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README.md: document how to remove alpha channel and --rotation=ifvalid

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Johannes Schauer Marin Rodrigues 2021-08-14 10:19:46 +02:00
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@ -80,7 +80,17 @@ Bugs
- Input images with alpha channels are not allowed. PDF only supports
transparency using binary masks but is unable to store 8-bit transparency
information as part of the image itself. But img2pdf will always be lossless
and thus, input images must not carry transparency information.
and thus, input images must not carry transparency information. You can
remove the alpha channel for example with imagemagick:
convert input.png -background white -alpha remove -alpha off output.png
- An error is produced if the input image is broken. This commonly happens if
the input image has an invalid EXIF Orientation value of zero. Even though
only nine different values from 1 to 9 are permitted, Anroid phones and
Canon DSLR cameras produce JPEG images with the invalid value of zero.
Either fix your input images with `exiftool` or similar software before
passing the JPEG to `img2pdf` or run `img2pdf` with `--rotation=ifvalid`.
- img2pdf uses PIL (or Pillow) to obtain image meta data and to convert the
input if necessary. To prevent decompression bomb denial of service attacks,