Why the output image will be rotated?

Question

I build a watermark creator by OpenCV. The program could create watermark in the 4 corner of a source image. I haven\'t change the source image direction and the output imag

Answer 1

As @MarkRansom says, some cameras store an EXIF "Orientation" field in the headers of images.

You have a few options, using exiftool, jhead or ImageMagick all of which are installed on many Linux distros and are available for macOS and Windows.

So, to see the field with exiftool:

exiftool -Orientation image.jpg
   Orientation                     : Horizontal (normal)

Or with jhead:

jhead -v image.jpg | grep -i orien
Orientation = 1

Or with ImageMagick:

identify -verbose image.jpg | grep -i orient
   Orientation: TopLeft
   exif:Orientation: 1

So, you could do that, and correct for it with OpenCV using a rotate(). Or you could correct it with ImageMagick outside of OpenCV:

mogrify -auto-orient image.jpg             # correct orientation of single image
convert image.jpg -auto-orient result.jpg  # alternative, more verbose but same as above  
mogrify -auto-orient *.jpg                 # correct orientation of all images

Beware that mogrify will change all your images irreversibly, so be sure that you make a backup before testing.

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Note that jhead is probably the easiest to install, lightest weight option of the three in Windows. In Linux or macOS, exiftool is the lightest weight as it is just a Perl script. ImageMagick is vastly more powerful, and heavier weight and somewhat harder to install - IMHO.

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