This is caused by the lossless rotation algorithm used in libjpeg. Rotating in the reverse direction will make the margin go away.
From libjpeg (man 1 jpegtran), which is used for the rotation: > The transpose transformation has no restrictions regarding image > dimensions. The other transformations operate rather oddly if the image > dimensions are not a multiple of the iMCU size (usually 8 or 16 pixels), > because they can only transform complete blocks of DCT coefficient data in > the desired way. > > jpegtran's default behavior when transforming an odd-size image is designed > to preserve exact reversibility and mathematical consistency of the trans‐ > formation set. As stated, transpose is able to flip the entire image area. > Horizontal mirroring leaves any partial iMCU column at the right edge > untouched, but is able to flip all rows of the image. Similarly, vertical > mirroring leaves any partial iMCU row at the bottom edge untouched, but is > able to flip all columns. The other transforms can be built up as sequences > of transpose and flip operations; for consistency, their actions on edge > pixels are defined to be the same as the end result of the corresponding > transpose-and-flip sequence.
signature.asc
Description: Digital signature
