taken" and the other means "how to rotate the image before displaying it". I am surprised that libavif enc/dec behavior is inconsistent though.

[joojoooo]

I think I handled Exif orientation based on web resources such as https://www.daveperrett.com/articles/2012/07/28/exif-orientation-handling-is-a-ghetto/#eh-exif-orientation.

The first image on this website is wrong, 6 and 8 are swapped. You put comments next to the returned orientations but they don't tell what the EXIF orientation does, they tell what AVIF IROT and IMIR do. The comments are correct and the only wrong values are 6 and 8 which don't match the action described in the comment, 6 is EXIF Orientation for Rotate 90° clockwise, you mistook it for 90° anti-clockwise, but is equal to 8 instead (270° clockwise). I think you just looked at the degrees and didn't think about it being clockwise/anti-clockwise

I am surprised that libavif enc/dec behavior is inconsistent though.

I'm surprised no one noticed for so long. In other parts of the code, orientation is correct:

libavif/src/exif.c

Lines 126 to 129 in 7d2c765

	case 8: // The 0th row is the visual left-hand side of the image, and the 0th column is the visual bottom.
	image->transformFlags = otherFlags | AVIF_TRANSFORM_IROT;
	image->irot.angle = 1;
	image->imir.axis = 0; // ignored

Here you can easily see orientation 8 should correspond to IROT angle = 1

libavif/src/exif.c

Lines 116 to 119 in 7d2c765

	case 6: // The 0th row is the visual right-hand side of the image, and the 0th column is the visual top.
	image->transformFlags = otherFlags | AVIF_TRANSFORM_IROT;
	image->irot.angle = 3;
	image->imir.axis = 0; // ignored

And 6 to angle = 3

Maybe one means "what was the camera orientation when the picture was taken" and the other means "how to rotate the image before displaying it".

No, both QuickTime:Rotation and Orientation are used to display the image correctly, they have the same purpose, if camera manufacturers decide to link it to the orientation the picture was taken that doesn't matter, what matters is it ends up being displayed correctly. This is a website showing how browsers prioritize one of the two based on the image format.

[y-guyon]

The first image on this website is wrong, 6 and 8 are swapped

I disagree. This blog post illustrates what I meant:

I think I looked at the rotation to apply to go from the first row to the second row (90° CW for 8), but it is actually to go from the second row to the third row (90° CCW for 8).

On the other hand, only 6 and 8 are swapped, so you may be right, I got confused.

[joojoooo]

I disagree

I agree you disagree, the images are correct, I got confused myself as well 😂

There is also a duplicate of avifImageGetExifOrientationFromIrotImir() function doing the exact same thing correctly:

libavif/src/exif.c

Lines 159 to 162 in 0c01186

	if (image->irot.angle == 1) {
	if (!(image->transformFlags & AVIF_TRANSFORM_IMIR)) {
	return 8; // The 0th row is the visual left-hand side of the image, and the 0th column is the visual bottom.
	}

[wantehchang]

@joojoooo Thank you for a thorough investigation of this bug.

Yannis: We can consider exposing avifImageIrotImirToExifOrientation() as a public API function and replacing avifImageGetExifOrientationFromIrotImir() with it.