Editing JPEG (section)

===Lossless editing===

Several alterations to a JPEG image can be performed losslessly (that is, without recompression and the associated quality loss) as long as the image size is a multiple of 1 MCU block (Minimum Coded Unit) (usually 16 pixels in both directions, for 4:2:0 [[chroma subsampling]]). Utilities that implement this include:
* [[jpegtran]] and its GUI, Jpegcrop.
* [[IrfanView]] using "JPG Lossless Crop (PlugIn)" and "JPG Lossless Rotation (PlugIn)", which require installing the JPG_TRANSFORM plugin.
* [[FastStone Image Viewer]] using "Lossless Crop to File" and "JPEG Lossless Rotate".
* [[XnViewMP]] using "JPEG lossless transformations".
* [[ACDSee]] supports lossless rotation (but not lossless cropping) with its "Force lossless JPEG operations" option.

Blocks can be rotated in 90-degree increments, flipped in the horizontal, vertical and diagonal axes and moved about in the image. Not all blocks from the original image need to be used in the modified one.

The top and left edge of a JPEG image must lie on an 8&nbsp;×&nbsp;8 pixel block boundary (or 16&nbsp;×&nbsp;16 pixel for larger MCU sizes), but the bottom and right edge need not do so. This limits the possible '''lossless crop''' operations, and prevents flips and rotations of an image whose bottom or right edge does not lie on a block boundary for all channels (because the edge would end up on top or left, where&nbsp;– as aforementioned&nbsp;– a block boundary is obligatory).

Rotations where the image is not a multiple of 8 or 16, which value depends upon the chroma subsampling, are not lossless. Rotating such an image causes the blocks to be recomputed which results in loss of quality.<ref>{{cite web|url=http://petapixel.com/2012/08/14/why-you-should-always-rotate-original-jpeg-photos-losslessly/|title=Why You Should Always Rotate Original JPEG Photos Losslessly|date=14 August 2012|website=Petapixel.com|access-date=16 October 2017|archive-date=17 October 2017|archive-url=https://web.archive.org/web/20171017093502/https://petapixel.com/2012/08/14/why-you-should-always-rotate-original-jpeg-photos-losslessly/|url-status=live}}</ref>

When using lossless cropping, if the bottom or right side of the crop region is not on a block boundary, then the rest of the data from the partially used blocks will still be present in the cropped file and can be recovered. It is also possible to transform between baseline and progressive formats without any loss of quality, since the only difference is the order in which the coefficients are placed in the file.

Furthermore, several JPEG images can be losslessly joined, as long as they were saved with the same quality and the edges coincide with block boundaries.