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The PCLm file Magic Number format should be:

%PDF-1.4
%PCLm-VERSION_NUMBER

However, this cannot be done using QPDF. For adding additional headers, QPDFWriter object has a function setExtraHeaderText(std::string const&). When used, the PCLm Magic Number gets added to the third line of the PCLm file, rather than the second line. The resulting file looks like:

%PDF-1.4
%BINARY_ENCODING
%PCLm-1.0

For streams with large size, i.e. large width and height, the DCT_DECODE compression for compressing the image fails. For instance, for this 600dpi raster file and strip height = 32, the PCLm file is empty, whereas for strip height = 6 and below, we get the PCLm file successfully.

https://github.com/sahilarora535/cups-filters/blob/ccb47b24178fc454713fd7b1051d80c1389db81b/filter/rastertopdf.cpp#L690-L697

The reason can be that there is a limit on the dimensions of the image using libjpeg. width and height are of type JDIMENSION in libjpeg. See the following blob in libjpeg which defines the type JDIMENSION:

https://github.com/LuaDist/libjpeg/blob/6c0fcb8ddee365e7abc4d332662b06900612e923/jmorecfg.h#L221-L232

The following Wiki article:

https://github.com/sahilarora535/cups-filters/wiki/Google-Summer-of-Code-2017-%7C-The-Linux-Foundation---Adding-PCLm-support-to-cups-filters

states: "CUPS-FILTERS is a software which is shipped with CUPS [....]"

This is not correct. CUPS-FILTERS is NOT shipped with CUPS!

CUPS-FILTERS nowadays is maintained by the Linux Foundation (even though that parts of its code are based on sources which were inherited from CUPS resp. donated and transferred to the Linux Foundation by Apple for future maintenance).

Most Linux distributions integrate and patch the original CUPS and bundle it with CUPS-FILTERS so that, in essence, CUPS-FILTERS constitutes as a third-party add-on module to the original CUPS maintained by Apple so that CUPS can be used on Linux.

The above statement is may mislead users to direct complaints and bug reports regarding CUPS-FILTERS (wrongly) towards Apple and the original CUPS maintainers.

Hence it would be nice if you changed the above Wiki sentence to reflect this.