How to tell if your converter re-encoded every page

The complaint turns up constantly, and it is usually discovered far too late: a whole library converted, the originals deleted, and then somebody zooms in on a caption and the lettering has gone mushy.

The frustrating part is that it was avoidable, and checking takes a minute.

Why it happens at all

Re-encoding a page means decoding the original JPEG and writing a new one. The new file is smaller, which looks like a feature, and JPEG is lossy, so the detail spent on that saving is gone permanently. Do it twice and you are compressing an already-compressed image.

Most converters do this by default because it is simpler than the alternative and it makes the output size look impressive. Very few say so.

The thing is, for a CBR-to-CBZ conversion it is entirely unnecessary. Both are just archives. The page images can be moved across untouched.

The one-minute check

Take one book you have already converted, and its original.

1. Copy both, and rename the copies from .cbz or .cbr to .zip.
2. Open both. macOS will expand them into folders.
3. Compare the same page in each.

ls -l original/page-042.jpg converted/page-042.jpg

Identical byte counts mean the image was copied and nothing was lost. A smaller converted file means it was re-encoded, and the difference is quality you no longer have.

For a stricter answer, compare the actual contents rather than the sizes:

cmp original/page-042.jpg converted/page-042.jpg && echo "identical"

What to look for by eye

Re-encoding shows up first in the places with hard edges against flat color: lettering inside speech balloons, panel borders, and large areas of single-tone sky or skin. Zoom to 200% on a caption. Blocky halos around the letters are compression artifacts, not scanning.

What honest output looks like

A converter should be able to tell you, per page, whether the image was copied across or had to be re-encoded—and if it had to be, why. That is what CBZ Forge reports at the end of every conversion, page by page, instead of a single progress bar and a finished sound.

Where a page can be copied losslessly, it is. Where it cannot, you find out which page and what happened to it.