Bug 43168, really crop image in PDF

hello,

i'm actually looking at https://bugs.freedesktop.org/show_bug.cgi?id=45168 .
"LibreOffice-Printing and PDF export enhancement/medium NEW Add Option to
crop Images in PDF Export"

As I understood, if an new option to be created is selected in the PDF
export dialog, the pdf writer must put in the pdf a smaller version of the
cropped image in the document.

If that's not right , please correct me.

If that's what expected, then :
A- have you any pointer on how to really crop the incoming image ?

B- the function
http://opengrok.libreoffice.org/xref/core/vcl/source/gdi/pdfwriter_impl.cxx#120his
almost an export test.
It's launch the first time you export a PDF.
I've defined DO_TEST_PDF, and the file pdf_export_test.pdf present in my
$HOME is corrupt. Evince & acroread 9.4.2 could not open it.
The idea is to modify the test function to include a smaller image.
If there is a more appropriate test in place where to add more test, fill
free to share it with me ;)

Michaël