JPEGhack

Once upon a time, I was thinking about lossless jpeg editing. The sort that jpegtran can do.

jpegtran (1)         - lossless transformation of JPEG files

Specifically, jpegtran can (within limits) rotate/flip/mirror losslessly, and also crop - for values of "lossless" that only apply to the cropped image data.

And I wondered if the reverse could be true. If jpeg image data can be cropped losslessly, then it cannot rely on surrounding data. Thus surely paste is equally possible.

I researched, and found... http://www.imagemagick.org/Usage/formats/#jpg_lossless. Scroll down to the 'Mixed JPEG Quality' section where it's suggested

* use jpegtran to merge the q60 on top of the q100

So, jpegtran can do that, huh?

nope.

But an experimental version can. http://jpegclub.org/jpegtran/

That reads to me that the 'drop' option has been experimental since 2000 at least... but, let's take a look. There are helpful binaries and a sample script provided, but I had something a bit more fun in mind.

One download later, and some fiddling around... and here is this!

Counting from the bottom where it's most evident, the rows of 80 pixels high each are of quality 1, 4, 5, 6, 7, 8, 9, 10, 15, 20, 50, and hte final 144 pixels at the top are original (ie, camera sourced jpeg with text composited in and saved at quality 100).

The resulting image is 153k in size. A direct conversion of the source (captioned) image to a constant quality at quality of 72 results in an image of equivalent size.

Additional notes

In creating this, I found a few things had to be kept in mind...

• Make sure the crops and drops are aligned on the jpeg iMCU boundaries, or headaches ensue since the crop size will be silently altered in position (as, I think, is the drop). I made all my changes on the boundary of 16x16 blocks, though 8x8 is also possible (depends on the jpeg)
• The caption text is a light grey - I found that pure white would cause jpeg errors when dropping (DCT coefficient out of range). I don't know if this is a bug, or just a limitation in what's possible due to jpeg encoding.