Last time I used convert
for such a task I explicitly specified the size of the destination via resizing:
$ i=150; convert a.png b.png -compress jpeg -quality 70 \
-density ${i}x${i} -units PixelsPerInch \
-resize $((i*827/100))x$((i*1169/100)) \
-repage $((i*827/100))x$((i*1169/100)) multipage.pdf
The convert
command doesn't always use DPI as default density/page format unit, thus we explicitly specify DPI with the -units
option (otherwise you may get different results with different versions/input format combinations). The new size (specified via -resize
) is the dimension of a DIN A4 page in pixels. The resize argument specifies the maximal page size. What resolution and quality to pick exactly depends on the use case - I selected 150 DPI and average quality to save some space while it doesn't look too bad when printed on paper.
Note that convert
by default does not change the aspect ratio with the resize operation:
Resize will fit the image into the requested size.
It does NOT fill, the requested box size.
(ImageMagick manual)
Depending on the ImageMagick version and the involved input formats it might be ok to omit the -repage
option. But sometimes it is required and without that option the PDF header might contain too small dimensions. In any case, the -repage
shouldn't hurt.
The computations use integer arithmetic since bash
only supports that. With zsh
the expressions can be simplified - i.e. replaced with $((i*8.27))x$((i*11.69))
.
Lineart Images
If the PNG files are bi-level (black & white a.k.a lineart) images then the img2pdf
tool yields superior results over ImageMagick convert
. That means img2pdf
is faster and yields smaller PDFs.
Example:
$ img2pdf -o multipage.pdf a.png b.png
or:
$ img2pdf --pagesize A4 -o multipage.pdf a.png b.png
There are many more resize options in imagemagick. -scale
looks like what you need.
Also, do not use JPEG for pixel-perfect "miniatures"; use PNG instead (or if impossible, GIF). JPEG is designed for photos and uses lossy compression, resulting in distortion clearly noticeable after scaling your provided JPEG sample.
This article on image scaling might be interesting.
Best Answer
This should work with all the image types that ImageMagick can handle without having to specify
*.png
,*.jpg
,*.jpeg
etc:This will move the original images into a dated directory in case the results are not desirable. Also, this will fail if, for whatever reason, the image files have newlines in their names.
This script could do with some more error messages to give a helpful indication if anything went wrong. But for now if there is any error moving the images (everything between the
set -e
andset +e
), the script will exit. Hopefully this will avoid doing any irreversible damage.Update
Now with
awk
script shamelessly plagiarised from @terdon's answer.