Paul Gideon Dann’s patchset for Poppler to enable Okular (Qt5) to use Cairo rather than Splash to render PDF files

If you view the same PDF file in Okular (KDE) and Evince (GNOME), you may notice that fonts and lines are rendered better in Evince. Both applications use Poppler to render text and graphics in PDF files, but Poppler uses a different rendering backend in the two applications. For Evince Poppler uses the Cairo library, whereas for Okular Poppler uses Splash, a backend inherited from Poppler’s predecessor Xpdf (still in development). Unfortunately for KDE users, Cairo often does a better job than Splash. However, independent software engineer Paul Gideon Dann came to the rescue by producing the patchset poppler-cairo-backend to modify Poppler in order to make it use the Cairo library instead of Splash when Poppler is used by Okular. To quote the README file for Paul’s patchset:

Purpose of this Patchset

Currently, the default backend for the Qt5 wrapper (used by Okular) is Splash. Unfortunately, Splash does not support subpixel rendering of fonts, so those of us using KDE are stuck with somewhat ugly-looking fonts. This patchset adds support for the Cairo backend to the Qt5 wrapper. It also forces subpixel rendering in the Cairo backend. The upshot of this is that we get beautiful fonts in Okular.

The README focuses on fonts, but in fact the rendering of lines in graphics in PDF files can also be improved by the application of the patchset.

Apparently the Poppler maintainer feels that the introduction of a dependency on Cairo to the Qt5 wrapper (even an optional dependency) in Poppler would be controversial, and he is not willing to merge the patchset. For Okular users who already have Cairo installed (e.g. for Firefox, Inkscape, Scribus and so on), and who are noticing inadequate rendering of some PDF files, Paul’s patchset is worth trying.

In Gentoo Linux, which is a source code-based distribution, it is very easy to apply the patchset. For example, I did the following to apply the patchset for Poppler 0.80.0 in a ~amd64 (Testing Branch) installation:

1. Created a package-specific and version-specific directory to hold the patchset:

root # mkdir -p /etc/portage/patches/app-text/poppler-0.80.0

2. Downloaded the patchset for Poppler 0.80.0 from the following Web page:

https://github.com/giddie/poppler-cairo-backend/tree/76e607bcf010d6d9b8df5cb0f851ef9c91d4caf2

3. Copied the patchset to the directory created in Step 1:

root # cp /home/fitzcarraldo/Downloads/*.patch /etc/portage/patches/app-text/poppler-0.80.0/
root # ls -1 /etc/portage/patches/app-text/poppler-0.80.0
0001-Cairo-backend-added-to-Qt5-wrapper.patch
0002-Setting-default-Qt5-backend-to-Cairo.patch
0003-Apply-subpixel-rendering-in-Cairo-Backend.patch

4. Checked first that the patchset could be applied successfully before actually using it:

root # cd /usr/portage/app-text/poppler
root # ebuild poppler-0.80.0.ebuild clean prepare
 * poppler-0.80.0.tar.xz BLAKE2B SHA512 size ;-) ...                                     [ ok ]
 * checking ebuild checksums ;-) ...                                                     [ ok ]
 * checking auxfile checksums ;-) ...                                                    [ ok ]
 * checking miscfile checksums ;-) ...                                                   [ ok ]
>>> Unpacking source...
>>> Unpacking poppler-0.80.0.tar.xz to /var/tmp/portage/app-text/poppler-0.80.0/work
>>> Source unpacked in /var/tmp/portage/app-text/poppler-0.80.0/work
>>> Preparing source in /var/tmp/portage/app-text/poppler-0.80.0/work/poppler-0.80.0 ...
 * Applying poppler-0.60.1-qt5-dependencies.patch ...                                    [ ok ]
 * Applying poppler-0.28.1-fix-multilib-configuration.patch ...                          [ ok ]
 * Applying poppler-0.78.0-respect-cflags.patch ...                                      [ ok ]
 * Applying poppler-0.61.0-respect-cflags.patch ...                                      [ ok ]
 * Applying poppler-0.57.0-disable-internal-jpx.patch ...                                [ ok ]
 * Applying 0001-Cairo-backend-added-to-Qt5-wrapper.patch ...                            [ ok ]
 * Applying 0002-Setting-default-Qt5-backend-to-Cairo.patch ...                          [ ok ]
 * Applying 0003-Apply-subpixel-rendering-in-Cairo-Backend.patch ...                     [ ok ]
 * User patches applied.
>>> Source prepared.

5. Re-merged Poppler to apply the patchset to the Poppler source code and rebuild the patched package:

root # emerge -1v poppler

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild   R    ] app-text/poppler-0.80.0:0/90::gentoo  USE="cairo cjk cxx introspection jpeg jpeg2k lcms png qt5 tiff utils -curl -debug -doc -nss" 0 KiB

Total: 1 package (1 reinstall), Size of downloads: 0 KiB

>>> Verifying ebuild manifests
>>> Emerging (1 of 1) app-text/poppler-0.80.0::gentoo
>>> Installing (1 of 1) app-text/poppler-0.80.0::gentoo
>>> Jobs: 1 of 1 complete                           Load avg: 1.06, 1.11, 0.95
>>> Auto-cleaning packages...

>>> No outdated packages were found on your system.

 * GNU info directory index is up-to-date.

6. Re-merged Okular so that it uses the patched Poppler dependency:

root # emerge -1v okular

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild   R    ] kde-apps/okular-19.08.1:5::gentoo  USE="chm crypt djvu image-backend pdf postscript tiff -debug -epub -handbook -markdown -mobi -mobile -plucker -share -speech -test" 0 KiB

Total: 1 package (1 reinstall), Size of downloads: 0 KiB

>>> Verifying ebuild manifests
>>> Emerging (1 of 1) kde-apps/okular-19.08.1::gentoo
>>> Installing (1 of 1) kde-apps/okular-19.08.1::gentoo
>>> Jobs: 1 of 1 complete                           Load avg: 1.17, 1.13, 1.04
>>> Auto-cleaning packages...

>>> No outdated packages were found on your system.

 * GNU info directory index is up-to-date.

My thanks go to Paul for taking the time to produce the patchset.

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About Fitzcarraldo
A Linux user with an interest in all things technical.

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